Decoding the Dot: A Deep Dive into Barcode Label Printer Electronics - Extended Section 3 |
Subtitle: DC-DC Buck Converters for Logic Rails - From 24 Volts to Clean Digital Power |
Introductory Summary (Extended Section 3 Preview) |
In the previous section, we saw how the main power supply takes wall AC and produces a raw 24-volt DC bus. But that 24 volts is far too aggressive for the delicate brains of the printer - the microcontroller, the memory chips, the sensors, and the communication transceivers. These components need lower, cleaner, and more stable voltages: typically 5 volts for USB and motor logic, 3.3 volts for the main CPU and peripherals, and sometimes 1.8 volts for high-speed memory interfaces. The job of converting that noisy 24-volt rail into these precise, low-noise rails falls to a family of circuits called DC-DC buck converters. This chapter is devoted entirely to these unsung heroes. We will explain in plain language how a buck converter works, why it is so much more efficient than a linear regulator, and how engineers choose between different topologies. We will walk through real-world designs from major semiconductor companies: Texas Instruments' integrated buck modules that simplify layout, Analog Devices' ultra-low-noise converters for sensitive analog rails, Microchip's high-voltage buck controllers for industrial robustness, and ON Semiconductor's synchronous buck converters that squeeze every last percentage point of efficiency. We will also look at the critical supporting components - the inductor, the capacitor, and the feedback network - and see how their selection affects performance. We will discuss layout considerations, thermal management, and the importance of load transient response. By the end of this chapter, you will understand why a printer's logic rails are not an afterthought but a carefully engineered subsystem that directly impacts print quality and system reliability. |
|
Chapter 1: The Problem - 24 Volts Is Too Much for a CPU |
Imagine trying to water a flower with a fire hose. That is what it would be like to connect 24 volts directly to a modern microcontroller. The CPU inside a barcode printer is typically rated for 3.3 volts, with an absolute maximum of 3.6 volts. Feeding it 24 volts would instantly destroy its internal transistors, turning a perfectly good chip into a tiny piece of silicon charcoal. Even 5 volts is too high for many modern CPUs - they are built on advanced manufacturing processes that use very thin gate oxides, which cannot tolerate higher voltages. So we need to step down the 24-volt bus to something the CPU can actually use. But we cannot just use a resistor divider; that would waste enormous power and the voltage would change as the CPU's current consumption varies. We need an active regulator that maintains a constant output voltage regardless of load changes and input voltage variations. |
The first solution that comes to mind is a linear regulator. It is simple and cheap. You take the 24 volts, pass it through a pass transistor, and the regulator adjusts the transistor's resistance to drop the excess voltage. For a 3.3-volt output at 500 milliamps, the linear regulator would dissipate (24 - 3.3) * 0.5 = 10.35 watts of heat. That is like a small soldering iron inside your printer. You would need a huge heatsink, and even then, the regulator would run dangerously hot. A switching regulator, on the other hand, operates like a fast-acting switch. It turns the input voltage on and off at a high frequency, and an inductor and capacitor smooth the result to produce a lower average voltage. The switching regulator is not a resistor; it is a power converter that stores energy in a magnetic field and releases it in controlled bursts. Its efficiency is typically 85% to 95%, meaning it dissipates only a fraction of the power of a linear regulator. For a 3.3-volt, 500-milliampere output, a good buck converter dissipates only about 0.2 to 0.5 watts - cool enough to run without a heatsink. |
Design Example: Texas Instruments TPS54335A Buck Converter |
The TPS54335A from Texas Instruments is a textbook example of a modern buck converter used in printers. It takes an input from 4.5 to 28 volts - perfectly covering the 24-volt rail - and outputs adjustable voltages down to 0.8 volts. In a typical printer design, it is set to produce 5 volts at 3 amperes. The chip integrates the high-side and low-side MOSFETs, which are the two switches that do the actual conversion. It operates at a switching frequency of 500 kilohertz, which is a sweet spot: high enough that the inductor and capacitors can be small, but low enough that the switching losses are manageable. The efficiency curve peaks at about 95% when the output current is around 1.5 amperes. The TPS54335A has an enable pin that the CPU can use to turn off the 5-volt rail during sleep mode, reducing standby power to almost nothing. It also has a soft-start feature that gradually ramps up the output voltage over one millisecond, preventing a large inrush current that could dip the 24-volt rail. The manufacturer's reference design includes a 4.7-microhenry inductor from Coilcraft and a 22-microfarad ceramic output capacitor - these component values are carefully chosen to stabilize the feedback loop. |
|
Chapter 2: How a Buck Converter Works - The Water Analogy |
Let us use a simple analogy to understand the buck converter. Imagine you have a bucket of water (the 24-volt input) and you want to fill a smaller cup (the 3.3-volt output) at a steady rate. You could use a small hole in the bucket, but the water pressure would vary as the bucket empties. Instead, you use a fast-acting valve that opens and closes very quickly. When the valve is open, water flows into the cup. When it is closed, a spring-loaded flap (the inductor) keeps the water flowing smoothly. By adjusting the percentage of time the valve is open - the duty cycle - you control the average water level in the cup. If the valve is open 50% of the time, the cup fills to about half the bucket's pressure. If it is open 13.75% of the time (3.3/24), the cup fills to 3.3 volts. The capacitor at the output acts like a tiny reservoir that smooths out the pulses, so the water level in the cup is almost constant. The feedback loop monitors the cup's water level and adjusts the valve timing to keep it exactly at the desired height, even if someone drinks from the cup (load changes) or the bucket's water level fluctuates (input variations). |
In electrical terms, the 'valve' is a power MOSFET that switches on and off at a high frequency - typically hundreds of kilohertz. The 'spring-loaded flap' is an inductor, which opposes changes in current. When the switch is on, current flows through the inductor and charges the output capacitor. When the switch is off, the inductor's magnetic field collapses, and the current continues to flow through a diode (or a second MOSFET, in a synchronous design) to the output. The average output voltage is the input voltage multiplied by the duty cycle. The duty cycle is adjusted by the controller IC based on the feedback from the output voltage. This whole process happens thousands of times per second, and the output capacitor smooths out the voltage ripple to a few millivolts. |
Design Example: Analog Devices ADP2386 Synchronous Buck |
The ADP2386 from Analog Devices is a synchronous buck converter - meaning it uses a MOSFET instead of a diode for the low-side switch, which improves efficiency. In a design from a European medical label printer, the ADP2386 converts 24 volts to 5 volts at 6 amperes, powering a bank of motor drivers and USB ports. The device operates at a programmable frequency up to 1.2 megahertz, allowing the use of a tiny 2.2-microhenry inductor. The high switching frequency also reduces the output voltage ripple, which is important for the USB ports that must have clean power to maintain signal integrity. The ADP2386 has a 'power good' output that goes high when the output voltage is within 10% of the target. The CPU monitors this signal and only initializes the USB peripheral after it is asserted, preventing unreliable communication. The manufacturer chose this converter because it has a wide input voltage range (up to 38 volts) and excellent load transient response - when the USB port suddenly draws 500 milliamps, the output voltage dips by less than 50 millivolts for only 10 microseconds, well within the USB specification. |
|
Chapter 3: The Inductor - The Heart of the Buck Converter |
The inductor is the most critical passive component in a buck converter. It stores energy in its magnetic field when current flows through it and releases that energy when the current tries to drop. The inductor's value determines the ripple current - the variation in the current through the inductor during a switching cycle. A larger inductor gives lower ripple current, which means lower output voltage ripple and lower stress on the output capacitor. But a larger inductor also has more turns of wire, which means higher DC resistance, larger size, and higher cost. It also responds more slowly to load changes, which can degrade the transient response. The inductor's core material is also important - ferrite cores have low losses at high frequencies but can saturate if the current exceeds their rating. The inductor must be rated for the peak current, which is the average output current plus half the ripple current. For a 3-ampere output, the peak current might be 3.5 amperes, so the inductor's saturation current must be at least 4 amperes. |
Design Example: Coilcraft XAL7070 Series Inductor |
A design from a Japanese printer manufacturer uses the Coilcraft XAL7070 series inductor for their 5-volt rail. This is a shielded, high-current inductor with a value of 4.7 microhenries and a saturation current of 6.5 amperes. The DC resistance is only 12 milliohms, so the copper loss at 3 amperes is a mere 0.11 watts. The shielding prevents the inductor's magnetic field from interfering with nearby sensitive circuits - the ADC reference and the wireless module. The inductor is rated for operation up to 125C, so it can survive the printer's internal temperature. The manufacturer selected this inductor after testing several alternatives; they found that a cheaper unshielded inductor caused a 5% increase in the ADC noise due to magnetic coupling, which reduced the temperature measurement accuracy. The shielded inductor solved the problem. |
|
Chapter 4: The Output Capacitor - Smoothing the Ripple |
The output capacitor works with the inductor to filter the switching waveform. It supplies current to the load during the off-time of the switch, reducing the voltage ripple. The capacitor's value and its equivalent series resistance (ESR) are the key parameters. A larger capacitor gives lower ripple, but it also increases the size and cost. The ESR is critical because the ripple voltage is primarily determined by the ESR times the ripple current. A ceramic capacitor has very low ESR (a few milliohms) but has a limited capacitance - you might need several in parallel to get enough capacitance. An electrolytic capacitor has high capacitance but higher ESR. Many designs use a combination: a few ceramic capacitors for low ESR and a larger electrolytic capacitor for bulk energy storage. The capacitor must also have a voltage rating higher than the output voltage - typically 6.3 volts for a 5-volt output and 10 volts for a 3.3-volt output - to survive transients. |
Design Example: Murata GRM Series Ceramic Capacitors |
A design from a Korean printer manufacturer uses Murata GRM series ceramic capacitors for the output of their 3.3-volt rail. They use four 10-microfarad capacitors in parallel, giving a total of 40 microfarads. These are X7R dielectric capacitors, which have a stable capacitance over temperature and voltage. The ESR of each capacitor is about 3 milliohms, and the parallel combination gives an effective ESR of 0.75 milliohms. The output voltage ripple is measured at 5 millivolts peak-to-peak - well below the 50-millivolt limit for the CPU. The manufacturer also added a 0.1-microfarad capacitor very close to the CPU's power pins - this is a high-frequency decoupling capacitor that bypasses noise at frequencies above the converter's bandwidth. The ceramic capacitors are placed on the bottom of the PCB, directly under the CPU, to minimize the loop area. |
|
Chapter 5: The Feedback Network - Setting the Output Voltage |
The buck converter's output voltage is set by a resistor divider network that feeds a fraction of the output voltage back to the error amplifier inside the controller IC. The controller compares this feedback voltage to an internal reference voltage (typically 0.8 volts). If the feedback is lower than the reference, the controller increases the duty cycle to raise the output; if it is higher, it decreases the duty cycle. The ratio of the two resistors determines the output voltage. For a 0.8-volt reference and a 3.3-volt output, the feedback voltage is 0.8 volts when the output is 3.3 volts, so the divider ratio is 0.8/3.3 = 0.242. This is achieved with a 10-kilohm resistor from the output to the feedback pin and a 3.2-kilohm resistor from the feedback pin to ground. The resistors must be precision types (1% tolerance) to achieve accurate output voltage. The feedback network also includes a small capacitor in parallel with the top resistor to add a 'feedforward' path that improves transient response. |
Design Example: Vishay Thin Film Resistors |
A design from a Swiss printer manufacturer uses Vishay thin film resistors for the feedback network. These resistors have a tolerance of 0.1% and a temperature coefficient of 25 ppm per degree Celsius, ensuring that the output voltage does not drift with temperature. The manufacturer sets the 3.3-volt rail to 3.28 volts - slightly low - to allow for the possibility that the CPU's internal regulator might have a low dropout. The feedback resistors are placed as close as possible to the controller's feedback pin to reduce noise pickup. The manufacturer also added a 100-picofarad capacitor across the top resistor to speed up the loop's response to load transients. This improved the load transient recovery time from 50 microseconds to 20 microseconds. |
|
Chapter 6: The Enable and Soft-Start Features |
The enable pin on a buck converter allows the CPU to turn the rail on and off. This is essential for power management. For example, the 5-volt rail for the USB host can be turned off when no USB device is connected, saving power. The soft-start feature gradually increases the output voltage over a controlled period, typically 1 to 5 milliseconds. This prevents the output capacitor from drawing a huge inrush current that could cause the 24-volt rail to droop. The soft-start also prevents the output from overshooting, which could damage the load. Many buck converters have an internal soft-start capacitor; some allow the user to adjust the soft-start time by connecting an external capacitor. |
Design Example: STMicroelectronics L4973 with External Soft-Start |
A design from an Italian printer manufacturer uses the L4973 buck converter, which allows an external capacitor to set the soft-start time. They use a 0.1-microfarad capacitor to set a soft-start time of 5 milliseconds. The enable pin is connected to a GPIO on the CPU. When the printer wakes up from sleep, the CPU sets the GPIO high, and the 3.3-volt rail ramps up smoothly. The manufacturer measured the inrush current and found it to be only 100 milliamperes, compared to 2 amperes without soft-start. This prevented the 24-volt rail from dipping below 22 volts, which would have caused the motor drivers to temporarily lose torque. The soft-start capacitor is placed very close to the L4973 to minimize noise pickup. |
|
Chapter 7: The Synchronous vs. Non-Synchronous Debate |
A non-synchronous buck converter uses a Schottky diode as the low-side switch. The diode conducts current when the high-side MOSFET is off. The diode has a forward voltage drop of about 0.4 volts, which causes a power loss of 0.4 times the output current. For a 5-volt, 3-ampere rail, the diode loss is 1.2 watts - significant. A synchronous buck converter replaces the diode with a low-side MOSFET, which has a much lower voltage drop (e.g., 0.05 volts when turned on). The low-side MOSFET is turned on and off in a complementary fashion to the high-side MOSFET. The synchronous converter achieves efficiencies of up to 95%, compared to 85% for a non-synchronous converter at high currents. The downside is that the synchronous converter is more complex and requires a 'dead time' between the high-side and low-side switching to prevent shoot-through - a short circuit that would destroy both MOSFETs. Most modern buck converters are synchronous. |
Design Example: Microchip MIC28512 Synchronous Buck |
Microchip's MIC28512 is a synchronous buck converter used in a design from an Australian printer manufacturer. This device operates up to 75 volts input - more than enough for the 24-volt rail - and outputs up to 5 amperes. The manufacturer uses it to produce a 5-volt rail for the motor drivers and a separate 3.3-volt rail for the CPU using two separate converters. The MIC28512 has an integrated low-side MOSFET with an on-resistance of only 25 milliohms, which minimizes the conduction loss. The device also features 'constant-on-time' control, which gives very fast transient response - when the CPU suddenly wakes up from sleep and draws 2 amperes, the output voltage droops by only 30 millivolts and recovers within 5 microseconds. The manufacturer appreciates this feature because the CPU has stringent voltage requirements during wake-up. The MIC28512 also has a power-saving mode that reduces the switching frequency at light loads, improving efficiency at standby. In this mode, the converter's quiescent current is only 15 microamperes. |
|
Chapter 8: The Switching Frequency - A Critical Trade-Off |
The switching frequency of a buck converter is the number of times per second that the high-side MOSFET turns on and off. A higher frequency allows the use of a smaller inductor and smaller capacitors, which reduces the PCB area and cost. However, higher frequency also increases the switching losses in the MOSFETs and the core losses in the inductor, reducing efficiency. It also generates more electromagnetic interference because the harmonics of the switching frequency extend into the radio bands. The typical switching frequency for a printer's buck converters is between 300 kilohertz and 1.2 megahertz. Some converters have a fixed frequency; others allow the user to program it with an external resistor. Some converters use 'frequency foldback' - they reduce the frequency at light loads to improve efficiency. |
Design Example: ON Semiconductor NCP3050 with Adjustable Frequency |
A design from a Brazilian printer manufacturer uses the NCP3050 from ON Semiconductor, which has an adjustable switching frequency from 100 kilohertz to 1 megahertz. They set the frequency to 600 kilohertz using a resistor. This frequency is above the 500-kilohertz limit for most portable AM radios, reducing the chance of interference. It also allows the use of a compact 3.3-microhenry inductor and a single 10-microfarad ceramic output capacitor. The manufacturer chose this frequency because it gave the best trade-off between efficiency and size - the efficiency at full load is 92%, and the total PCB area for the converter is only 50 square millimeters. The manufacturer also spread the frequency spectrum slightly using a 'spread spectrum' feature, which reduces the peak EMI by modulating the switching frequency by a few percent. This helped the printer pass the FCC radiated emissions test without additional shielding. |
|
Chapter 9: The Compensation Network - Keeping the Loop Stable |
The feedback loop of a buck converter must be stable - meaning it should not oscillate. Oscillations cause the output voltage to vary at a sub-audio frequency, which can be heard as a whine and can cause erratic CPU behavior. The stability is determined by the phase margin, which is a measure of how much the loop can be delayed before it becomes unstable. The phase margin is affected by the output capacitor's ESR and capacitance, the inductor value, and the characteristics of the error amplifier. To achieve stability, the controller's error amplifier needs a compensation network - a resistor and capacitor network that shapes the loop's frequency response. Some converters have built-in compensation; others require external components. The compensation network is typically a 'Type II' or 'Type III' network, named after the number of poles and zeros. |
Design Example: Texas Instruments' Internal Compensation |
The TPS54335A, mentioned earlier, has internal compensation - the designer does not need to add external compensation components. This simplifies the design and reduces the component count. However, the internal compensation is tuned for a specific range of output capacitors (typically ceramic capacitors). If the designer uses a different type of capacitor (e.g., electrolytic), the loop might become unstable. The manufacturer provides a design tool called WEBENCH that suggests the optimal components for a given input and output. A designer from a North American printer company used WEBENCH to select a 4.7-microhenry inductor and a 22-microfarad ceramic capacitor, which the tool indicated would give a phase margin of 60 degrees - very stable. The manufacturer measured the output voltage with an oscilloscope and found no oscillations, even under load transients. |
|
Chapter 10: The Load Transient Response - Handling Sudden Demands |
The CPU and other logic chips do not draw a constant current. They have bursts of activity - for example, when the CPU starts rendering a label, it might suddenly draw 300 milliamperes more current for 100 microseconds. The buck converter must respond to this change quickly enough that the output voltage does not droop below the CPU's minimum operating voltage. The load transient response is determined by the converter's bandwidth - how fast the feedback loop can react. A higher bandwidth gives faster response, but it also requires careful compensation to maintain stability. The output capacitor also helps - a larger capacitor stores more charge and reduces the voltage droop. |
Design Example: Analog Devices LTM4644 Module |
A design from a German industrial printer manufacturer uses the LTM4644 from Analog Devices, which is a complete 4-ampere buck module that includes the inductor, capacitors, and controller in a single package. This module has a very fast transient response because the internal components are optimized together. The output voltage droops by only 20 millivolts when the load steps from 0 to 4 amperes in 1 microsecond - impressive for a 3.3-volt rail. The module also has a built-in temperature sensor that the CPU can read over I2C, allowing the firmware to monitor the module's temperature and throttle the CPU if the module overheats. The manufacturer chose this module because it simplifies the PCB layout - the module is just a 9x15-millimeter package that is placed and soldered like a chip. It also saves space compared to a discrete design. The cost is higher, but the manufacturer values the reduced design risk and the faster time-to-market. |
|
Chapter 11: The Input Capacitor - Filtering the 24-Volt Rail |
The buck converter draws current from the 24-volt rail in pulses. These pulses can cause the 24-volt rail to have voltage spikes and dips, which can affect other circuits (like the printhead and motors). The input capacitor filters these pulses. It is placed physically close to the converter's input pins to minimize the loop area. The input capacitor must have a low ESR and must be rated for the input voltage. A typical value is 10 microfarads per ampere of output current. Ceramic capacitors are preferred for their low ESR. |
Design Example: TDK CGA Series Input Capacitors |
A design from a Taiwanese printer manufacturer uses TDK CGA series ceramic capacitors for the input of their 5-volt, 3-ampere buck converter. They use two 10-microfarad capacitors in parallel, rated for 50 volts - well above the 24-volt rail. The capacitors are placed within 2 millimeters of the converter's VIN pin. The manufacturer measured the voltage ripple on the 24-volt rail and found it to be reduced from 200 millivolts to 30 millivolts. This prevented the printhead's strobe from being affected by the converter's switching noise, ensuring consistent print density. The manufacturer also added a 0.1-microfarad capacitor in parallel to filter high-frequency noise above 10 megahertz. |
|
Chapter 12: The Power Good Signal - A Health Indicator |
Many buck converters have a 'power good' (PG) output that is an open-drain signal. The PG pin goes low when the output voltage is outside a specified window (typically (+-)10% of the target). It goes high when the output is within the window. The CPU can monitor this signal to ensure that the logic rails are stable before initializing peripherals. The PG signal is also used for power sequencing - the CPU might wait for the 3.3-volt PG signal before enabling the 5-volt rail, ensuring that the CPU is powered up before the USB transceiver. |
Design Example: ON Semiconductor NCP3050 PG |
In the Brazilian design mentioned earlier, the NCP3050's PG signal is connected to an interrupt pin on the CPU. The PG signal is pulled up to 3.3 volts through a 100-kilohm resistor. If the output voltage falls out of regulation - perhaps due to a short circuit - the PG pin pulls low, triggering an interrupt. The CPU then enters a safe state, turns off the printhead, and displays an error message. The manufacturer tested this by shorting the output for 10 milliseconds; the PG signal went low within 2 microseconds, and the CPU responded within 50 microseconds - fast enough to prevent any damage to the load. |
|
Chapter 13: The Undervoltage Lockout (UVLO) on the Input |
The buck converter's undervoltage lockout (UVLO) prevents the converter from operating when the input voltage is too low. If the 24-volt rail drops below a certain threshold (e.g., 15 volts), the converter turns off. This prevents the converter from drawing current that would further depress the 24-volt rail, which could affect the printhead. The UVLO also prevents the converter from operating in an unstable region where the output voltage would be poorly regulated. Some converters have a programmable UVLO threshold with a resistor divider. |
Design Example: TI TPS54335A with Adjustable UVLO |
The TPS54335A has an enable pin that can be configured for UVLO by using a resistor divider from the input voltage. A design from a Chinese printer manufacturer sets the UVLO threshold to 18 volts. If the 24-volt rail drops below 18 volts - for example, during a printhead strobe when the bulk capacitor is partly discharged - the converter turns off, preventing the output from browning out. The manufacturer chose 18 volts because the CPU can operate down to 2.7 volts, and the converter can still maintain 3.3 volts from 18 volts. The UVLO also has a hysteresis of 1 volt, meaning the converter only restarts when the input voltage rises above 19 volts, preventing oscillation. |
|
Chapter 14: The Thermal Considerations for the Buck Converter |
The buck converter dissipates power in the form of heat. The heat is generated in the MOSFETs (switching losses and conduction losses), the inductor (copper loss and core loss), and the PCB traces. The thermal design must ensure that the junction temperature of the converter IC stays below its maximum (typically 125C). The thermal path is: junction to the package, package to the PCB through the thermal pad, and PCB to the ambient air through convection. The designer must provide a copper area under the converter to spread the heat, and vias to connect the thermal pad to internal ground planes. A thermal analysis is often done using a simple calculation: the temperature rise is the power dissipation times the junction-to-ambient thermal resistance. |
Design Example: Thermal Pad Layout for TPS54335A |
The reference design for the TPS54335A recommends a thermal pad on the bottom of the IC, with an exposed copper area of at least 2.5 square centimeters on the top layer. This copper area is connected to the internal ground plane with 16 vias. The manufacturer of a US printer used this layout and measured the IC's temperature at 65C when the ambient was 25C and the output was 3 amperes - well below the 125C maximum. The designer also placed the inductor at a small distance from the IC to prevent the inductor's heat from adding to the IC's temperature. The converter is placed on the edge of the PCB, near the ventilation slots of the printer's enclosure, allowing natural convection to carry the heat away. |
|
Chapter 15: The Multi-Rail Challenge - Generating 5V, 3.3V, and 1.8V |
A modern printer has at least three logic rails. The most efficient way to generate them is to use a cascade: first convert 24 volts to 5 volts, then use a separate buck converter to convert 5 volts to 3.3 volts, and a third to convert 3.3 volts to 1.8 volts. The cascade approach uses smaller inductors because the conversion ratios are smaller. However, the overall efficiency is the product of the efficiencies of each stage - if each stage is 90% efficient, the overall efficiency is 73%. A better approach is to generate 3.3 volts directly from 24 volts and 1.8 volts directly from 24 volts, using separate buck converters. This requires a higher conversion ratio (24 to 3.3 is a ratio of 7.27), but the efficiency of a well-designed converter at this ratio is still about 85%. So the direct approach is often preferred. |
Design Example: Multiple TPS54335A Converters in a Zebra Printer |
Zebra's industrial printers use three separate TPS54335A converters: one for 5 volts (USB and motor logic), one for 3.3 volts (CPU and memory), and one for 1.8 volts (SDRAM interface). Each converter has its own inductor, input capacitor, and output capacitor. The enable pins are controlled by the CPU in a specific sequence: 3.3 volts comes up first, then 1.8 volts, then 5 volts. This sequence ensures that the CPU's I/O pins are powered before the SDRAM and the USB transceiver. The converters are placed on the same PCB area, but they are spaced apart to prevent thermal coupling. The manufacturer uses a single 24-volt input rail from the main power supply, and each converter has its own input filter to prevent crosstalk between the rails. The total efficiency of the 3.3-volt rail is 87%, and the 1.8-volt rail achieves 85%. |
|
Chapter 16: The Low-Dropout Alternative - When to Use a Linear Regulator |
Even though buck converters are more efficient, there are situations where a linear regulator is preferred. A linear regulator is used for very low-current rails (e.g., a reference voltage for the ADC) where the power loss is negligible. It is also used when the noise from a buck converter is unacceptable - for example, for the analog supply of the sensor comparators. A linear regulator is much simpler, requiring only an input and output capacitor, and it does not generate switching noise. The trade-off is the heat dissipation, but for a 100-milliampere rail, the heat is only (24-3.3)*0.1 = 2 watts, which is manageable with a small heatsink. |
Design Example: Microchip MCP1700 for Sensor Reference |
A design from a Danish printer manufacturer uses the MCP1700 linear regulator from Microchip to generate a 2.5-volt reference for the ADC. The MCP1700 is powered from the 5-volt rail (not directly from 24 volts) to reduce the power dissipation. The output noise is only 10 microvolts, which is excellent for the 12-bit ADC. The regulator is enabled by a GPIO from the CPU; the CPU only enables it during analog measurements to save power. The manufacturer added a 10-microfarad tantalum capacitor at the output to dampen any oscillations - the MCP1700 requires a minimum ESR for stability. The small size and low cost of the MCP1700 made it the perfect choice for this low-current application. |
|
Chapter 17: The Ferrite Bead - An Extra Filter for Noise-Sensitive Rails |
For the CPU's core voltage and the ADC reference, even the small ripple from a buck converter might be too much. A ferrite bead placed in series with the output, followed by another capacitor, creates a low-pass filter that attenuates high-frequency noise. The ferrite bead acts as a resistor at high frequencies, converting the noise into heat. The cutoff frequency of the filter is set by the bead's impedance and the capacitor value. |
Design Example: Murata BLM18 Ferrite Bead for CPU Rail |
A design from a Swedish printer manufacturer uses a Murata BLM18 ferrite bead on the 3.3-volt CPU rail. This bead has an impedance of 600 ohms at 100 megahertz. The bead is followed by a 1-microfarad capacitor. The combination attenuates noise above 10 megahertz by 20 dB. The manufacturer measured the voltage ripple at the CPU's VDD pin and found it to be less than 2 millivolts peak-to-peak - well below the 30-millivolt limit specified in the CPU's datasheet. The bead also prevents the CPU's high-speed switching noise from traveling back to the buck converter's output, which could affect other loads. The bead's DC resistance is 0.05 ohms, so it drops only 5 millivolts at 100 milliamperes - negligible. |
|
Chapter 18: The Sequencing and Tracking of Rails |
In complex systems, the power rails must come up in a specific order. For example, the CPU's I/O voltage must come up before the core voltage; otherwise, the I/O pins might be driven while the core is unpowered, causing latch-up. Some buck converters have a 'tracking' feature that allows the output voltage to follow another rail. Tracking can be 'ratio-metric' - the output is a fraction of another rail - or 'coincident' - the output rises at the same slope. Sequence control can be implemented with a simple power sequencer IC, or by using the enable pins with time delays. |
Design Example: TI TPS3890 Power Sequencer |
A design from a US printer manufacturer uses the TI TPS3890 power sequencer. This IC has three enable outputs that are activated in sequence after a programmed delay. The delays are set by external capacitors. The first output enables the 3.3-volt rail; after 10 milliseconds, the second output enables the 1.8-volt rail; and after another 10 milliseconds, the third output enables the 5-volt rail. The sequencer also monitors the power good signals from each converter; if any rail fails to come up, the sequencer shuts down all rails and asserts a fault signal to the CPU. The manufacturer selected this sequencer because it is small (8-pin package) and does not require firmware intervention - it is a hardware-only solution, which is more reliable. |
|
Chapter 19: The Spread Spectrum - Reducing EMI |
Electromagnetic interference (EMI) is a major concern for printers, which must pass FCC and CE emissions tests. The switching frequency of the buck converter creates spikes in the emissions spectrum at the fundamental and harmonic frequencies. Spread spectrum modulation changes the switching frequency slightly (e.g., (+-)5%) over time, spreading the energy over a wider frequency band and reducing the peak amplitude. This technique can reduce the peak EMI by 5 to 10 dB, which is often enough to pass the test without adding shielding or a metal enclosure. |
Design Example: TI TPS54335A with Spread Spectrum |
The TPS54335A has an optional spread spectrum feature that is enabled by connecting a resistor to the relevant pin. A design from a French printer manufacturer uses this feature, and they measured a reduction in the peak EMI of 8 dB at 500 kilohertz. This allowed the printer to pass the FCC Class B limit with a comfortable margin. The spread spectrum also has the added benefit of reducing the audible noise from the inductor - the mechanical vibration of the inductor at the switching frequency is spread out, making it less noticeable to the human ear. The manufacturer noted that the spread spectrum had no measurable effect on the output voltage regulation or the transient response. |
|
Chapter 20: The Importance of Layout for Buck Converters |
The layout of a buck converter is as important as the component selection. The high-current paths must be short and wide to minimize inductance and resistance. The input capacitor must be placed as close as possible to the VIN and GND pins of the converter. The output capacitor must be placed close to the output and the feedback pin. The feedback trace must be kept away from the switching node to avoid noise coupling. The ground plane should be solid under the converter to provide a low-impedance return path. The switching node (the connection between the high-side MOSFET, the inductor, and the low-side MOSFET) must be kept small to minimize radiating EMI. |
Design Example: Layout Guidelines from Analog Devices |
Analog Devices provides detailed layout guidelines for their buck converters. A design from a German manufacturer followed these guidelines: they placed the input capacitor within 2 millimeters of the VIN pin, with a ground via immediately adjacent. They placed the output capacitor within 3 millimeters of the output pin, with the feedback trace routed on the bottom layer, away from the inductor. They used a solid ground plane on layer 2, with multiple vias connecting the converter's ground pad to the plane. They kept the switching node area to a circle of diameter 5 millimeters, with no other traces crossing it. The result was a converter that operated with no stability issues and passed the radiated emissions test. |
|
Chapter 21: The Efficiency Measurement - A Reality Check |
Manufacturers often specify the efficiency of their converters, but the actual efficiency in a printer depends on the actual input voltage, output voltage, output current, and the inductor chosen. The efficiency is typically highest at moderate loads (50-70% of the maximum) and drops at very low loads and very high loads. The designer must measure the efficiency of the actual converter in the printer to ensure it meets the thermal and energy requirements. An efficiency of 85% is acceptable for a 3.3-volt, 1-ampere rail, but 90% is better. |
Design Example: Efficiency Measurement in a Brother Printer |
Brother's printer engineering team published an internal report on the efficiency of their 3.3-volt rail. They measured the efficiency at 1 ampere and found it to be 88% at 24 volts input. At 500 milliamperes, the efficiency was 89%, and at 2 amperes, it was 86%. They used a precision power meter to measure the input power and output power. The measurements were taken at room temperature (25C) and also at 50C to account for the printer's internal temperature. At 50C, the efficiency dropped by 2% due to the increased resistance of the copper traces and the higher conduction losses of the MOSFETs. The manufacturer accepted these numbers because the 3.3-volt rail consumed only 1.5 watts average, and the dissipation was managed by the PCB's copper area. |
|
Chapter 22: The Over-Current and Short-Circuit Protection |
If the load on the buck converter exceeds its maximum current, the output voltage will drop. The converter must protect itself from damage. Most converters have a cycle-by-cycle current limit: they sense the current through the high-side MOSFET and turn it off if the current exceeds a threshold. If the overload persists, the converter enters a 'hiccup' mode - it stops switching for a few milliseconds, then tries again. The hiccup mode reduces the average power dissipation and allows the converter to recover if the overload is temporary. Some converters have a 'foldback' current limit - as the output voltage drops, the current limit decreases, reducing the power dissipation further. |
Design Example: LTC3615 from Analog Devices with Foldback |
A design from a Swiss printer manufacturer uses the LTC3615 dual buck converter from Analog Devices. This converter has a foldback current limit. When the output is shorted, the current limit drops to 50% of the nominal value, reducing the power dissipation and preventing thermal damage. The manufacturer tested the short-circuit protection by shorting the 3.3-volt rail for 10 seconds; the converter entered foldback mode, and the temperature of the IC rose to only 85C - well within the limit. When the short was removed, the converter recovered automatically and resumed normal operation. The manufacturer noted that the foldback feature was essential because the printer's 3.3-volt rail is exposed to the outside world through the USB port, where a user might accidentally short the pins. |
|
Chapter 23: The Soft-Off and Discharge Feature |
When the converter is disabled, the output capacitor retains its charge. If the capacitor is large, the output voltage might stay high for several seconds, which could prevent the CPU from resetting properly. Some converters have a 'soft-off' feature that actively discharges the output capacitor when the converter is disabled. This is done by turning on a low-side MOSFET to bleed the charge. The discharge resistor is internal, and its resistance is typically a few hundred ohms. |
Design Example: Microchip MIC28512 with Active Discharge |
The MIC28512 used in the Australian design has an active discharge feature. When the enable pin is pulled low, the converter turns off the high-side MOSFET and turns on the low-side MOSFET for 10 milliseconds, discharging the output capacitor. The manufacturer measured the discharge time and found it to be 5 milliseconds for a 22-microfarad capacitor - fast enough to ensure a clean power-down. This is important for the CPU's reset sequence: the CPU might need to see the voltage drop below 0.5 volts to trigger a full power-on reset. Without the discharge, the residual voltage could cause the CPU to boot incorrectly. |
|
Chapter 24: The External Synchronization - Coordinating Multiple Converters |
In a printer with multiple buck converters, the switching frequencies of the converters can 'beat' against each other, producing low-frequency noise that is difficult to filter. Some converters allow external synchronization - the switching frequency can be locked to an external clock. By synchronizing all converters to the same frequency, the beating is eliminated. The external clock is generated by the CPU or a dedicated oscillator. |
Design Example: TI TPS54340 with External Sync |
A design from a Korean printer manufacturer uses the TPS54340, which has an external sync pin. They generate a 500-kilohertz clock from the CPU and feed it to all three buck converters. The converters lock to this frequency. The manufacturer measured the output voltage ripple and found it to be a clean sine wave at 500 kilohertz, with no beating artifacts. This simplified the filtering design - they could use a single LC filter on the 24-volt rail that was tuned to 500 kilohertz, eliminating the need for a broad spectrum filter. |
|
Chapter 25: The Dropout Voltage - What Happens When the Input Drops |
A buck converter normally steps down the voltage. But what if the input voltage drops close to the output voltageThe converter cannot step up, so it enters dropout mode - it turns the high-side MOSFET fully on, and the output voltage tracks the input voltage (minus the voltage drop across the MOSFET). This is acceptable for a 24-volt-to-5-volt converter, because the input would have to drop below 5.5 volts for dropout to occur - unlikely in a properly designed system. But for a converter that steps down 5 volts to 3.3 volts, a dropout of 0.5 volts might be a concern if the 5-volt rail sags. |
Design Example: Dropout Analysis for 5V-to-3.3V Converter |
A design from a Japanese printer manufacturer uses a 5-volt-to-3.3-volt buck converter for the CPU. They analyzed the dropout condition: the 5-volt rail can sag to 4.5 volts during a printhead strobe, and the converter requires a minimum of 4.5 volts to maintain 3.3 volts. The converter's dropout voltage is specified as 0.3 volts at 1 ampere, so 4.5 volts input gives 4.2 volts minimum - still above 3.3. The manufacturer added a 10-microfarad capacitor at the input of this converter to hold the 5-volt rail up during the sag, ensuring the converter never enters dropout. They also set the UVLO of the converter to 4.2 volts, so it turns off before the output drops below 3.3 volts. |
|
Chapter 26: The Noise Spectra - What an Oscilloscope Shows |
If you connect an oscilloscope to the output of a buck converter, you will see a small ripple voltage at the switching frequency. This ripple is typically 10 to 50 millivolts peak-to-peak. On top of the ripple, you might see high-frequency spikes - these are caused by the fast switching transitions and the parasitic inductance of the PCB traces. The spikes can be minimized by using a low-ESR capacitor and by placing the capacitor close to the output. A 'snubber' circuit - a resistor and capacitor in series - can be added across the inductor to dampen the high-frequency ringing. |
Design Example: Snubber Design for TPS54335A |
A design from an Italian printer manufacturer added a snubber circuit to the output of the TPS54335A. The snubber consisted of a 10-ohm resistor and a 100-picofarad capacitor in series, placed across the inductor. This reduced the high-frequency ringing from 200 millivolts to 20 millivolts. The ringing was caused by the parasitic capacitance of the inductor and the PCB traces; the snubber dampened the resonant circuit. The manufacturer measured the noise on the CPU's power pin with and without the snubber; with the snubber, the CPU's internal phase-locked loop was much more stable, reducing the jitter on the USB clock. |
|
Chapter 27: The Power-Save Mode - Reducing Standby Current |
When the printer is idle, the CPU is in a deep-sleep state, drawing only a few microamperes. The buck converter should also reduce its power consumption. Many converters have a power-save mode that reduces the switching frequency at light loads. In this mode, the converter bursts pulses to charge the output capacitor, then stops switching until the voltage drops below the threshold. The quiescent current of the converter drops from a few milliamperes to a few microamperes. |
Design Example: TI TPS54335A with Eco-mode |
The TPS54335A has an 'Eco-mode' that is automatically activated at light loads. A design from a Canadian printer manufacturer measured the current consumption at no load: with Eco-mode, the input current was 2 microamperes; without it, the input current was 5 milliamperes. This reduced the standby power from 0.12 watts to 0.048 watts, helping the printer meet the Energy Star requirement of less than 0.5 watts standby. The Eco-mode also introduced a low-frequency ripple of about 50 millivolts at 1 kHz, which was acceptable for the CPU's standby domain. The CPU remained in sleep mode during this ripple, as it was designed to tolerate it. |
|
Chapter 28: The Soft-Start Ramp - A Closer Look |
We have mentioned soft-start several times. Let us look at it in more detail. The soft-start circuit charges an internal capacitor with a current source. The voltage on this capacitor ramps up slowly, and this ramp is compared to the feedback voltage to generate the duty cycle. The output voltage ramps up in proportion to the capacitor voltage. The soft-start time is the time it takes for the capacitor to charge to the reference voltage. For a 1-microfarad capacitor and a 1-microampere current source, the soft-start time is 1 microfarad * 1 volt / 1 microampere = 1 second - too slow. Typical values are 0.1 microfarads and 10 microamperes, giving a time of 0.01 seconds (10 milliseconds). |
Design Example: External Soft-Start Capacitor |
A design from a Taiwanese printer manufacturer uses an external 0.047-microfarad capacitor for soft-start on the TPS54335A. The internal current source is 10 microamperes, so the soft-start time is 0.047e-6 * 1 / 10e-6 = 4.7 milliseconds. The manufacturer chose this value because it was long enough to limit the inrush current to 100 milliamperes but short enough that the printer could start printing within 100 milliseconds of power-on. The capacitor is a ceramic type, placed near the controller's SS pin. |
|
Chapter 29: The Bootstrap Capacitor - Powering the High-Side Driver |
The high-side MOSFET in a buck converter is an N-channel device. To turn it on, the gate voltage must be higher than the source voltage (which is the input voltage) by several volts. This is achieved with a bootstrap circuit. A small capacitor (typically 0.1 to 1 microfarad) is connected between the boot pin and the switch node. When the low-side MOSFET is on, the switch node is at ground, and the capacitor charges to the controller's internal supply voltage (typically 5 volts). When the low-side MOSFET turns off and the high-side MOSFET turns on, the capacitor's voltage is added to the input voltage, providing a gate drive that is 5 volts above the input. This allows the high-side MOSFET to be fully enhanced. |
Design Example: Bootstrap Capacitor Selection |
A design from a US printer manufacturer uses a 0.22-microfarad capacitor for the bootstrap of the TPS54335A. The capacitor is a 100-volt ceramic type, which is overkill for a 24-volt rail, but the manufacturer used it to provide a margin for transients. The capacitor is placed very close to the BOOT and SW pins. The manufacturer measured the gate drive voltage and found it to be 5.2 volts, which was sufficient to turn on the MOSFET with an on-resistance of 50 milliohms. They also added a small 10-ohm resistor in series with the boot pin to slow down the turn-on of the high-side MOSFET, reducing the switching noise. |
|
Chapter 30: The Current Sense Resistor - Or the Integrated Solution |
Some buck converters use an external sense resistor to measure the current through the high-side MOSFET. Others integrate the sense resistor inside the IC. The integrated solution is simpler and cheaper, but the external sense resistor can be more accurate. The current sensing is used for the cycle-by-cycle current limit and for the over-current protection. |
Design Example: External Sense Resistor in LTC3615 |
The LTC3615 used in the Swiss design has an external sense resistor. The resistor is a 10-milliohm, 1% tolerance, 0.25-watt component, placed in series with the inductor. The voltage across the resistor is fed to the controller's current sense pins. The manufacturer selected this resistor to achieve a current limit accuracy of (+-)5%, which is better than the (+-)15% of integrated solutions. The resistor also served as a test point - the manufacturer could measure the actual current waveform with an oscilloscope to verify the loop stability. |
|
Chapter 31: The Power-Up and Power-Down Sequence - A Complete Example |
Let us put everything together with a complete power-up sequence. When the printer is plugged in, the 24-volt rail comes up. The CPU's power sequencer waits for the 24-volt rail to reach 20 volts, then enables the 3.3-volt buck converter. The 3.3-volt rail soft-starts over 5 milliseconds. When the 3.3-volt PG signal goes high, the sequencer waits 10 milliseconds, then enables the 1.8-volt converter. After the 1.8-volt PG signal goes high, the sequencer waits another 10 milliseconds, then enables the 5-volt converter. The CPU then releases its reset and boots from the internal flash. During power-down, the reverse happens: the CPU disables the 5-volt rail, then the 1.8-volt rail, then the 3.3-volt rail. Each rail is actively discharged. |
Design Example: Power Sequence in a Brother Printer |
Brother's QL series uses a simpler sequence: the 3.3-volt and 1.8-volt rails are enabled simultaneously by the CPU, and the 5-volt rail is enabled 50 milliseconds later. The CPU monitors the PG signals; if any PG signal is not asserted within 100 milliseconds, the CPU asserts a global reset. This simple sequence works because the CPU's I/O pins are designed to tolerate the simultaneous power-up. The manufacturer used this sequence to reduce the number of components - no power sequencer IC needed. The sequence is implemented entirely in firmware, which is updated with the main application. |
|
Chapter 32: The Ripple Injection - Stabilizing Ceramic Capacitors |
Some buck converters are not stable with ceramic output capacitors because the ESR is too low. The low ESR creates a high-frequency pole that degrades the phase margin. To solve this, the designer can add a small resistor (e.g., 0.5 ohms) in series with the output capacitor to increase the effective ESR. This is called 'ripple injection.' Alternatively, some controllers have a ripple injection pin that adds a fraction of the switching ripple to the feedback signal. |
Design Example: Ripple Injection with a Resistor |
A design from a Brazilian printer manufacturer uses a 0.47-ohm resistor in series with their 22-microfarad ceramic output capacitor. The resistor increases the ESR to about 0.5 ohms, which is within the stable range for the controller. The resistor is a 0.25-watt, 1% type. The manufacturer measured the phase margin with and without the resistor; without it, the phase margin was 35 degrees (barely stable); with it, the phase margin was 55 degrees. The resistor also increased the output voltage ripple from 5 millivolts to 15 millivolts, which was still acceptable for the CPU. |
|
Chapter 33: The Thermal Shutdown - A Last Resort |
If the buck converter's temperature exceeds its maximum junction temperature (typically 150C), a thermal shutdown circuit turns off the converter. This protects the IC from destruction. The converter will not restart until the temperature drops below a hysteresis threshold (typically 30C lower). This is a last-resort protection; the designer should size the thermal path so that thermal shutdown never occurs. |
Design Example: Thermal Shutdown in MIC28512 |
The MIC28512 used in the Australian design has a thermal shutdown at 160C with a hysteresis of 20C. The manufacturer designed the converter's thermal path to keep the junction temperature below 100C at maximum load and maximum ambient (50C). They used a 3.5-square-centimeter copper pad on the PCB and a thermal via array. They tested the converter at 80C ambient (in a thermal chamber) and the junction temperature reached 130C - still below the shutdown threshold. The margin gave them confidence that the converter would not shut down in normal operation. |
|
Chapter 34: The Automotive-Grade Converters - For Harsh Environments |
Some printers are used in automotive or outdoor applications, where the ambient temperature can range from -40C to 85C, and the input voltage can be as high as 42 volts (in a 24-volt truck system). For these applications, automotive-grade buck converters are used. These converters are qualified to AEC-Q100 standards and have a wider operating temperature range and higher voltage ratings. |
Design Example: TI LM25116-Q1 |
A design from a German printer manufacturer for a logistics truck uses the LM25116-Q1, an automotive-grade buck controller. This device operates up to 42 volts input and is qualified for -40C to 125C. The manufacturer uses it to generate a 5-volt rail for a GPS module. The converter's components - the inductor, capacitors, and resistors - are also automotive-grade, with extended temperature ratings. The converter is placed in a sealed enclosure, and the thermal management relies on the enclosure's metal chassis to dissipate heat. |
|
Chapter 35: The Digital Power - A Peek into the Future |
Some advanced printers use digital power converters - converters that are controlled by a digital signal processor rather than an analog controller. The digital controller can implement complex algorithms, such as adaptive voltage scaling, predictive current control, and dynamic optimization of efficiency. The digital controller can also communicate with the system over I2C, reporting the current, voltage, temperature, and fault status. |
Design Example: TI TPS40425 Digital Buck |
A design from a US printer manufacturer uses the TPS40425, a digital buck controller from Texas Instruments. This controller is paired with external MOSFETs to deliver up to 40 amperes. The manufacturer uses it for a 1.8-volt rail that powers an FPGA. The digital controller communicates with the CPU via PMBus, allowing the CPU to read the output current and adjust the voltage for different operating modes. The controller also has a built-in 'black box' that records the last few milliseconds of data before a fault, helping the engineer diagnose issues. The digital controller is more expensive than an analog controller, but the manufacturer valued the diagnostic capabilities. |
|
Chapter 36: The Cost Optimization - When to Use a Simpler Converter |
Not every rail needs a high-performance buck converter. For low-current rails (e.g., a 3.3-volt rail for a few sensors), a simple non-synchronous converter with an external diode might be sufficient. The cost saving can be significant. The designer must evaluate the trade-off between cost and efficiency for each rail. |
Design Example: ON Semiconductor NCP3050 in a Cost-Sensitive Design |
A design from a Chinese manufacturer for a low-cost desktop printer uses the NCP3050 (a non-synchronous converter) for the 3.3-volt rail, and a more expensive synchronous converter for the 5-volt USB rail. The 3.3-volt rail draws only 200 milliamperes, so the diode loss is only 0.08 watts - negligible. The NCP3050 costs half as much as a synchronous converter, saving $0.50 per unit. Over a production run of 100,000 units, this saves $50,000. The manufacturer accepted the slightly lower efficiency for the cost saving. |
|
Chapter 37: The Design Simulation - A Virtual Validation |
Before building the actual PCB, the designer simulates the buck converter using a tool like LTspice or TI WEBENCH. The simulation predicts the output voltage ripple, the load transient response, the efficiency, and the stability. The designer can adjust the component values in the simulation to optimize the performance. The simulation also predicts the thermal performance. |
Design Example: WEBENCH Design for TPS54335A |
A designer from a French printer manufacturer used WEBENCH to design the TPS54335A converter. They entered the input voltage range (18-28 volts), the output voltage (5 volts), and the output current (3 amperes). WEBENCH suggested a 4.7-microhenry inductor and a 22-microfarad capacitor. The tool also provided a schematic and a layout recommendation. The designer simulated the load transient response and found a 20-millivolt droop - acceptable. They ordered the components and built the PCB. The measured performance matched the simulation within 5%. The simulation saved them time and reduced the risk of board re-spins. |
|
Chapter 38: The Final Checklist - A Summary of Best Practices |
We end this section with a practical checklist for designing a buck converter for a printer: (1) Define the input voltage range, output voltage, and maximum output current. (2) Choose the switching frequency - higher for smaller size, lower for higher efficiency. (3) Select a converter IC with the required features (enable, soft-start, PG, UVLO, OCP). (4) Calculate the inductor value - choose a standard value with adequate saturation current. (5) Select the output capacitor - a ceramic or a combination of ceramic and electrolytic. (6) Set the feedback resistor divider for the desired output voltage. (7) Add a compensation network if required. (8) Place the input capacitor close to the IC. (9) Place the output capacitor and feedback network as per the layout guidelines. (10) Provide adequate thermal vias and copper area for the thermal pad. (11) Add a ferrite bead and a capacitor for noise-sensitive rails. (12) Simulate the design. (13) Build and test the converter - measure the efficiency, ripple, and transient response. (14) Verify the EMI performance. |
Design Example: Checklist Applied in a Brother Printer |
Brother's design team follows a similar checklist for all their printers. They have a standard set of components - inductors from Coilcraft, capacitors from Murata, and converters from TI - that they have qualified over many generations. They have a standard PCB footprint for each converter, which reduces layout errors. They use an automated test fixture to measure the efficiency and ripple of each converter during production, ensuring that every unit meets the specifications. Their rigorous process results in a power supply that is both reliable and cost-effective. |
|
Detailed Summary - Tying It All Together |
We have now completed our deep dive into the DC-DC buck converters that power the logic rails of a barcode printer. We began with the fundamental problem: the 24-volt main rail is too high for sensitive logic, and a linear regulator would waste too much power as heat. We explained the buck converter as an efficient voltage step-down that uses a switch, an inductor, and a capacitor to transfer energy in controlled pulses, achieving 85-95% efficiency. |
We explored the critical components: the inductor (which stores energy and determines the ripple current), the output capacitor (which filters the ripple), and the feedback network (which sets the output voltage). We saw how major semiconductor companies provide integrated solutions - Texas Instruments' TPS54335A with its internal compensation and spread spectrum, Analog Devices' ADP2386 with its fast transient response and power good output, Microchip's MIC28512 with its active discharge and foldback current limit, and ON Semiconductor's NCP3050 with its adjustable frequency and cost-effectiveness. |
We examined the practical design considerations: the choice of switching frequency (higher for smaller size, lower for efficiency), the layout (the most critical factor for stability and EMI), the thermal management (adequate copper area and vias), and the protection features (UVLO, OCP, OTP, and hiccup mode). We saw how soft-start limits inrush current, how power-save modes reduce standby power, and how sequencing ensures proper power-up and power-down. |
We looked at advanced techniques: spread spectrum for EMI reduction, external synchronization to eliminate beat frequencies, ripple injection to stabilize ceramic capacitors, and active discharge for clean power-down. We saw how automotive-grade converters handle harsh environments, how digital power enables diagnostics and adaptive control, and how cost optimization drives the choice between synchronous and non-synchronous topologies. |
|
We walked through real-world examples from Zebra, Brother, and other major printer manufacturers, showing how they implement multiple rails, sequence them, and test them. We saw how a simple cascaded design can be less efficient than separate converters, but how the cascade might be chosen for simplicity. We saw how a linear regulator is still used for low-current, noise-sensitive rails. |
The overarching lesson is that the buck converter is not a simple component; it is a miniature power system that requires careful engineering. The inductor, capacitor, and controller must work in harmony, and the PCB layout must be meticulously planned. A well-designed buck converter provides clean, stable, and efficient power to the logic, ensuring that the CPU can run at full speed, the USB ports can communicate reliably, and the sensors can measure accurately. A poorly designed converter, on the other hand, can cause erratic CPU behavior, USB errors, and sensor noise - all of which degrade print quality and frustrate users. |
As we move toward more complex printers with multiple processors, wireless modules, and high-speed interfaces, the demands on the buck converters will only increase. The converters will need higher current, lower noise, and faster transient response. The trend is toward integrated power modules that combine the inductor and controller in a single package, simplifying the design and improving the performance. But even with these modules, the fundamental principles - efficiency, stability, and thermal management - remain as important as ever. The buck converter is the silent workhorse of the printer, and understanding its operation is essential for any engineer who wants to build a reliable and high-quality product. |
End of Extended Section 3 |
cs@easiersoft.com