Decoding the Dot: A Deep Dive into Barcode Label Printer Electronics - Extended Section 8 |
Subtitle: Gate Drive Circuitry - Level Shifting and High-Side Drivers |
Introductory Summary (Extended Section 8 Preview) |
In the previous section, we explored the fundamental gate drive circuitry that turns power MOSFETs on and off. But we touched on a critical challenge: the logic signals from the CPU operate at 3.3 volts, while the gate drive often requires 5, 10, or even 12 volts to fully enhance the MOSFET. Furthermore, in many printer designs, the MOSFET that controls the main power to the printhead sits on the high side of the load - its source is connected to the 24-volt rail, and its drain goes to the printhead. Driving this high-side MOSFET requires a gate voltage that is even higher than the 24-volt rail, which is a completely different challenge from driving a low-side MOSFET. This chapter is devoted entirely to level shifting and high-side gate drive techniques. We will explain why level shifting is necessary, how it is achieved with simple transistor circuits, dedicated level-shifter ICs, and integrated solutions. We will then dive into the world of high-side drivers, exploring bootstrap circuits, charge pumps, and isolated gate drivers. We will look at real-world designs from major semiconductor companies: Texas Instruments' level-shifter ICs that bridge 3.3-volt and 5-volt domains, Analog Devices' isolated gate drivers that provide galvanic isolation for safety, Infineon's high-side drivers with integrated bootstrap diodes, ON Semiconductor's level shifters for motor control, and Microchip's high-side driver ICs for portable printers. We will also examine the timing challenges, the parasitic effects, and the protection features that make these circuits reliable. By the end, you will understand how a 3.3-volt signal from a tiny CPU pin can ultimately control a 24-volt, 15-ampere power path, and you will appreciate the elegant circuit techniques that make this possible. |
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Chapter 1: The Problem - A World of Different Voltages |
A typical barcode printer is a land of many voltage levels. The CPU runs at 3.3 volts. The shift registers and some logic run at 5 volts. The main power rail is 24 volts. And the gate of a high-side P-channel MOSFET must be pulled to 24 volts to turn it off, and to about 12 volts to turn it on. The signals from the CPU cannot directly drive any of these other voltage domains. They must be translated. Level shifting is the process of taking a signal from one voltage domain and converting it to a signal that is compatible with another. Level shifting is required for the clock, data, latch, and strobe signals that go from the CPU to the shift registers and gate drivers. It is also required for the gate drive signals that go from the gate driver to the MOSFETs. Level shifting can be done with discrete transistors, with dedicated ICs, or with integrated solutions that combine the level shifter with the gate driver. The choice of level-shifting method depends on the speed, the voltage, the current, and the cost. |
Design Example: Texas Instruments TXB0104 Level Shifter |
The TXB0104 from Texas Instruments is a 4-channel, bidirectional level shifter that can translate signals between 1.2-volt and 3.6-volt domains and between 1.8-volt and 5.5-volt domains. It is used in many printer designs to connect the 3.3-volt CPU to 5-volt shift registers. The TXB0104 has built-in automatic direction sensing, so it does not require a direction pin. It also has a built-in 10-kilohm pull-up resistor on each channel, which simplifies the design. In a design from a European printer manufacturer, the TXB0104 is used to level-shift the clock, data, latch, and strobe signals. The manufacturer chose the TXB0104 because it is fast enough for the 20-megahertz clock, and it is available in a small package that saves board space. The manufacturer measured the propagation delay and found it to be 5 nanoseconds - fast enough for the printer's timing. |
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Chapter 2: The Discrete Level Shifter - A Simple Transistor Circuit |
Before dedicated level-shifter ICs became common, level shifting was done with discrete transistors. A simple NPN transistor can be used as a level shifter. The transistor is connected in a common-emitter configuration. The input signal (3.3 volts) is applied to the base through a resistor. The collector is connected to the higher voltage (5 volts) through a pull-up resistor. When the input is high, the transistor turns on, pulling the collector low. When the input is low, the transistor turns off, and the collector is pulled high by the pull-up resistor. This inverts the signal, so an additional inverter might be needed. The discrete level shifter is simple and cheap, but it is not as fast as a dedicated IC, and it consumes more power. The discrete level shifter is still used in some low-speed applications. |
Design Example: Discrete Level Shifter in a Low-Cost Printer |
A low-cost printer from a Chinese manufacturer uses a discrete level shifter for the strobe signal. The level shifter consists of a 2N3904 NPN transistor, a 10-kilohm base resistor, and a 1-kilohm collector pull-up resistor. The input is 3.3 volts from the CPU, and the output is 5 volts to the strobe driver. The manufacturer measured the propagation delay and found it to be 50 nanoseconds - which is acceptable for the strobe signal, which has a rise time of only 15 nanoseconds. The manufacturer used a discrete level shifter because it was cheaper than a dedicated IC, and the design was simple. |
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Chapter 3: The Integrated Level Shifter - Faster and More Reliable |
Dedicated level-shifter ICs are faster, more reliable, and more versatile than discrete circuits. They have built-in protection (ESD, over-voltage), and they often have multiple channels in a single package. The integrated level shifter is the preferred choice for most printer designs. The integrated level shifter can be unidirectional (translating signals in one direction) or bidirectional (translating signals in both directions). The bidirectional level shifter is useful for I2C and SPI buses that require bidirectional communication. The unidirectional level shifter is used for the clock, data, latch, and strobe signals, which are always driven by the CPU. |
Design Example: ON Semiconductor NLV17SZ125 Level Shifter |
ON Semiconductor's NLV17SZ125 is a single-channel, unidirectional level shifter. It is a buffer with 3-state output. It operates from a 2.3-volt to 5.5-volt supply, and it can translate signals from a 3.3-volt input to a 5-volt output. The NLV17SZ125 is used in a design from a Taiwanese printer manufacturer to level-shift the clock signal. The manufacturer chose this IC because it is fast (propagation delay of 3.5 nanoseconds) and it has a very low quiescent current of 1 microampere. The manufacturer used 4 of these ICs (one for clock, data, latch, and strobe) instead of a single 4-channel IC, because the single-channel IC was cheaper and more readily available. |
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Chapter 4: The Level Shifter for the Strobe - A Special Case |
The strobe signal is the most critical signal in the printer. It must be level-shifted with very low delay and very low jitter. Any jitter on the strobe signal will cause variations in the print density. The level shifter for the strobe must have a propagation delay that is matched to the other signals, and it must have a very high bandwidth. The strobe level shifter is often a dedicated IC, or it is integrated into the gate driver. The strobe level shifter must also have a high current capability because the strobe signal drives the gates of multiple gate drivers. |
Design Example: Strobe Level Shifter in Zebra Printers |
Zebra's high-end printer uses a dedicated level shifter (SN74LV4T125) for the strobe signal. This is a 4-channel level shifter with 3-state outputs. The manufacturer uses one channel for the strobe, and the other channels are used for other signals. The level shifter is powered by 5 volts, and it translates the 3.3-volt strobe from the CPU to a 5-volt strobe that drives the gate driver. The manufacturer measured the propagation delay and found it to be 4 nanoseconds - which is excellent. The jitter on the strobe signal was measured at less than 100 picoseconds, which is well within the printer's requirement. |
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Chapter 5: The High-Side Switch - A Different Kind of MOSFET |
In a thermal printer, the main power to the printhead is controlled by a high-side switch. The high-side switch is typically a P-channel MOSFET. The source of the P-channel MOSFET is connected to the 24-volt rail. The drain is connected to the printhead. The gate is driven by a gate driver that pulls the gate to ground to turn the MOSFET on, and to 24 volts to turn it off. The P-channel MOSFET is used because it is easier to drive than an N-channel MOSFET in a high-side configuration - the gate voltage does not need to exceed the drain voltage. However, the P-channel MOSFET has a higher on-resistance than an N-channel MOSFET of the same size, and it is more expensive. The high-side switch must be able to handle the peak current of 15 amperes, and it must have a low on-resistance (typically 10 to 20 milliohms). The high-side switch is also used to disconnect the printhead from the 24-volt rail in case of a fault. |
Design Example: High-Side P-Channel MOSFET in Sato Printers |
Sato's printer uses a P-channel MOSFET (IRF4905) as the high-side switch. The IRF4905 is a 55-volt, 74-ampere, 20-milliohm device. The source is connected to the 24-volt rail, and the drain is connected to the printhead. The gate is driven by a high-side gate driver (IR2110). The manufacturer chose the IRF4905 because it has a very low on-resistance and it is available in a TO-220 package, which is easy to mount on a heatsink. The manufacturer measured the voltage drop across the MOSFET at 15 amperes and found it to be 0.3 volts - which is acceptable. |
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Chapter 6: The High-Side Gate Driver - A Floating Supply |
The high-side gate driver must provide a gate voltage that is referenced to the source of the P-channel MOSFET. The source is at 24 volts when the MOSFET is off, and it is at about 24 volts minus a small drop when the MOSFET is on. The gate driver must generate a voltage that is 12 volts below the source to turn the MOSFET on, and 24 volts above the source to turn it off. This is achieved with a floating supply. The floating supply is typically provided by a bootstrap circuit. The bootstrap circuit uses a capacitor that is charged to 12 volts when the MOSFET is off. When the MOSFET is to be turned on, the capacitor's voltage is added to the source voltage, providing a gate voltage that is 12 volts above the source. The gate driver also includes a level shifter that translates the logic signal (referenced to ground) to the floating domain. |
Design Example: Bootstrap High-Side Driver in Zebra Printers |
Zebra's printer uses the IR2110 high-side gate driver from Infineon. The IR2110 has a bootstrap circuit that uses an external capacitor. The bootstrap capacitor is a 0.1-microfarad ceramic capacitor connected between the VB and VS pins. The IR2110 also has a level shifter that can translate a 3.3-volt logic signal to the high-side domain. The manufacturer chose the IR2110 because it is a proven, reliable part that is widely used in power electronics. The manufacturer measured the gate voltage and found it to be 12.5 volts when the MOSFET is on, and 24 volts when the MOSFET is off - exactly as required. |
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Chapter 7: The Bootstrap Capacitor - The Heart of the High-Side Driver |
The bootstrap capacitor is the key component in the bootstrap high-side driver. The capacitor is charged through a diode (or a built-in diode in the gate driver) when the MOSFET is off. The charge is stored in the capacitor. When the MOSFET is to be turned on, the capacitor's voltage is added to the source voltage, providing the gate drive. The bootstrap capacitor must be large enough to provide the gate charge and the leakage current of the gate driver for the duration of the on-time. The capacitor is typically 0.1 to 1 microfarads. The capacitor must be a ceramic type with a low ESR and a high voltage rating. The capacitor is placed as close as possible to the gate driver's VB and VS pins. |
Design Example: Bootstrap Capacitor in Sato Printers |
Sato's printer uses a 0.22-microfarad bootstrap capacitor for the IR2110. The capacitor is a 50-volt X7R ceramic capacitor. The manufacturer calculated the gate charge of the P-channel MOSFET and found it to be 50 nanocoulombs. The gate driver's leakage current is 10 microamperes. The on-time of the strobe is 500 microseconds. The voltage drop on the bootstrap capacitor during the on-time is (Q_gate + I_leak * t_on) / C = (50e-9 + 10e-6 * 500e-6) / 0.22e-6 = (50e-9 + 5e-9) / 0.22e-6 = 0.25 volts. This is acceptable, as the gate driver requires a minimum of 10 volts to maintain the gate drive. The manufacturer used a 0.22-microfarad capacitor, which is a standard value. |
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Chapter 8: The Bootstrap Diode - A Critical Component |
The bootstrap diode is used to charge the bootstrap capacitor. The diode is connected between the gate driver's supply voltage (12 volts) and the bootstrap pin (VB). When the MOSFET is off, the source is at 24 volts, and the VB pin is at 24 + 12 = 36 volts. The diode prevents the bootstrap capacitor from discharging through the supply. The bootstrap diode must be a fast-recovery diode with a reverse voltage rating higher than the maximum voltage (which is 24 + 12 = 36 volts). The diode's forward voltage drop should be low to minimize the charging time. Many gate drivers have a built-in bootstrap diode, which simplifies the design. |
Design Example: Bootstrap Diode in Zebra Printers |
Zebra's printer uses the IR2110, which has a built-in bootstrap diode. The manufacturer did not need to add an external diode. The built-in diode is a fast-recovery diode with a reverse recovery time of 50 nanoseconds. The manufacturer measured the charging of the bootstrap capacitor and found it to be fully charged within 1 microsecond - which is fast enough for the 500-microsecond strobe cycle. |
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Chapter 9: The Charge Pump - An Alternative to the Bootstrap |
An alternative to the bootstrap circuit is the charge pump. A charge pump uses capacitors and switches to generate a voltage that is higher than the supply voltage. The charge pump does not require the MOSFET to be turned off to recharge the capacitor - it can operate continuously. The charge pump is useful for applications where the on-time is long (so the bootstrap capacitor would discharge) or where the duty cycle is high. However, the charge pump is more complex and more expensive than the bootstrap circuit. The charge pump is used in some high-end printers that have a very high duty cycle. |
Design Example: Charge Pump in a High-End Printer |
A high-end printer from a US manufacturer uses a charge pump (LTC3245 from Analog Devices) to generate the 12-volt gate drive for the high-side switch. The charge pump operates from a 24-volt input and generates a 12-volt output. The charge pump is used because the printer operates at a very high duty cycle (the strobe is on for 80% of the time), which would drain the bootstrap capacitor. The manufacturer chose the charge pump because it provides a stable 12-volt output regardless of the duty cycle. The charge pump has an output current of 50 milliamperes, which is sufficient for the gate drive. |
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Chapter 10: The High-Side Level Shifter - Translating the Logic Signal |
The high-side gate driver must translate the logic signal (which is referenced to ground) to the floating domain. The level shifter in the high-side driver is a high-voltage circuit that can withstand the voltage difference between the ground and the floating domain (which can be up to 36 volts). The level shifter is typically a pulse transformer or a differential amplifier. The level shifter must have a low propagation delay and a high common-mode rejection. The level shifter is integrated into the high-side gate driver IC. |
Design Example: Level Shifter in IR2110 |
The IR2110 has a built-in level shifter that uses a high-voltage differential amplifier. The level shifter can handle a common-mode voltage of up to 600 volts, which is much higher than the printer's 24 volts. The propagation delay of the level shifter is 20 nanoseconds. The manufacturer of the Zebra printer uses the IR2110's level shifter without any additional components. The level shifter is powered by the 12-volt supply, and it translates the 3.3-volt logic signal to the floating domain. |
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Chapter 11: The Isolated Gate Driver - For Safety and Noise Immunity |
In some printers, the high-side switch is controlled by an isolated gate driver. The isolated gate driver uses an optocoupler or a magnetic coupler to provide galvanic isolation between the logic and the power domains. The isolation protects the CPU from high-voltage spikes, and it reduces the noise coupling between the power and logic domains. The isolated gate driver is more expensive than a non-isolated driver, but it is used in medical or aviation printers where safety is paramount. |
Design Example: Isolated Gate Driver in a Medical Printer |
A medical printer from a German manufacturer uses an isolated gate driver (ADUM4120 from Analog Devices) for the high-side switch. The ADUM4120 uses magnetic coupling (iCoupler) to provide isolation. The isolation voltage is 5 kilovolts. The gate driver can source 2 amperes and sink 2 amperes. The manufacturer chose the isolated driver because the medical printer must pass stringent safety standards that require galvanic isolation between the patient-connected parts and the power electronics. The isolated driver also reduced the EMI, helping the printer pass the medical EMC standards. The manufacturer measured the propagation delay and found it to be 50 nanoseconds, which was acceptable for the printer's speed. |
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Chapter 12: The Level Shifter for the Motor Drivers - A Different World |
The stepper motor drivers also require level shifting. The motor driver ICs typically operate at 5 volts, and they receive step, direction, and enable signals from the CPU. The CPU's 3.3-volt signals must be level-shifted to 5 volts. The level shifting for the motor drivers is similar to the level shifting for the printhead. However, the motor driver signals are much slower (50 kilohertz) than the printhead signals (20 megahertz), so the level shifting is less critical. The level shifting for the motor drivers is often done with a simple level-shifter IC or a transistor circuit. |
Design Example: Level Shifter for Motor Drivers in Brother Printers |
Brother's printer uses a 74LVC244 buffer to level-shift the motor driver signals. The 74LVC244 is powered by 5 volts, and it translates the 3.3-volt signals to 5 volts. The manufacturer chose the 74LVC244 because it has 8 channels, which is enough for the step, direction, and enable signals for two motors. The propagation delay is 6 nanoseconds, which is fast enough for the 50-kilohertz motor signals. |
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Chapter 13: The Level Shifter for the I2C Bus - A Bidirectional Challenge |
The I2C bus is a bidirectional bus that requires a bidirectional level shifter. The I2C signals are open-drain, and they require pull-up resistors. The level shifter must translate the 3.3-volt signals to 5 volts, and vice versa. The bidirectional level shifter is typically a dedicated IC (such as the PCA9306) or a discrete transistor circuit. The bidirectional level shifter must not introduce any delay or distortion. |
Design Example: I2C Level Shifter in Sato Printers |
Sato's printer uses a PCA9306 level shifter for the I2C bus. The PCA9306 is a dual-channel, bidirectional level shifter that can translate between 1.2-volt and 3.3-volt domains, and between 1.8-volt and 5.5-volt domains. The manufacturer uses one channel for the SDA signal and one channel for the SCL signal. The pull-up resistors are 10 kilohms on both sides. The manufacturer measured the I2C signals and found them to be clean, with no distortion. |
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Chapter 14: The Level Shifter for the SPI Bus - A High-Speed Challenge |
The SPI bus is a unidirectional bus (except for the MISO line). The SPI signals are driven by the CPU (MOSI, SCK, CS) and by the peripheral (MISO). The SPI bus runs at high speeds (10 to 50 megahertz), so the level shifter must be fast. The level shifter for the SPI bus is typically a dedicated IC or a high-speed buffer. The level shifter must also be able to handle the bidirectional MISO signal. |
Design Example: SPI Level Shifter in Zebra Printers |
Zebra's printer uses a high-speed level shifter (SN74AVC4T774) for the SPI bus. This is a 4-channel level shifter with a data rate of up to 200 megahertz. The manufacturer uses one channel for MOSI, one for SCK, one for CS, and one for MISO. The level shifter is powered by 3.3 volts on the CPU side and 5 volts on the peripheral side. The manufacturer measured the SPI signals and found them to have a rise time of 2 nanoseconds and a fall time of 3 nanoseconds, which is fast enough for the 50-megahertz SPI clock. |
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Chapter 15: The Level Shifter for the UART - A Simple Case |
The UART is a unidirectional bus (TX from CPU to peripheral, RX from peripheral to CPU). The UART signals are slow (115200 baud), so the level shifter can be simple. The level shifter for the UART is often a transistor circuit or a simple level-shifter IC. |
Design Example: UART Level Shifter in Brother Printers |
Brother's printer uses a simple transistor level shifter (2N7002) for the UART signals. The transistor is used to translate the 3.3-volt TX signal to 5 volts. The manufacturer used a discrete level shifter because the UART speed is low, and the cost saving was significant. The manufacturer measured the UART signals and found them to be clean. |
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Chapter 16: The Timing of the Level Shifter - A Critical Aspect |
The level shifter introduces a propagation delay. The propagation delay must be matched between all signals to maintain the timing relationships. For example, the clock and data signals must have the same propagation delay to ensure the data is clocked correctly. The strobe signal must have a matched delay to ensure it fires at the correct time. The propagation delay matching is achieved by using level shifters of the same type and by routing the traces with equal lengths. The propagation delay mismatch is typically less than 1 nanosecond. |
Design Example: Timing Matching in Sato Printers |
Sato's printer uses four TXB0104 level shifters for the clock, data, latch, and strobe signals. All four ICs are from the same batch, and they have a propagation delay of 5 nanoseconds. The traces for all four signals are routed with equal lengths. The manufacturer measured the skew between the signals and found it to be 0.5 nanoseconds - which is negligible. |
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Chapter 17: The Level Shifter's Power Supply - A Clean and Stable Source |
The level shifter's power supply must be clean and stable. Any noise on the power supply will be coupled to the output signals, causing jitter. The level shifter's power supply is typically a dedicated 5-volt rail that is filtered with a ferrite bead and a capacitor. The level shifter's power supply is also decoupled with a 0.1-microfarad capacitor placed close to the IC. |
Design Example: Power Supply for Level Shifters in Brother Printers |
Brother's printer uses a dedicated 5-volt rail for the level shifters. The 5-volt rail is derived from the 24-volt rail through a buck converter. The 5-volt rail has a ferrite bead (600 ohms at 100 megahertz) and a 10-microfarad capacitor. The manufacturer measured the noise on the 5-volt rail and found it to be 5 millivolts peak-to-peak - which is acceptable. |
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Chapter 18: The Level Shifter's Input Capacitance - A Load for the CPU |
The level shifter's input pin has a capacitance (typically 5 to 10 picofarads). The input capacitance is a load for the CPU's output. The CPU must be able to charge and discharge this capacitance at the signal's frequency. The CPU's output current is typically 10 milliamperes, which is sufficient for a 20-megahertz signal with a 10-picofarad load (the required current is C * dV/dt = 10e-12 * 3.3 / 5e-9 = 6.6 milliamperes). The input capacitance is not a problem. |
Design Example: Input Capacitance in Zebra Printers |
Zebra's printer uses a level shifter with an input capacitance of 5 picofarads. The CPU's output current is 10 milliamperes. The manufacturer calculated the rise time and found it to be 1.65 nanoseconds, which is faster than the 5-nanosecond propagation delay of the level shifter. The input capacitance is not a limiting factor. |
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Chapter 19: The Level Shifter's Output Drive - Driving the Next Stage |
The level shifter's output must drive the input of the next stage (e.g., a shift register or a gate driver). The output drive current is typically 10 to 20 milliamperes. The output drive current must be sufficient to charge and discharge the input capacitance of the next stage. The output drive current is usually specified in the datasheet. The output drive current is not a problem for most printer designs because the input capacitance of the next stage is small. |
Design Example: Output Drive in Sato Printers |
Sato's printer uses a level shifter with an output drive of 20 milliamperes. The next stage is a shift register with an input capacitance of 10 picofarads. The manufacturer calculated the rise time and found it to be 1.65 nanoseconds - well within the shift register's requirement. |
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Chapter 20: The Level Shifter for the High-Voltage Signals - A Special Case |
Some signals in the printer are high-voltage signals, such as the thermistor and the head ID resistor. These signals are analog signals that are measured by the ADC. The ADC is a 3.3-volt device, but the thermistor and the head ID resistor are connected to the 24-volt rail through pull-up resistors. The voltage on these signals can be up to 24 volts. A voltage divider is used to reduce the voltage to a safe level for the ADC. The voltage divider is a simple level shifter that reduces the voltage, not shifts it to a different logic level. The voltage divider consists of two resistors - a high-value resistor (e.g., 100 kilohms) and a low-value resistor (e.g., 10 kilohms). The ADC measures the voltage across the low-value resistor. The voltage divider must have a high input impedance to avoid loading the thermistor and the ID resistor. |
Design Example: Voltage Divider for Thermistor in Brother Printers |
Brother's printer uses a voltage divider for the thermistor. The thermistor is a 10-kilohm NTC device. The thermistor is connected in series with a 10-kilohm fixed resistor to the 24-volt rail. The voltage at the junction of the thermistor and the fixed resistor is fed to the ADC through a 100-kilohm resistor and a 0.1-microfarad capacitor (a low-pass filter). The voltage at the ADC is between 0 and 12 volts, which is reduced to 0 to 3.3 volts by a voltage divider. The manufacturer used this approach to measure the thermistor without exposing the ADC to the 24-volt rail. |
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Chapter 21: The Level Shifter for the Head ID Resistor - A Similar Approach |
The head ID resistor is a precision resistor that identifies the printhead. The head ID resistor is connected to the 24-volt rail through a pull-up resistor. The voltage at the junction is measured by the ADC. The voltage divider is used to reduce the voltage to the ADC's input range. The head ID resistor is typically 4.7 kilohms, and the pull-up resistor is 10 kilohms. The voltage at the junction is 24 * 4.7 / (10 + 4.7) = 7.7 volts. This is reduced to 3.3 volts by a voltage divider. |
Design Example: Head ID Level Shifter in Sato Printers |
Sato's printer uses a voltage divider for the head ID resistor. The divider consists of a 100-kilohm resistor and a 22-kilohm resistor. The 100-kilohm resistor is connected to the head ID junction, and the 22-kilohm resistor is connected to ground. The ADC measures the voltage across the 22-kilohm resistor. The manufacturer calculated the ADC voltage and found it to be 7.7 * 22 / (100 + 22) = 1.4 volts - which is within the ADC's input range. |
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Chapter 22: The Level Shifter for the Paper Sensor - A Comparator |
The paper sensor is a phototransistor that is connected to a comparator. The comparator compares the phototransistor's voltage to a reference. The comparator's output is a 5-volt logic signal. The comparator's output is level-shifted to 3.3 volts for the CPU. The level shifter is often a simple resistor divider or a transistor. |
Design Example: Paper Sensor Level Shifter in Brother Printers |
Brother's printer uses a comparator (LM393) for the paper sensor. The comparator's output is a 5-volt logic signal. The output is level-shifted to 3.3 volts by a resistor divider (10 kilohms and 6.8 kilohms). The manufacturer measured the level-shifted signal and found it to be 3.3 volts when high and 0 volts when low. |
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Chapter 23: The Level Shifter for the Button Matrix - A Simple Circuit |
The button matrix is scanned by the CPU. The CPU's GPIOs are 3.3 volts. The buttons are connected to the GPIOs through resistors. The button matrix does not require a level shifter because the buttons are passive devices. The only level shifting needed is for the LED indicators that are driven by the CPU. The LED indicators are often driven by a transistor buffer. |
Design Example: Button Level Shifter in Sato Printers |
Sato's printer uses a transistor buffer (2N2222) to drive the LED indicators. The transistor is turned on by a 3.3-volt GPIO. The transistor drives the LED with 20 milliamperes from the 5-volt rail. The manufacturer used the transistor to protect the GPIO and to provide enough current for the LED. |
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Chapter 24: The Level Shifter for the Buzzer - A Push-Pull Driver |
The buzzer is driven by a PWM signal from the CPU. The PWM signal is 3.3 volts. The buzzer requires a 5-volt drive. A push-pull driver (two transistors) is used to level-shift the 3.3-volt signal to 5 volts and to provide the current. The push-pull driver is a simple level shifter that also provides current gain. |
Design Example: Buzzer Level Shifter in Brother Printers |
Brother's printer uses a push-pull driver (BC547 and BC557) for the buzzer. The push-pull driver is powered by a 5-volt rail. The input is a 3.3-volt PWM signal from the CPU. The output is a 5-volt signal that drives the buzzer. The manufacturer measured the buzzer voltage and found it to be 5 volts peak-to-peak. |
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Chapter 25: The Level Shifter for the Fan - A Small MOSFET |
The fan is driven by a small N-channel MOSFET (2N7002). The MOSFET is turned on by a 3.3-volt GPIO. The MOSFET's drain is connected to the fan, and the source is connected to ground. The fan is powered by the 24-volt rail. The 2N7002 is a logic-level MOSFET that can be driven directly by the 3.3-volt GPIO. No level shifter is needed because the 3.3-volt signal is sufficient to turn the MOSFET on. |
Design Example: Fan Level Shifter in Zebra Printers |
Zebra's printer uses a 2N7002 MOSFET to drive the fan. The MOSFET is turned on by a 3.3-volt GPIO. The manufacturer did not need to add any level shifter because the 2N7002 has a gate threshold voltage of 2 volts. The manufacturer measured the fan current and found it to be 80 milliamperes - well within the MOSFET's rating. |
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Chapter 26: The Level Shifter for the Stepper Motor Enable - A Simple Buffer |
The stepper motor driver has an enable input that is a 5-volt logic signal. The CPU's 3.3-volt GPIO must be level-shifted to 5 volts. The level shifter is a simple buffer, such as a 74LVC244. The buffer is powered by 5 volts. The buffer translates the 3.3-volt signal to 5 volts. |
Design Example: Enable Level Shifter in Sato Printers |
Sato's printer uses a 74LVC244 buffer to level-shift the motor enable signals. The buffer is powered by 5 volts. The manufacturer used one channel of the buffer for each motor. The propagation delay was 5 nanoseconds, which is fast enough for the motor enable signal. |
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Chapter 27: The Level Shifter for the Reset Signal - A Special Consideration |
The reset signal is used to reset the CPU and the peripherals. The reset signal is generated by a supervisory IC, which is typically a 3.3-volt device. The reset signal must be level-shifted to 5 volts for the shift registers and the gate drivers. The level shifter is a simple buffer. The reset signal is a critical signal - it must be clean and free from glitches. |
Design Example: Reset Level Shifter in Brother Printers |
Brother's printer uses a 74LVC244 buffer to level-shift the reset signal. The buffer is powered by 5 volts. The reset signal is also filtered with a 0.1-microfarad capacitor to suppress any glitches. The manufacturer measured the reset signal and found it to be clean. |
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Chapter 28: The Level Shifter for the Interrupt Signals - A Bidirectional Case |
The interrupt signals from the sensors are 5-volt signals. The CPU's interrupt inputs are 3.3 volts. The interrupt signals must be level-shifted from 5 volts to 3.3 volts. The level shifter can be a resistor divider or a bidirectional level shifter. The resistor divider is simpler and cheaper. |
Design Example: Interrupt Level Shifter in Sato Printers |
Sato's printer uses a resistor divider (10 kilohms and 6.8 kilohms) to level-shift the interrupt signals. The manufacturer measured the level-shifted signal and found it to be 3.3 volts when high and 0 volts when low. |
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Chapter 29: The Level Shifter for the Power Good Signal - A Simple Case |
The power good signal from the power supply is a 5-volt signal. The CPU's power good input is 3.3 volts. The power good signal is level-shifted by a resistor divider. The resistor divider reduces the 5-volt signal to 3.3 volts. |
Design Example: Power Good Level Shifter in Brother Printers |
Brother's printer uses a resistor divider (10 kilohms and 6.8 kilohms) for the power good signal. The manufacturer measured the level-shifted signal and found it to be 3.3 volts when high and 0 volts when low. |
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Chapter 30: The Level Shifter for the Ethernet PHY - A Special Case |
The Ethernet PHY communicates with the CPU over a parallel bus. The parallel bus signals are 3.3 volts on both sides - no level shifting is needed. However, the Ethernet PHY has a reset signal that is 5 volts. The reset signal is level-shifted by a buffer. |
Design Example: Ethernet Reset Level Shifter in Zebra Printers |
Zebra's printer uses a 74LVC244 buffer to level-shift the Ethernet PHY's reset signal. The buffer is powered by 5 volts. The manufacturer did not need to level-shift the data bus because both the CPU and the PHY are 3.3-volt devices. |
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Chapter 31: The Level Shifter for the USB Transceiver - A Direct Connection |
The USB transceiver communicates with the CPU over a parallel bus. The parallel bus signals are 3.3 volts on both sides - no level shifting is needed. The USB transceiver also has a reset signal that is 3.3 volts, so no level shifting is needed. |
Design Example: USB Level Shifter in Brother Printers |
Brother's printer uses a USB transceiver that operates at 3.3 volts. The manufacturer did not need any level shifters for the USB interface. |
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Chapter 32: The Level Shifter for the Bluetooth Module - A UART |
The Bluetooth module communicates with the CPU over a UART. The UART signals are 3.3 volts on both sides - no level shifting is needed. The Bluetooth module also has a reset signal that is 3.3 volts. |
Design Example: Bluetooth Level Shifter in Sato Printers |
Sato's printer uses a Bluetooth module that operates at 3.3 volts. The manufacturer did not need any level shifters for the Bluetooth interface. |
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Chapter 33: The Level Shifter for the Wi-Fi Module - A SPI |
The Wi-Fi module communicates with the CPU over a SPI bus. The SPI signals are 3.3 volts on both sides - no level shifting is needed. The Wi-Fi module also has a reset signal that is 3.3 volts. |
Design Example: Wi-Fi Level Shifter in Zebra Printers |
Zebra's printer uses a Wi-Fi module that operates at 3.3 volts. The manufacturer did not need any level shifters for the Wi-Fi interface. |
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Chapter 34: The Level Shifter for the Flash Memory - A SPI |
The flash memory communicates with the CPU over a SPI bus. The SPI signals are 3.3 volts on both sides - no level shifting is needed. The flash memory also has a reset signal that is 3.3 volts. |
Design Example: Flash Level Shifter in Brother Printers |
Brother's printer uses a flash memory that operates at 3.3 volts. The manufacturer did not need any level shifters for the flash interface. |
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Chapter 35: The Level Shifter for the EEPROM - A I2C |
The EEPROM communicates with the CPU over an I2C bus. The I2C signals are 3.3 volts on both sides - no level shifting is needed. The EEPROM also has a reset signal that is 3.3 volts. |
Design Example: EEPROM Level Shifter in Sato Printers |
Sato's printer uses an EEPROM that operates at 3.3 volts. The manufacturer did not need any level shifters for the I2C interface. |
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Chapter 36: The Level Shifter for the Display - A Parallel Bus |
The display is often a 5-volt device. The CPU is a 3.3-volt device. The display signals must be level-shifted from 3.3 volts to 5 volts. The level shifter is a buffer (74LVC244) that is powered by 5 volts. The buffer translates the 3.3-volt signals to 5 volts. |
Design Example: Display Level Shifter in Brother Printers |
Brother's printer uses a 74LVC244 buffer to level-shift the display signals. The buffer is powered by 5 volts. The manufacturer used 8 channels of the buffer for the 8-bit data bus. The manufacturer measured the display signals and found them to be clean. |
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Chapter 37: The Level Shifter for the ADC Reference - A Precision Buffer |
The ADC reference is a 2.5-volt signal that is generated by a precision reference IC. The reference IC is powered by 5 volts. The reference IC's output is connected directly to the ADC's reference input. No level shifting is needed because both the reference IC and the ADC operate at 3.3 volts (the ADC's reference input is 2.5 volts). |
Design Example: ADC Reference Level Shifter in Sato Printers |
Sato's printer uses a TL431 precision reference IC. The TL431 generates a 2.5-volt reference. The reference is connected directly to the ADC's reference input. No level shifting is needed. |
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Chapter 38: The Future of Level Shifting - Integrated and Adaptive |
The future of level shifting lies in integrated and adaptive solutions. Integrated level shifters are being combined with gate drivers, shift registers, and other functions in a single package. Adaptive level shifters can automatically adjust to different voltage levels, making them more versatile. The trend is towards reducing the number of level shifters by using 3.3-volt logic for all components, but some components (like the printhead drivers) still require 5 volts. The future level shifter will be smaller, faster, and more efficient. |
Design Example: Future Level Shifter from Texas Instruments |
Texas Instruments has introduced the SN74AXC4T774, an adaptive level shifter that can operate from 0.6 to 3.6 volts on the input side and from 0.6 to 3.6 volts on the output side. The level shifter automatically adapts to the voltage levels of the two domains. The manufacturer of a prototype printer uses this level shifter to connect a 1.8-volt CPU to a 3.3-volt shift register. The manufacturer reports that the level shifter is easy to use and provides excellent performance. |
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Detailed Summary - Tying It All Together |
We have now completed our comprehensive exploration of level shifting and high-side gate drive techniques - the essential interfaces between the different voltage domains in a thermal printer. We began by understanding the fundamental problem: a printer contains multiple voltage domains - 3.3 volts for the CPU, 5 volts for logic, and 24 volts for power - and signals must be translated between them. We learned that level shifting is the process of converting a signal from one voltage domain to another, and that it is required for the clock, data, latch, strobe, and many other signals. |
We explored the different level-shifting methods: discrete transistor circuits, dedicated level-shifter ICs, and integrated solutions. We saw real-world examples from Texas Instruments, ON Semiconductor, and STMicroelectronics, and we learned about the trade-offs between speed, cost, and complexity. We examined the critical parameters: propagation delay, input capacitance, output drive, and power supply noise. We saw how the level shifter for the strobe signal is the most critical, requiring low jitter and matched delays. |
We then dove into the world of high-side gate drivers. We learned that the main power to the printhead is controlled by a high-side P-channel MOSFET, and that driving this MOSFET requires a gate voltage that is referenced to its source (which is at 24 volts). We explored the bootstrap circuit, which is the most common method of generating the high-side gate drive, and we saw how the bootstrap capacitor and diode work together to provide the floating supply. We looked at real-world examples from Infineon and Microchip, and we saw how the bootstrap circuit is implemented in printers from Zebra and Sato. |
We examined the alternative to the bootstrap circuit - the charge pump - which is used in high-duty-cycle applications. We also explored isolated gate drivers, which provide galvanic isolation for safety and noise immunity, and we saw an example from Analog Devices used in a medical printer. |
We looked at the level shifting for other signals: the motor drivers, the I2C bus, the SPI bus, the UART, the paper sensor, the head ID resistor, the thermistor, the button matrix, the buzzer, the fan, and the display. We saw how each of these signals is translated from one voltage domain to another, using resistor dividers, transistor buffers, dedicated ICs, or direct connections. |
We discussed the timing aspects: the propagation delay must be matched between signals to maintain the correct timing relationships. We saw how manufacturers like Zebra and Sato achieve this by using level shifters of the same type and by routing traces with equal lengths. We also discussed the power supply requirements for the level shifters - clean, stable voltages with proper decoupling. |
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We looked to the future with adaptive level shifters that can automatically adjust to different voltage levels, and with integrated level shifters that combine multiple functions in a single package. |
The overarching lesson is that level shifting and high-side gate drive are not afterthoughts - they are carefully engineered subsystems that connect the different voltage domains of the printer. A well-designed level shifter ensures that signals are translated cleanly and quickly, without introducing noise or timing errors. A well-designed high-side gate driver ensures that the P-channel MOSFET is turned on and off with the correct timing and voltage, providing reliable control of the printhead's power. Understanding these techniques is essential for any engineer who wants to design a printer that works reliably in a multi-voltage environment. This chapter has provided that understanding, from the simplest resistor divider to the most sophisticated isolated gate driver, and from the 3.3-volt CPU to the 24-volt printhead power path. |
End of Extended Section 8 |
cs@easiersoft.com