Power Supply - The Quiet Heart: How Clean, Stable Power Keeps the Barcode Scanner Alive |
Subtitle: A Deep Dive into Power Regulation, Decoupling, Noise Suppression, and Battery Management - with Real-World Examples from Symbol, Zebra, Honeywell, Datalogic, Texas Instruments, and Maxim Integrated |
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Opening Summary |
The analogue front-end of a barcode scanner is a delicate instrument. It measures currents in the nanoampere range and voltages in the millivolt range. Any noise on the power supply - any ripple, any spike, any droop - can corrupt these tiny signals, leading to decoding errors and failed reads. The power supply is the quiet heart of the scanner. It must provide clean, stable, and well-regulated voltage to all the active components, from the photodiode's bias to the comparator's reference. |
This article is dedicated to the power supply - its design, its challenges, and its critical role in the barcode scanner. We will explore the different types of power supplies used in scanners: the linear regulator (LDO) for low noise, the switching regulator (buck or boost) for efficiency, and the battery for portable devices. We will examine the key parameters: output voltage, current capability, line regulation, load regulation, ripple, and noise. We will look at how major companies have implemented power supplies in their products. We will see how Symbol (now Zebra) used a simple 5-volt linear regulator in the LS2208. We will explore Honeywell's use of a low-dropout regulator for their imagers. We will examine Datalogic's use of a switching regulator for battery-powered scanners. We will also look at reference designs from Texas Instruments and Maxim Integrated, which showcase the latest power supply technologies. |
By the end of this journey, you will understand that the power supply is not just a battery or a wall adapter but a carefully engineered subsystem that must be designed to meet the unique demands of the barcode scanner. |
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Full Article |
Section 1: The Power Supply's Mission - To Provide Clean, Stable Voltage |
The power supply's mission is to provide a clean, stable voltage to all the active components of the barcode scanner. The active components include the photodiode (which needs a bias voltage), the op-amps (which need a dual or single supply), the comparator, the microcontroller, and the illumination source (LED or laser). |
The power supply must meet several requirements: |
Voltage Accuracy: The output voltage must be within a tight tolerance (e.g., +/- 5%). |
Current Capability: The power supply must be able to supply the peak current of all the components. |
Line Regulation: The output voltage must be stable when the input voltage changes. |
Load Regulation: The output voltage must be stable when the load current changes. |
Ripple and Noise: The output voltage must have very low ripple and noise. |
Efficiency: The power supply must be efficient to save battery power. |
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Section 2: The Linear Regulator - The Classic Low-Noise Choice |
The linear regulator (also known as the LDO, or low-dropout regulator) is the classic choice for low-noise power supplies. A linear regulator uses a pass transistor to drop the input voltage to the desired output voltage. The pass transistor is controlled by a feedback loop that compares the output voltage to a reference. |
The linear regulator has several advantages: it is simple, cheap, and very low-noise. It has excellent line and load regulation. Its main disadvantage is its low efficiency, especially when the input voltage is significantly higher than the output voltage. The excess voltage is dissipated as heat. |
Section 3: Symbol's LS2208 - The 78L05 Linear Regulator |
Symbol's LS2208 uses a 78L05 linear regulator. The 78L05 is a classic 5-volt linear regulator. It can supply up to 100 milliamperes of current. The LS2208's power consumption is about 300 milliwatts, so the 78L05 is adequate. |
The 78L05 is a simple, robust, and inexpensive solution. It provides a clean, stable 5-volt rail for the analogue and digital circuits. |
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Section 4: The Switching Regulator - The Efficient Choice |
The switching regulator (also known as the buck, boost, or buck-boost converter) is a more efficient alternative to the linear regulator. A switching regulator uses a transistor that switches on and off at a high frequency. The switching action is filtered by an inductor and a capacitor to produce a DC output. |
The switching regulator has several advantages: it is highly efficient (80-95%), it can step up or step down the voltage, and it can handle higher currents. Its main disadvantage is the higher output ripple and noise, which can be problematic for sensitive analogue circuits. |
Section 5: Honeywell's 1900 - The Switching Regulator for Battery Power |
Honeywell's 1900 imager is a battery-powered device. It uses a switching regulator (a boost converter) to generate the 5-volt rail from the battery's 3.7-volt lithium-ion cell. The boost converter steps up the battery voltage to 5 volts. A linear regulator is then used to generate the 3.3-volt rail for the analogue circuitry. |
The switching regulator provides high efficiency, extending the battery life. The linear regulator provides a clean, low-noise supply for the sensitive analogue circuits. |
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Section 6: Datalogic's PowerScan - The Multi-Rail Supply |
Datalogic's PowerScan series uses a multi-rail power supply. The scanner requires several different voltages: 5 volts for the analogue circuits, 3.3 volts for the digital circuits, 12 volts for the laser driver, and 150 volts for the avalanche photodiode (APD). |
The multi-rail supply uses a combination of switching regulators and linear regulators. The switching regulators provide the high voltages and high currents. The linear regulators provide the clean, low-noise voltages for the analogue circuits. |
Section 7: The Battery - The Portable Power Source |
For portable scanners, the power source is a battery. The battery is typically a lithium-ion (Li-ion) or lithium-polymer (Li-Po) cell. The battery voltage is typically 3.7 to 4.2 volts. The battery capacity is typically 2000 to 4000 milliampere-hours (mAh). |
The battery must be charged by a battery charger. The battery charger is typically a switching regulator that converts the AC mains voltage or the USB voltage to the appropriate charging voltage. |
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Section 8: The Power Management - The Energy Saver |
Power management is a critical feature for battery-powered scanners. The scanner must conserve power to extend the battery life. The power management is handled by the microcontroller. The microcontroller can put the scanner into a low-power 'sleep' mode when it is not in use. |
In the sleep mode, the microcontroller turns off the illumination, the motor, and the analogue circuits. The microcontroller itself enters a low-power state. The scanner wakes up when the trigger is pressed or when an object is detected. |
Section 9: The Decoupling Capacitors - The Noise Filters |
Decoupling capacitors are essential for suppressing noise on the power supply. A decoupling capacitor is a small capacitor (typically 100 nanofarads) that is placed as close as possible to the power pin of each integrated circuit. The capacitor provides a local reservoir of charge and filters out high-frequency noise. |
A larger capacitor (typically 10 microfarads) is used for bulk decoupling. The bulk capacitor provides a reservoir of charge for sudden current demands. |
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Section 10: The Ferrite Bead - The High-Frequency Noise Suppressor |
A ferrite bead is a passive component that suppresses high-frequency noise. It is used in series with the power supply line. The ferrite bead acts as a high-frequency resistor, dissipating the high-frequency noise as heat. |
The ferrite bead is often used in conjunction with a decoupling capacitor to create a low-pass filter. |
Section 11: The Power Supply Rejection Ratio - The Op-Amp's Defense |
The power supply rejection ratio (PSRR) is a measure of an op-amp's ability to reject noise on its power supply. The PSRR is typically 80-100 dB. This means that a 1-volt noise on the supply is reduced to 0.1 millivolt at the output. |
A high PSRR is essential for sensitive analogue circuits. The TLV272 and the AD8615 both have high PSRR. |
Section 12: The Ground - The Reference Plane |
The ground is the reference plane for all voltages. It must be clean and stable. The ground is typically a copper plane on the PCB. The ground plane provides a low-impedance return path for the current. |
The analogue and digital grounds must be separated. The analogue ground is the reference for the sensitive analogue circuits. The digital ground is the reference for the noisy digital circuits. The two grounds are connected at a single point (star ground). |
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Section 13: The Star Ground - A Single Connection Point |
The star ground is a single connection point for the analogue and digital grounds. The star ground prevents the digital currents from flowing through the analogue ground, which would create noise. |
The star ground is typically located near the power supply. |
Section 14: The Shield Can - The EMI Shield |
The shield can is a metal enclosure that covers the analogue circuits. The shield can prevents electromagnetic interference (EMI) from reaching the sensitive analogue circuits. The shield can also prevents the analogue circuits from radiating EMI. |
The shield can is connected to the ground plane. |
Section 15: Symbol's LS2208 - The Power Supply Implementation |
Symbol's LS2208 uses a simple, cost-effective power supply. The scanner is powered by a 5-volt external supply (typically from a USB port or a wall adapter). The 5-volt supply is regulated by a 78L05 linear regulator. |
The 78L05 provides a clean, stable 5-volt rail. The rail is decoupled with a 100-nanofarad capacitor and a 10-microfarad capacitor. The analogue and digital grounds are separated. |
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Section 16: Honeywell's 1900 - The Power Supply Implementation |
Honeywell's 1900 imager uses a more sophisticated power supply. The scanner is powered by a 3.7-volt lithium-ion battery. The battery voltage is boosted to 5 volts by a switching regulator. The 5-volt rail is then regulated to 3.3 volts by a linear regulator. |
The 3.3-volt rail powers the analogue circuits. The 5-volt rail powers the LEDs and the motor. The power supply is controlled by the microcontroller. |
Section 17: Datalogic's PowerScan - The Power Supply Implementation |
Datalogic's PowerScan series uses a complex multi-rail power supply. The scanner is powered by a 3.7-volt lithium-ion battery. The battery voltage is boosted to 5 volts, 12 volts, and 150 volts by a series of switching regulators. |
The 5-volt rail powers the analogue circuits. The 12-volt rail powers the laser driver. The 150-volt rail powers the APD. The power supply is controlled by the microcontroller. |
Section 18: The Power Supply and the Photodiode Bias |
The photodiode requires a bias voltage. The bias voltage is typically 5 volts. The bias voltage is provided by the TIA's virtual ground. The photodiode is reverse-biased by connecting its cathode to the TIA's inverting input (which is at virtual ground) and its anode to ground. |
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Section 19: The Power Supply and the TIA |
The TIA requires a clean, stable power supply. The TIA is powered by the 5-volt rail. The rail is decoupled with a 100-nanofarad capacitor. |
The TIA's power supply rejection ratio (PSRR) is important. A high PSRR ensures that the TIA rejects any noise on the power supply. |
Section 20: The Power Supply and the Comparator |
The comparator also requires a clean, stable power supply. The comparator is powered by the 5-volt rail. The rail is decoupled with a 100-nanofarad capacitor. |
The comparator's PSRR is also important. |
Section 21: The Power Supply and the Illumination |
The illumination (LED or laser) requires a significant amount of power. The illumination is typically powered by a separate rail. For LEDs, the rail is typically 5 volts. For lasers, the rail is typically 12 volts. |
The illumination current is pulsed to reduce the average power consumption. |
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Section 22: The Power Supply and the Motor |
The motor that drives the scanning mirror also requires significant power. The motor is typically powered by the 5-volt or 12-volt rail. |
The motor current is also pulsed. |
Section 23: The Power Supply and the Microcontroller |
The microcontroller requires a clean, stable power supply. The microcontroller is typically powered by the 3.3-volt or 5-volt rail. The rail is decoupled with a 100-nanofarad capacitor. |
The microcontroller's power consumption is relatively low. |
Section 24: The Power Supply and the USB Interface |
The USB interface provides both power and data. The USB port provides a 5-volt supply. The USB 5-volt supply is used to charge the battery and to power the scanner when it is connected to a host. |
The USB supply is typically regulated by a linear regulator. |
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Section 25: The Power Supply and the Battery Charger |
The battery charger is a circuit that charges the battery. The battery charger is typically a switching regulator. The charger converts the USB 5-volt supply to the appropriate charging voltage (4.2 volts for a lithium-ion battery). |
The battery charger includes a current limiter and a voltage limiter. The charger also includes a temperature monitor to prevent the battery from overheating. |
Section 26: The Power Supply and the Power Management IC |
The power management IC (PMIC) is a dedicated chip that manages the power supply. The PMIC integrates the switching regulators, the linear regulators, the battery charger, and the power management logic into a single chip. |
The PMIC simplifies the design and reduces the component count. It is used in many modern scanners. |
Section 27: The Power Supply and the Ripple |
The ripple is the AC component of the output voltage. The ripple is caused by the switching action of the switching regulator. The ripple is typically 10-100 millivolts peak-to-peak. |
The ripple must be filtered out by the linear regulators and the decoupling capacitors. |
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Section 28: The Power Supply and the Noise |
The noise is the random fluctuation of the output voltage. The noise is caused by the thermal noise of the components and the switching regulator's switching noise. |
The noise must be kept low. A high PSRR of the op-amps helps to reject the noise. |
Section 29: The Power Supply and the Transient Response |
The transient response is the power supply's ability to respond to a sudden change in load current. The transient response is important for the illumination and the motor, which draw sudden bursts of current. |
The transient response is determined by the power supply's bandwidth and the output capacitance. |
Section 30: The Power Supply and the Efficiency |
The efficiency is the ratio of the output power to the input power. The efficiency is important for battery-powered scanners. A higher efficiency extends the battery life. |
The switching regulators are more efficient than the linear regulators. |
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Section 31: The Power Supply in Texas Instruments' TIDA-00857 |
Texas Instruments' TIDA-00857 reference design uses a simple power supply. The design is powered by a 5-volt external supply. The 5-volt supply is regulated by a TPS715A low-dropout regulator. The TPS715A provides a clean, stable 5-volt rail. |
Section 32: The Power Supply in Maxim Integrated's Reference Design |
Maxim Integrated's reference design uses a more sophisticated power supply. The design is powered by a 3.7-volt lithium-ion battery. The battery voltage is boosted to 5 volts by a MAX17062 switching regulator. The 5-volt rail is then regulated to 3.3 volts by a MAX8891 linear regulator. |
The MAX17062 provides high efficiency. The MAX8891 provides low noise. |
Section 33: The Power Supply and the ESD Protection |
The power supply input is susceptible to ESD. The input is protected by a clamping diode. The clamping diode shunts the ESD current to ground. |
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Section 34: The Power Supply and the EMI Filtering |
The power supply input is also susceptible to EMI. The input is filtered by an EMI filter. The EMI filter is a low-pass filter that attenuates high-frequency noise. |
Section 35: The Power Supply and the PCB Layout |
The PCB layout of the power supply is critical. The switching regulator's loop area must be minimized. The decoupling capacitors must be placed as close as possible to the power pins. The ground plane must be continuous. |
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Section 36: The Power Supply - A Summary of Best Practices |
Based on our exploration, let us summarize the best practices for designing the power supply for a barcode scanner: |
1. Use a Linear Regulator for Analogue Circuits: The linear regulator provides low noise. |
2. Use a Switching Regulator for High Power: The switching regulator provides high efficiency. |
3. Use Decoupling Capacitors: The decoupling capacitors filter out the noise. |
4. Use a Ferrite Bead: The ferrite bead suppresses high-frequency noise. |
5. Use a Star Ground: The star ground prevents ground loops. |
6. Use a Shield Can: The shield can prevents EMI. |
7. Consider Power Management: Power management extends the battery life. |
8. Test the Power Supply: The power supply must be tested for ripple, noise, and transient response. |
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Final Summary |
The power supply is the quiet heart of the barcode scanner. It provides the clean, stable voltage that the sensitive analogue circuits need to operate correctly. The power supply must be carefully designed to meet the scanner's requirements for voltage accuracy, current capability, ripple, noise, and efficiency. |
We have seen how major companies have implemented power supplies in their products. Symbol's LS2208 uses a simple linear regulator. Honeywell's 1900 uses a switching regulator for efficiency. Datalogic's PowerScan uses a complex multi-rail supply. Texas Instruments and Maxim Integrated provide reference designs with advanced power supply technologies. |
The power supply is not just a battery or a wall adapter. It is a carefully engineered subsystem that is critical to the scanner's performance. |
cs@easiersoft.com