Constant Current Drive: The Foundation of Stable Illumination in Barcode Readers |
Executive Summary |
This article provides a comprehensive, accessible exploration of constant current drive circuits for illumination sources in barcode readers. We examine why stable light output is essential for reliable barcode decoding and how constant current regulation achieves this stability. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Texas Instruments, Microchip Technology, Toshiba, and various pioneering patent holders. We explore the fundamental principles of current mirrors, the integration of constant current drivers with DC-DC converters, multi-channel driver architectures, and the practical implementation of programmable current sources. The article covers both simple discrete designs and sophisticated integrated solutions, with special attention to the practical trade-offs between efficiency, accuracy, cost, and complexity. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting constant current drive circuits for barcode reader illumination systems. |
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Chapter 1: Why Constant Current Matters |
The illumination source in a barcode reader is a current-driven device. The brightness of an LED, and the output power of a laser diode, are directly proportional to the forward current flowing through the device. This fundamental relationship makes current regulation the most critical aspect of illumination subsystem design. |
If the current is not well-regulated, the light output will vary. Variations in light output cause variations in the reflected signal, which can lead to decoding errors. A reader that works perfectly in the lab may fail in the field if its LED current drifts with temperature or battery voltage. |
The problem is that the forward voltage of an LED is not constant. It varies with temperature, from device to device, and with aging. If you apply a fixed voltage to an LED, the current will change as the forward voltage changes. A constant current source, on the other hand, forces a precise current through the LED regardless of the forward voltage. This ensures consistent light output under all conditions. |
This principle is well understood in the industry. As described in a patent for a synthetic barcode module, 'The LED is a current-driven device whose brightness is proportional to its forward current. Forward current can be controlled either by applying a voltage source and using a ballast resistor or, preferably, by regulating LED current with a constant-current source'. |
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Chapter 2: The Basic Current Mirror - A Building Block |
The current mirror is one of the most fundamental building blocks of constant current drive circuits. It is a simple circuit that copies a reference current from one branch to another, providing a stable output current that is independent of the load. |
A basic current mirror consists of two transistors with their gates (or bases) connected together. One transistor is diode-connected, meaning its gate and drain are shorted. A reference current flows through this diode-connected transistor, establishing a gate voltage. This gate voltage is applied to the second transistor, which then conducts the same current as the first, provided the two transistors are matched. |
The beauty of the current mirror is that the output current is determined by the reference current, not by the load. If the load changes, the current mirror adjusts its output voltage to maintain the current. This makes it an ideal circuit for driving LEDs, where the forward voltage can vary. |
A patent from 2009 describes a driving circuit that uses a current mirror to drive LED series. The circuit includes a constant current source and a current mirror circuit with two NMOS transistors. The constant current source supplies a current to the drain of the first transistor, and the drain of the second transistor is connected to the cathode of the LED series. |
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Chapter 3: The Constant Current Driver - A Practical Implementation |
The constant current driver is a practical implementation of the current mirror concept. It typically consists of a reference current source, a current mirror, and a control circuit that adjusts the output voltage to maintain the current. |
The constant current source provides a stable reference current. This reference current is mirrored by the current mirror to produce the output current that drives the LED. The control circuit monitors the voltage across the current mirror and adjusts the supply voltage to ensure that the current mirror operates properly. |
In the driving circuit described in the patent, the DC-DC converter adjusts its output voltage based on the voltages across the current mirror. The goal is to keep the output voltage just high enough to maintain the correct current, without wasting power. |
The patent explains that the output voltage of the DC-DC converter is adjusted so that the output voltage and the input voltage of the current mirror are equal. This ensures that the current mirror operates in the saturation region, where the current is stable and independent of voltage variations. |
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Chapter 4: The Current Mirror and the Saturation Region |
The key to a stable current mirror is ensuring that the transistors operate in the saturation region. In the saturation region, the drain current is essentially independent of the drain-source voltage. This means that variations in the load voltage do not affect the current. |
The patent explains this principle: 'According to the current-voltage characteristics of a transistor, when the source-drain voltage Vds of a transistor exceeds a certain threshold value Vth, the source-drain current Ids is saturated. That is, if the source-drain voltage Vds is higher than a certain value, even when the source-drain voltage Vds changes more or less, the source-drain current Ids essentially does not have an error (variation)'. |
The driving circuit maintains the source-drain voltage of the first transistor above the threshold value so that it works in the saturated region. By comparing the output voltage and the input voltage of the current mirror, the circuit controls the output voltage of the DC-DC converter according to the actual fluctuation of the output current. |
Chapter 5: Integrated Constant Current Drivers - The Single-Chip Solution |
Many modern barcode readers use integrated constant current drivers that combine the current mirror, the reference current source, and the control circuit on a single chip. These integrated solutions are more compact, more reliable, and often more accurate than discrete designs. |
Texas Instruments offers a wide range of LED driver ICs that integrate constant current control. The LP8556, for example, is a high-efficiency LED backlight driver for tablets that includes a boost converter, a current sink, and a PWM dimming controller on a single chip. |
The company provides extensive documentation for these devices, including data sheets, white papers on common LED functions and driver design considerations, and application notes with guidelines and tips for LED driver applications. |
The integration of the constant current driver with the DC-DC converter is a key feature of many modern designs. This integration allows the converter to provide only the voltage needed for the LED, reducing power consumption and extending battery life. |
Chapter 6: Multi-Channel Constant Current Drivers |
Many barcode readers use multiple LEDs for illumination. These LEDs may be arranged in an array around the camera module, or they may be distributed across the field of view. Driving multiple LEDs requires a multi-channel constant current driver. |
A patent from 2009 describes a driving circuit for plural LED series. The circuit includes multiple constant current drivers, each connected to a separate LED series. A voltage comparator selects the lowest voltage among the current mirrors, and a selector selects the highest voltage among the current mirrors. The DC-DC converter adjusts its output voltage based on these selected voltages. |
This architecture ensures that all LED series receive the correct current, even if they have different forward voltages. The DC-DC converter provides a voltage that is high enough to drive the LED series with the highest forward voltage, while the current mirrors ensure that each series receives the correct current. |
The patent notes that the constant current drivers, the voltage comparator, the selector, and the DC-DC converter are integrated together into one IC chip, providing a compact and efficient solution. |
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Chapter 7: Toshiba's TB62708N - A 16-Channel Driver |
Toshiba's TB62708N is a practical example of a multi-channel constant current driver. This device is a Bi-CMOS integrated circuit that consists of a 16-bit shift register, latches, and constant current drivers. |
The TB62708N is designed for LED and LED display (cathode common) applications. It features constant current output that can be set up with one external resistor for currents from 5 to 90 mA. The output current matching is excellent, with a typical accuracy of (+-)6.0%. |
The device includes a 5V CMOS compatible input, a maximum clock frequency of 15 MHz, and a supply voltage for the LED of 0 to 17 V. The package is a 30-pin SDIP, making it suitable for through-hole mounting. |
The TB62708N is a good example of how a dedicated driver IC can simplify the design of a multi-LED illumination system. The designer simply provides the serial data, clock, and latch signals, and the IC handles the constant current regulation for all 16 channels. |
Chapter 8: The SN2726 - Another 16-Channel Driver |
The SN2726 is another example of a 16-channel constant current driver. This device is comprised of constant current drivers designed for LEDs and LED displays. |
The output current value of the SN2726 can be set using an external resistor, with a range from 2 mA to 60 mA. All outputs have virtually the same current levels, with a bit-to-bit accuracy of (+-)1% and a chip-to-chip accuracy of (+-)3%. |
The SN2726 incorporates a 16-bit constant-current output, a 16-bit shift register, a 16-bit latch, and a 16-bit AND-gate circuit. It has a fast response output, with an enable minimum of 50 ns, and a best data transfer frequency of 25 MHz. The device also features an output delay for low EMI and Schmitt trigger input for I/O logic. |
The SN2726 is available in a 24-pin SSOP package and is specified for an operating temperature range of -40 to 85C. It is a green, Pb-free device, making it suitable for environmentally conscious designs. |
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Chapter 9: Microchip's MIC5891 - A Latched Source Driver |
Microchip's MIC5891 is a different type of LED driver. It is an 8-bit serial-input latched source driver that is designed for high-voltage, high-current outputs. |
The MIC5891 is comprised of eight CMOS data latches, CMOS control circuitry, and bipolar Darlington transistor drivers for each latch. The bipolar/MOS construction provides extremely low power latches with maximum interface flexibility. |
The MIC5891 will typically operate at 5 MHz with a 5V logic supply. The CMOS inputs are compatible with standard CMOS, PMOS, and NMOS logic levels. TTL circuits may be used with appropriate pull-up resistors to ensure a proper logic-high input. |
A CMOS serial data output allows additional drivers to be cascaded when more than 8 bits are required. The MIC5891 has open-emitter outputs with suppression diodes for protection against inductive load transients. The output transistors are capable of sourcing 500 mA and will sustain at least 35V in the on-state. |
The MIC5891 is available in a 16-pin plastic DIP package and a 16-pin wide SOIC package. |
Chapter 10: The Programmable Current Source - An Alternative Approach |
Some barcode readers use a programmable current source to control the LED current. This approach provides more flexibility than a fixed current mirror, allowing the microcontroller to adjust the current on the fly. |
A patent from a barcode reader manufacturer describes a programmable current source that is coupled to a digital-to-analog converter. The analog output from the DAC provides an analog input to an amplifier, which drives a transistor that controls the current through the LED. |
The current is determined by the voltage from the DAC divided by a resistance. As long as a minimum voltage headroom is maintained across the programmable current source, the current is well-controlled. |
The programmable current source allows the controller to set the LED current to any value within the operating range. This is useful for adapting to different operating conditions, such as flash mode and torch mode. |
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Chapter 11: The DAC-Controlled Current Source - A Detailed Example |
The programmable current source described in the patent is a good example of how digital control can be applied to LED drive circuits. The current source is coupled to a digital-to-analog converter that forms part of the control circuit and receives control signals from the host microprocessor or controller. |
The analog output from the DAC provides an analog input to an amplifier having an output coupled to a gate input of a transistor. The transistor is coupled to the LED, and as the capacitor discharges through the LED, its rate of discharge is controlled by the bias on the transistor provided by the amplifier. |
The current is equal to the DAC voltage divided by a resistance, as long as a minimum voltage of about 0.4V is maintained across the programmable current source. This ensures that the current is accurately controlled. |
The DAC-controlled current source is used in both flash mode and torch mode. In flash mode, the discharge current is controlled by the DAC. In torch mode, the DAC controls the current that is supplied to the LED by the boost converter. |
Chapter 12: Constant Current Feedback Loops - The Scanner Bar Example |
A practical example of a constant current feedback loop can be found in a scanner bar design. In this design, the LED is driven with a constant current feedback loop that ensures the LED current is insensitive to its forward voltage. |
The circuit uses an op-amp and a transistor to form a constant current source. The instantaneous LED current is set by the voltage across a capacitor, which is buffered across a resistor, corresponding to a specific milliamps per volt ratio. At a typical current, there is a voltage drop across the resistor, and the op-amp sources sufficient voltage to maintain the current. |
The feedback loop ensures that the LED current is stable regardless of variations in the LED's forward voltage. This is particularly important in a scanner bar, where multiple LEDs must have consistent brightness. |
The design also includes a fade control function, where a network of resistors and capacitors forms a low-pass filter with different rise and fall time constants. This gives a fast fade-in and a slower fade-out, mimicking the behavior of incandescent bulbs. |
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Chapter 13: Cascading Drivers - Scaling the Illumination |
For readers with a large number of LEDs, multiple LED driver ICs can be cascaded. The serial data is shifted through the chain, allowing the microcontroller to control all the LEDs with the same few I/O pins. |
The SN2726 and TB62708N both support cascading. The serial data output of one device connects to the serial input of the next. The shift register clock and register clock are common to all devices. |
This approach is particularly useful for applications that require a large number of LEDs for illumination. The reader can have LEDs distributed around the camera module or across the field of view, and the cascaded shift register drivers provide efficient control. |
The MIC5891 also supports cascading. A CMOS serial data output allows additional drivers to be cascaded when more than 8 bits are required. |
Chapter 14: The DC-DC Converter - Providing the Voltage |
The constant current driver is only part of the illumination subsystem. The driver also needs a voltage supply that is appropriate for the LEDs being driven. This is typically provided by a DC-DC converter. |
The patent from Symbol Technologies describes a DC-to-DC boost circuit that provides a boosted voltage for charging the capacitor in a flash driver. The output voltage is in the range of 10-16 volts. |
The boost circuit is implemented with an integrated circuit, such as the LT1618 from Linear Technology. The LT1618 is a constant-current/constant-voltage step-up DC/DC converter that is well-suited for LED driver applications. |
The LT1618 has a feedback pin that can be used to set the output voltage by selecting values for a resistor network. A second pin is controlled with a DC voltage to control the output current of the voltage step-up converter. |
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Chapter 15: Current Sense Resistor - Measuring the Current |
A current sense resistor is often used in LED driver circuits to provide feedback for the constant current control loop. The voltage across the resistor is proportional to the current, and this voltage is used to regulate the current. |
The MIC2298 evaluation board uses a current sense resistor to set the LED current. The 100% current level can be set by the equation, and by choosing different resistance values, the designer can set the maximum current to the desired level. |
The current sense resistor should be carefully selected to minimize power loss while providing an adequate signal for the control loop. A smaller resistance reduces power loss but provides a smaller signal, making the current regulation more sensitive to noise. A larger resistance increases the signal level but also increases power loss. |
Chapter 16: LED Current Accuracy - A Key Specification |
The accuracy of the constant current driver is a key specification for barcode reader illumination. Poor current accuracy leads to variations in brightness, which can affect the reading performance. |
The SN2726 specifies a bit-to-bit current accuracy of (+-)1% and a chip-to-chip accuracy of (+-)3% for currents from 5 to 60 mA. This means that all outputs on the same chip will have virtually the same current, and different chips will have closely matched currents. |
The TB62708N specifies a current matching of (+-)6.0% for output currents up to 90 mA. This is adequate for many applications, but it is not as precise as the SN2726. |
The MIC5891 specifies that the output transistors are capable of sourcing 500 mA and will sustain at least 35V in the on-state. The accuracy of the current is not specified, as this is a latched driver rather than a precision constant current source. |
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Chapter 17: Power Dissipation and Thermal Management |
Constant current drivers dissipate power, and this power dissipation must be managed to prevent overheating. The SN2726, for example, has a thermal resistance that affects the maximum power it can dissipate. |
The TB62708N has a specified power dissipation of 2.08 W on a PCB at 25C and 1.56 W in free air. The IC must be operated within these limits to prevent damage. |
The MIC5891 has a note that 'Simultaneous operation of all drivers at maximum rated current requires a reduction in duty cycle due to package power limitations'. This means that the duty cycle must be reduced if all outputs are operating at maximum current. |
Chapter 18: ESD Protection - Safeguarding the Inputs |
ESD (electrostatic discharge) protection is essential for any electronic device that will be handled by users. The inputs to the LED driver IC must be protected from ESD events. |
The SN2726 features CMOS Schmitt trigger inputs, which provide some ESD protection. The TB62708N has CMOS compatible inputs with standard ESD protection. |
The MIC5891 has CMOS, PMOS, NMOS, and TTL compatible inputs, which are generally robust against ESD. The device also includes output transient protection diodes. |
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Chapter 19: The Current Mirror in Depth - Operation and Limitations |
To fully appreciate the constant current driver, it is helpful to understand the operation of the current mirror in more detail. |
The current mirror consists of two transistors with common gate and source terminals. One transistor is diode-connected, and the other is the output. The current through the first transistor is set by the reference current, and this current establishes the gate-source voltage. The same gate-source voltage is applied to the second transistor, so it conducts the same current, provided the transistors are matched and operate in the saturation region. |
The limitation of the current mirror is that it requires a minimum voltage across the output transistor to operate in the saturation region. If the voltage is too low, the transistor enters the linear region, and the current is no longer stable. |
The driving circuit described in the patent addresses this limitation by controlling the output voltage of the DC-DC converter to maintain the current mirror in the saturation region. |
Chapter 20: The Error Amplifier and PWM Control |
The control of the DC-DC converter is typically done with an error amplifier and a PWM controller. The error amplifier compares the output voltage to the input voltage of the current mirror, and the PWM controller adjusts the duty cycle of the switching element to maintain the correct voltage. |
The patent describes an error amplifier that compares the output voltage and the input voltage of the current mirror. The output signal of the error amplifier is input to a PWM circuit, which generates a pulse signal for PWM control. A driver then generates a control signal for switching control of the switching element. |
The PWM controller adjusts the duty cycle of the switching signal based on the error signal. If the output voltage is too low, the duty cycle is increased, and if it is too high, the duty cycle is decreased. This closed-loop control ensures that the output voltage is always correct. |
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Chapter 21: Constant Current Source - The Reference |
The constant current source that provides the reference current for the current mirror is a critical component. Its stability directly affects the stability of the output current. |
In the patent, the constant current source is a separate component that is integrated with the current mirror and the DC-DC converter on a single IC chip. The constant current source provides a stable current to the drain of the first transistor in the current mirror. |
The constant current source can be implemented with a bandgap voltage reference and a resistor, or with more complex circuits that provide temperature compensation. The key requirement is that the reference current is stable over temperature and supply voltage variations. |
Chapter 22: Voltage Comparator and Selector - Multi-Channel Control |
For multi-channel drivers, a voltage comparator and a selector are used to control the DC-DC converter. The voltage comparator selects the lowest output voltage among the current mirrors, and the selector selects the highest input voltage among the current mirrors. |
The patent describes a voltage comparator that compares the respective output voltages of the current mirror circuits and selects the lowest voltage. A selector compares the respective input voltages and selects the highest voltage. |
The DC-DC converter adjusts the output voltage so that the selected lowest voltage equals the selected highest voltage. This ensures that all current mirrors operate correctly and that the output voltage is not unnecessarily high. |
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Chapter 23: The Advantage of the Voltage Mirror Control |
The control method described in the patent has a significant advantage over traditional methods: it prevents degradation of power efficiency without setting the output voltage of the DC-DC converter unnecessarily high. |
The patent explains that by comparing the output voltage and the input voltage of the current mirror, it is possible to control the output voltage of the DC-DC converter according to the magnitude of the actual fluctuation of the output current of the constant current source. |
This prevents the output voltage from being set too high, which would waste power, or too low, which would cause the current mirror to operate outside the saturation region. The result is a more efficient and more reliable driving circuit. |
Chapter 24: Practical Design Example - The Synthetic Barcode Module |
A practical example of a constant current driver in a barcode application is the synthetic barcode module described in a patent. This module uses an LED and a constant current driver to simulate the reflections from a printed barcode. |
The module includes a microcontroller that causes the LED driver to energize the LED and transmit light pulses in a fashion to simulate the reflections from printed barcodes using standard barcode systems. The microcontroller is programmed to cause the LED driver to make the LED turn on and off with sufficient brightness, and at the correct timing, for the emitted light to be interpreted by a standard laser barcode scanner as the signal from a printed barcode. |
The LED driver supplies the correct amount of current to drive the LED. While a separate LED driver is shown, the driver could optionally be included or integrated into the microcontroller. The LED driver eliminates changes in current due to variations in forward voltage, which translates into a constant LED brightness. |
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Chapter 25: The Synthetic Barcode Concept |
The synthetic barcode concept is an interesting application of constant current drive technology. Instead of reading a printed barcode, the module generates light pulses that simulate the reflections from a barcode. |
The microcontroller modulates the light emission period by sending control signals to the LED driver. Periods during which light is emitted represent white spaces between bars, and periods during which no light is emitted represent black bars. |
The timing works well across a wide range of barcode scanners. The barcode scanner interprets the emitted light as an analog signal waveform of more or less rectangular-shaped pulses. |
The synthetic barcode module has several advantages. The encoded information is private, not legible to a human. The data may be updated and replaced using bidirectional communication. And vast amounts of data may be encoded in the module. |
Chapter 26: PWM Dimming - A Method of Brightness Control |
Pulse width modulation, or PWM, is a common method of controlling LED brightness. By varying the duty cycle of the signal that drives the LED, the average current can be adjusted. |
The patent for the synthetic barcode module notes that the LED driver may enable PWM dimming, which entails applying full current to the LED at a reduced duty cycle and at a high enough frequency to avoid pulsing that is visible to the human eye. |
The MIC2298 also supports dimming by PWM signals. The LED current is proportional to the duty cycle, allowing the microcontroller to adjust the brightness by varying the duty cycle. |
PWM dimming is efficient because the LED is either fully on or fully off, so there is no power dissipated in the drive circuit. The average current is controlled by the duty cycle, providing a wide range of brightness levels. |
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Chapter 27: Analog Dimming - An Alternative Approach |
An alternative to PWM dimming is analog dimming, where the LED current is varied continuously. This is done by adjusting the voltage on the dimming pin or the resistance on the brightness pin of the LED driver. |
The MIC2298 supports analog dimming by varying the voltage on the DIM pin or the resistance on the BRT pin. This allows the microcontroller to adjust the brightness with a simple analog signal. |
Analog dimming is less efficient than PWM dimming because the LED is not operated at its maximum efficiency when the current is reduced. However, it can be simpler to implement in some applications. |
The evaluation board for the MIC2298 includes plots of the LED current versus the dimming control parameters, providing designers with practical data for implementing their own designs. |
Chapter 28: The MIC2298 Evaluation Board - A Practical Tool |
The MIC2298 evaluation board is a practical tool for evaluating the performance of the MIC2298 LED driver. The board is set to give 500 mA nominal current into two series connected power LEDs at the 50% position. |
The board includes a jumper that can be removed to set the load current higher to 1A (flash) or moved to 20% to set a lower 200 mA current (torch). The external pins can be driven to set any current between the minimum and maximum values. |
The evaluation board is designed for high efficiency, with a peak efficiency exceeding 87% at full load. The board includes test points for measuring voltages and currents, and a circuit diagram is provided. |
The evaluation board provides a starting point for custom designs. The designer can use the board's circuit diagram as a reference and modify it to meet the specific requirements of the application. |
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Chapter 29: The PIC16F785 - Integrating Analog Peripherals |
The PIC16F785 from Microchip is an example of a microcontroller that integrates analog peripherals for switch-mode power supply applications. This device includes two on-chip op-amps, two on-chip comparators, two analog PWM modules, and an adjustable voltage reference. |
These peripherals can be digitally configured to implement a wide variety of SMPS circuit topologies. Once configured, the analog control circuitry can run independently of the MCU, freeing the MCU for other tasks such as communications and status monitoring. |
The PIC16HV785 device adds a shunt voltage regulator to reduce external component count, making it even more compact. |
This level of integration is particularly valuable in applications where space and power are at a premium. The same microcontroller that provides the system control can also be used for the LED driver, eliminating the need for additional ICs. |
Chapter 30: The CAT4016 - A Serial-Input Constant-Current Driver |
The ON Semiconductor CAT4016 is another example of a serially controlled constant-current LED driver. This device is designed for driving LEDs in bargraph displays and signage applications. |
The CAT4016 features serial data input, constant LED current output, and is suitable for applications such as process control, signage, and audio meter displays. |
The device is part of a family of LED drivers that are commonly used in applications requiring multiple LEDs to be controlled with a few microcontroller pins. The constant current outputs ensure that all LEDs have consistent brightness. |
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Chapter 31: The TLC59025 - A Similar Device from Texas Instruments |
The TLC59025 from Texas Instruments is a similar device to the CAT4016. It is a 16-channel constant-current LED driver that is used in applications such as LED signs and displays. |
The TLC59025 is a good example of how multiple manufacturers offer similar devices for the same applications. This provides designers with a range of options and ensures that there is a competitive market for these components. |
The device is used in many audio and industrial applications where precise control of multiple LEDs is required. |
Chapter 32: Multiplexing with LED Drivers |
LED drivers can be used to multiplex LEDs, reducing the number of drivers needed for a given number of LEDs. In a multiplexed system, the LEDs are arranged in a matrix, and the drivers are used to scan through the rows and columns. |
The forum post describes a design where a 16-output LED driver is used to control 3 banks of 12 LEDs. The lower 12 outputs drive the LED cathodes, and the top 3 outputs switch high side drivers to multiplex between the 3 banks of LEDs. |
This approach reduces the number of drivers required for a large number of LEDs. However, multiplexing can introduce flicker and noise, so it is not always suitable for all applications. The forum post notes that for audio meters, the LEDs should not be multiplexed to keep noise lower. |
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Chapter 33: Stacking Drivers - For Even More LEDs |
Multiple LED drivers can be stacked to control even more LEDs. The serial output of one driver is fed to the input of the next, allowing a chain of drivers to be controlled with the same few microcontroller pins. |
The forum post describes a generic LED meter board with one 16 LED driver that can be selected between a 2x8 or 1x16 meter. Multiple units for even more LEDs could be stacked by feeding the serial output of the LED latch to the input of the next one. |
This approach is flexible and scalable, allowing the designer to create systems with any number of LEDs. |
Chapter 34: Programmable Features - Linear and Logarithmic Scaling |
Some LED drivers offer programmable features such as linear and logarithmic scaling. This allows the driver to be used in applications where the LED brightness should follow a specific curve. |
The forum post describes a design where a programming pin could select between linear and log scale, and arbitrary non-standard voltage ranges could be used with the microcontroller's several A/D inputs. |
This flexibility makes the LED driver suitable for a wide range of applications, including audio meters where a logarithmic response is often desired. |
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Chapter 35: The Inductor Selection - A Critical Component |
In boost converter LED drivers, the inductor is a critical component. Its value affects the performance of the converter, including the efficiency, the output voltage ripple, and the maximum output current. |
Texas Instruments provides application notes on inductor selection in boost converters for LCD backlight applications. These notes provide guidance on selecting the appropriate inductor for a given application. |
The inductor must be chosen to handle the peak current without saturating, and it must have a low DC resistance to minimize power loss. The value of the inductor determines the ripple current, which affects the noise on the output voltage. |
Chapter 36: The Schottky Diode - A Key Component in Asynchronous Converters |
In asynchronous boost converters, a Schottky diode is used for rectification. The Schottky diode has a low forward voltage drop, which reduces power loss compared to a standard diode. |
Texas Instruments provides application notes on Schottky diode selection in asynchronous boost converters. These notes provide guidance on selecting the appropriate diode for a given application. |
The Schottky diode must be chosen to handle the peak current and the reverse voltage without breakdown. The forward voltage drop and the reverse leakage current are the key parameters that affect efficiency. |
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Chapter 37: Power Budgeting - A Critical Design Step |
Power budgeting is a critical step in the design of any battery-powered barcode reader. The power consumption of the LED driver is a significant part of the total power budget. |
Texas Instruments provides application notes on power budget for backlight drivers. These notes provide guidance on estimating the power consumption of the LED driver and budgeting for it. |
The power budget must account for the power consumed by the LEDs, the driver IC, the inductor, and the other components in the drive circuit. The budget must also account for the efficiency of the converter, which varies with the input voltage and the output current. |
Chapter 38: Efficiency Optimization - Getting the Most from the Driver |
Efficiency is a critical consideration in LED driver design. A more efficient driver consumes less power, generating less heat and extending battery life. |
Texas Instruments provides application notes on increasing full-load efficiency of a backlight driver. These notes provide guidance on optimizing the efficiency of the driver. |
The efficiency of the driver is affected by the choice of components, the PCB layout, and the operating conditions. The converter efficiency is typically highest at full load and decreases at light load. |
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Chapter 39: The Constant-Voltage Supply - An Alternative Approach |
While constant-current drive is the preferred method for LED illumination, some systems use a constant-voltage supply with a ballast resistor. This approach is simpler and cheaper, but it is less accurate and less efficient. |
The patent for the synthetic barcode module notes that forward current can be controlled either by applying a voltage source and using a ballast resistor or, preferably, by regulating LED current with a constant-current source. |
The ballast resistor is a simple resistor in series with the LED. The current is determined by the voltage drop across the resistor, which is the supply voltage minus the LED forward voltage, divided by the resistance. This approach is sensitive to variations in the supply voltage and the LED forward voltage. |
Chapter 40: The Switched Current Source - A Simple Alternative |
Another alternative to a precision constant-current source is a switched current source. This is a simple circuit where a transistor is used to turn the LED on and off, and the current is limited by a resistor. |
The patent for the synthetic barcode module notes that a switched current source or current sink may also be used to drive the LED. |
A switched current source is simple and inexpensive, but it does not provide the same level of current regulation as a dedicated constant-current driver. The current may vary with temperature and supply voltage, leading to variations in brightness. |
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Chapter 41: Integrated LED Drivers with Microcontrollers |
The trend in LED driver design is toward greater integration, with more functions being combined into single chips. Some microcontrollers now include LED driver functions, reducing the component count and simplifying the design. |
The patent for the synthetic barcode module notes that the LED driver could optionally be included or integrated into the microcontroller. This would reduce the component count and simplify the design. |
The PIC16F785 is a good example of a microcontroller with integrated analog peripherals for switch-mode power supply applications. This device can be used to implement a complete LED driver with a single chip. |
Chapter 42: The Bi-Directional Communication - For Reprogrammable Modules |
Some LED driver modules support bidirectional communication, allowing them to be reprogrammed in the field. This is particularly useful for synthetic barcode modules, where the data to be transmitted may change over time. |
The patent for the synthetic barcode module describes a module that may be reprogrammable. The microcontroller may be designed to accept new instructions and programming through the LED and signal conditioning means. Using an optical communication protocol, the microcontroller could be programmed to replace existing instructions with new data received through this optical input path. |
A protocol may allow the microcontroller to replace existing instructions with new data received through this path. Thus, the module can easily be reconfigured to emulate a different barcode. |
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Chapter 43: The Wearable Application - A Novel Use of LED Drivers |
The synthetic barcode module has a novel application: wearable devices. Because of its compact configuration, a synthetic barcode module may be applied to a wearable item such as a bracelet, incorporated into a personal identification card, or attached to a container for pharmaceuticals. |
The module can be used to transmit personal identification data, medical history data, and other information. The primary readout technology (a barcode scanner) is ubiquitous and inexpensive, making this an attractive approach for many applications. |
The total cost of ownership of such modules can be relatively low, comparable to that of RFID tags, because the hardware components (an LED, a signal conditioner, a microcontroller, and an LED driver) are all inexpensive and widely available. |
Chapter 44: The Integrated Source-Detector Matrix - A Different Approach |
An alternative to the conventional LED driver is the integrated source-detector matrix. This is an array of solid state photonic diodes that are configured as LEDs and can be individually driven to emit light or to detect light as may be desired. |
The integrated matrix has the advantage of providing nearly coextensive fields of view and illumination. An image being viewed is not flood illuminated, so the device is relatively insensitive to the diffusion and laminate effects that degrade the performance of conventional scanners. |
The matrix can be used in bar code readers, optical heads, and other optical scanners that can be portable and are required to operate with low power consumption. |
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Chapter 45: Summary - The Constant Current Drive in Perspective |
The constant current drive is the foundation of stable illumination in barcode readers. It ensures that the LED or laser diode provides consistent light output, regardless of variations in temperature, supply voltage, or device characteristics. |
We have examined how different companies and technologies have approached the challenge of constant current drive: |
Rohm Semiconductor developed a driving circuit with a current mirror and a DC-DC converter that is integrated on a single IC chip. The circuit adjusts the output voltage so that the current mirror always operates in the saturation region, ensuring stable current output. |
Texas Instruments offers a wide range of LED driver ICs, including the LP8556 for tablet backlighting and the TLC59025 for LED signs. They provide extensive documentation, including white papers and application notes on LED driver design. |
Microchip Technology offers the MIC5891 latched source driver and the PIC16F785 microcontroller with integrated analog peripherals. Their MIC2298 evaluation board provides a practical tool for evaluating LED driver performance. |
Toshiba offers the TB62708N, a 16-channel constant current driver with excellent current matching. |
ON Semiconductor offers the CAT4016, a serially controlled constant-current LED driver for bargraph and signage applications. |
The patent literature reveals a wealth of innovative solutions, from programmable current sources to synthetic barcode modules. |
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The key lessons from our exploration are: |
Constant current is essential. LED brightness is proportional to current, and current must be regulated to ensure stable light output. Voltage regulation is inadequate for this purpose. |
Current mirrors are a fundamental building block. They provide a simple and effective way to copy a reference current to one or more output channels. |
Integration is the trend. The trend is toward integrating the constant current driver, the DC-DC converter, and the control logic on a single IC chip. This reduces component count, simplifies the design, and improves reliability. |
Multi-channel drivers are needed for multiple LEDs. Many readers use multiple LEDs for illumination, and multi-channel drivers provide efficient control of these LEDs. |
Programmable current sources offer flexibility. Digital control of the LED current allows the reader to adapt to different operating conditions, such as flash mode and torch mode. |
Efficiency matters. In battery-powered readers, the efficiency of the LED driver is critical for extending battery life. |
Accuracy matters. The accuracy of the current driver determines the consistency of the illumination, which affects the reading performance. |
In the end, the constant current drive is a testament to the ingenuity of engineers who have solved a difficult problem in an elegant way. The choices made in the design of this subsystem affect every subsequent stage of the signal chain, from the photodetector to the decoder. And the art of the barcode reader lies in the integration of all these stages into a reliable and efficient whole. |
cs@easiersoft.com