LED Technology
Figure 2. 3A camera flash waveform of Figure 1’s parallel LED drivers looks the same regard- less of the amount of PWM off-time. Waveforms show that a 10µs pulse after (a) 10ms and after (b) one second are the same. The LT3932 LED flash also looks the same after a day or longer of PWM off-time.
time. Longer off-times (for frequencies below 100Hz) risk output capacitor charge loss due to leakage, preventing a quick response when the LED is turned back on.
Parallel LED drivers for higher current
LED drivers act as current sources, regulating the current sent out through the light emitting diodes. Since current only flows in a single direction to the output, multiple LED drivers can be placed in parallel and their currents sum through the load. Current sources do not need to be protected against current running backward through one converter or having mismatched outputs. Voltage regulators, on the other hand, are not inherently good at current sharing. If they are all trying to regulate the output voltage to a single point, and there are slight differences in their feedback networks, a regulator may draw reverse current.
An LED driver maintains its output current, regardless of other drivers that may supply additional current summed at the output load. This makes paralleling LED drivers quite simple. For example, the LED flash system of two parallel LT3932 LED drivers shown in Figure 1 efficiently drives 4 LEDs at 3A with short 10µs pulses spread out by long periods
of time - defined by the machine vision system. Each LT3932 converter sources half of the total string current during PWM on-time and turns off and saves its output state during PWM off- times. The off-time can be short or long, with no effect on the flash waveform repeatability. Parallel camera flash applications share nearly the same simplicity as single converters during long off-times. The converters observe the shared output voltage at the end of the last PWM on pulse, and keep the output capacitor charged to that state, even during long off-times. Each converter disconnects its PWM MOSFET from the shared load and keeps its output capacitor charged to approximately the last voltage state by sourcing current to that capacitor as it leaks energy. Any leakage experienced by these capacitors over long off-times is overcome by the small amount of maintenance current. When the next PWM on pulse starts, the PWM MOSFETs of each converter are turned on and the output capacitors start up in approximately the same state as the last pulse, regardless of whether 10ms has passed or a full day. Figures 2(a) and 2(b) demonstrate the LT3932 parallel LED drivers driving 4 LEDs at 3A with a 10µs machine vision camera pulse. The LED pulse is sharp and fast, regardless of whether there is a 10ms PWM off-time
Figure 3. An example of machine vision on an industrial conveyor belt. Inspection systems move at many different speeds, yet the flash technology must be fast and crisp.
Figure 4. Two DC2286A LT3932 demo circuits are easily connected in parallel to create the 3A to 4A machine vision LED flash application shown in Figure 1.
(100Hz) or a 1s PWM off-time (1Hz), which is ideal for machine vision systems.
Even higher current is possible Parallel LED drivers are not limited to two converters. Three or more converters can be paralleled to create even higher current waveforms with sharp edges. Since this system does not have a master or slave device, all of the converters source the same amount of current and share the load equally. It is recommended that all of the parallel LED driver converters share the same synchronised clock and remain in-phase. This ensures that all converters have approximately the same phasing on the ripple of their output capacitors so that ripple currents do not flow backward or between the different converters. It is important for the PWM pulse waveform to remain in-phase with the 2MHz synchronisation clock. This ensures that the LED flash waveform remains square and without jitter, producing the best image processing results.
The LT3932 demonstration circuit (DC2286A) is designed to drive 1A of LED current through one or two LEDs as a step-down LED driver. It can easily be altered and paralleled, as shown
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in Figure 1, for higher current, higher voltage, or parallel operation. Figure 4 demonstrates how two of these circuits are easily connected together to drive 10µs, 3A pulses through 4 LEDs from 24V input. For testing purposes, a pulse generator can be used for the synchronised clock signal, as shown in Figure 4. In a production machine vision system, a clock chip can be used to generate the synchronised sync and PWM pulses. For higher current pulses, add more demonstration circuit DC2286A converters using the same parallel scheme.
Conclusion
Machine vision systems can use parallel LED drivers to create the fast, square, high current waveforms required for automated image processing. The LT3932 LED driver’s proprietary camera flash technology can be extended to higher currents by connecting parallel converters. 3A and higher pulses on the order of microseconds are possible with parallel LT3932 converters, even with long off-times. LED camera flash waveforms remain square and without jitter, no matter how long the off-time between LED flashes may be.
www.analog.com Components in Electronics May 2023 55
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