LED Technology
LED driver for high power machine vision flash
By Keith Szolusha, LED drivers applications manager, and Kyle Lawrence, associate applications engineer, Analog Devices
M
achine vision systems use very short flashes of intense light to produce high speed images used in a wide variety of data processing
applications. For instance, fast moving conveyor belts are run through machine vision systems for quick label and defect inspections. IR and laser LED flashes are commonly used for proximity and motion-sensing machine vision. Security systems send out high speed, hard-to-detect LED flashes to sense motion and capture and store security footage. One challenge in all of these systems is creating the very high current and short-term (microseconds) LED camera flash waveforms, which can be spread out over long periods of time, such as 100ms to over 1s. Creating short, square LED flash waveforms separated by long periods of time is nontrivial. As the drive currents for the LEDs (or strings of LEDs) rise above 1A and the LED on-times shrink to microseconds, the challenge increases. Many LED drivers with high speed PWM capabilities may not efficiently handle long off-times and high currents for short amounts of time without degradation of the square-type waveform needed for proper high speed image processing.
Fortunately, the LT3932 high speed LED driver can provide machine vision camera flash for up to 2A LED strings, even with long off-times of 1 second, 1 hour, 1 day, or longer. The LT3932’s special camera flash feature allows it to maintain the output capacitor and control loop charge state, even during long off-times. After sampling the state of the output and control loop capacitors, the LT3932 continues to trickle-charge these components during long off-times to compensate for typical leakage currents, which is not accounted for by other LED drivers.
The proprietary flash technology of the LT3932 scales up when drivers are paralleled for increased LED flash current. The desired flash shape and integrity are maintained. Figure 1 shows how easy it is to parallel two
54 May 2023
Figure 1. Parallel LT3932 1.5A LED drivers yield 3A machine vision LED pulses with long off-times relative to standard PWM dimming frequencies.
drivers for a 3A camera flash - designs up to 4A are possible.
LED flash requirements for machine vision systems are far more demanding than a standard PWM dimming driver can meet. That is, most high-end LED drivers are designed to produce PWM dimming brightness control
Components in Electronics
at a PWM frequency of at least 100Hz. This is because lower frequencies can be perceived by the human eye as an annoying flicker or strobing, even if the LED waveforms are square and repeatable. At 100Hz, the theoretical maximum off-time is about 10ms. During the 10ms off-time, if designed correctly, an
LED driver loses minimal output capacitor charge, allowing it to start its control loop in approximately the same state in which it ended the last PWM ON pulse. A quick response and ramp-up of the inductor current and the next LED PWM ON pulse can be quick and repeatable, with minimised start-up
www.cieonline.co.uk.uk
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