Electronic – Is it OK to PWM a current source

constant-currentcurrent-sourceled-driver

I want to control power RGB LEDs, for efficiency and safety they are better to be driven by a current source. I can use either linear or buck converters. I made a linear current source using LM317.

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I PWMed the input (Vin) and it worked fine. Is it OK if I PWM the output(using a MOSFET right before the load)? As I learned in theory if the output is left open the output voltage reaches infinity (hear maybe a maximum of input voltage), so as I turn the MOSFET on a high voltage goes to the LED momentarily and might damage it after a while.

I think a decent step-down LED driver is PT4115E (Datasheet) it has a PWM pin. Are chips like this OK for driving LEDs that duty cycle may change several times per second?

Best Answer

Assuming you do not want to modify your current source, and only want to modify the load: Instead of inserting the switch in series, it is more important to provide an alternate path for the current to take. If I understand you correctly, you are considering the schematic on the left. Replacing it with the one on the right should work better. Your control signal will be inverted; when the signal is high, the load will be shorted so not drawing current.

schematic

simulate this circuit – Schematic created using CircuitLab

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