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Can I switch this constant current LED driver at 38kHz? (discuss.tchncs.de)
submitted 2 years ago by silvio2402@discuss.tchncs.de to c/askelectronics@discuss.tchncs.de
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My goal is to drive an infrared LED like (this SFH 4727AS) to control IR remote controlled devices (for example a TV).

I can already output the signal on a pin of my microcontroller. The signal has a carrier frequency of 38kHz.

I am however limited to a 5V 1A power supply and would like to drive the LED as close to the 5W psu limit as possible.

I first thought about using a MOSFET like this AO3400A together with a resistor, but decided it was worth using an LED driver, do you agree?

Now I'm looking at this PAM2804 constant current LED driver but I'm not sure if I can just supply my signal to the EN pin. The EN pin is meant for PWM-dimming, so does that mean the output is smoothed to prevent flickering?

I'd love to hear your thoughts about how I should go about this. Thanks

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[–] cmnybo@discuss.tchncs.de 0 points 2 years ago (2 children)

I would just use a MOSFET and resistor to drive the LED if you are only using it in short bursts.

If you want to drive the MOSFET directly from the microcontroller pin, it will need a series resistor to limit the current since the gate has quite a bit of capacitance. If your microcontroller pins can handle 15-20mA, it will be able to switch an AO3400A fast enough without needing a gate driver.

[–] silvio2402@discuss.tchncs.de 1 points 2 years ago* (last edited 2 years ago) (1 children)

Hey, thanks for your comment. I looked at using a resistor in series with the LED, but if my calculations were correct I could only power the LED less than 3W and 2W would be wasted.

R = (V~s~-V~f~)/I~f~ = (5V-2.8V)/1A = 2.2Ω

P~LED~ = V~f~*I~f~ = 2.8V*1A = 2.8W

P~Resistor~ = V~s~*I~f~-P~LED~ = 5V*1A-2.8W = 2.2W

Let me know if the calculations are correct.

Edit: Calculations

[–] cmnybo@discuss.tchncs.de 1 points 2 years ago

Those calculations are correct.

Since the remote control signals are short and low duty cycle, you could use a capacitor to provide the peak current for the LED without going over the maximum current of the power supply.