MP1584 Current Feedback: 1 W Red LED

A red 1 W LED works just as well as the amber LED from an MP1584 regulator hacked into current feedback mode:

MP1584 buck regulator - current feedback - red LED
MP1584 buck regulator – current feedback – red LED

I started with the same 1.65 Ω sense resistor and got the same 484 mA current, with the LED forward drop at a surprisingly high 3.3 V = 1.6 W. Ouch.

Adding a 1 Ω series resistor to get 2.65 Ω lowered the current to 300 mA with a forward drop of 2.45 V = 740 mW.

Running the numbers suggested a 2.3 Ω sense resistor made from a pair of parallel 4.7 Ω resistors, which produced 346 mA and an LED drop of 2.66 V = 920 mW. The resistor dissipates 280 mW.

The bench supply provides 6.3 V @ 200 mA = 1.26 W, so the overall efficiency is 94% and the LED burns 73% of the input.

I expected the red LED would have a lower forward drop than the amber LED, but it’s actually higher.

Word: Trust, but verify.

One thought on “MP1584 Current Feedback: 1 W Red LED

  1. Re color vs Vf. It’s going to depend on the chemistry. High intensity reds (AlGaAs) are pretty different from old-school LEDs (GaAs).

    I’d assume nothing, except if they are all the same part number, they should be close. Probably.

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