Converting old USB step down power to variable voltage

I have few step down boards designed to provide 5V as USB ource, but I don't really use it these days. these boards having a bigger inductor than the cheap and small DCDC converter today

These are small, and claim 2A, but if you use it to 1A continue, it would get hot pretty fast, on inductor, and the chip(the switching mosfet is in the chip.)


The main chip on these old boards are MP2307, data sheet claim to have 3A continuous.

With these old board, the inductor is larger, should have better performance.

Find the feedback resistor

MP2703 use pin 5 as feedback pin, with a set of voltage divider, as R1 and R2 in the reference design.






After few search, I found that the R2 (10K) is the R2 of reference design, which connect FB pin to ground. R6and R7 on my board is parallel connected as a R1 in reference design.

Change the voltage divider resistors.

In the data sheet, it shows how to calculate the out put voltage and some reference value.

 




I wish to use this board to power a plug form model engine, I have do some test with my power supply:


Bearly heated at 2.21A



1.19V 2.5A seems fine


1.39V 2.79A seems the be limit.





I attached a 10K VR to reach 1.39V I wished, R value is about 3.3K for 1.38V
The red led has a current limitation resistor at 1K, so it won't light up at 1.538V.




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