S21E/H1 Ramping Boost Driver- Increase Vmax

I was looking to get a little more voltage out of a 6V S21E/H1 XHP50 driver. It looks like the resistor circuit in there caps the Vout at a max of around 6.5. I’d like to bump is to 8 if possible. This driver uses the MP3431, so in theory I could do the same mod as discussed for the XHP35 boost driver, but the topology of this driver is pretty different and I’m struggling to find the correct resistor to swap.

Has anyone smart played with this driver yet and knows which resistor to swap?

Thanks for any input!

Do you have pictures of the driver ?

Here are some pics. Top, bottom, and the circuit I’m trying to find.

Thanks, though the traces a a bit difficult to see, and it looks like a 4 layer PCB which makes things harder to see.

FB pin is that one so you need to find the resistor between OUT and FB. Probing with a DMM in continuity mode should help.

1 Thank

I’ll keep poking at it… hopefully I get lucky…!

Thanks for your help.

OK I’ve mapped Pin 5 from the MP3431 to these two resistors, so one it R1 and the other is R2. Unfortunately neither one maps to ground. XD

The when I put the values in the calculator from the applications note i get the right output voltage if the Vref is 2.7, which is plausible.


Well, my guess at R2 was wrong. stacking a resistor there nerfs the driver. I’ll try the R1 position as that’s the only other resistor that has continuity to the number 5 pin shown above.