LM27402: Not starting up consistently with Vout=12V (original design was Vout=10V)

The values recommended by Webench are marked-up in red on the schematic shown below (14V<Vin<15V, Vout=12V, and Iout=8A).   Which changes are definitely required and which ones would be "optional"  ? ?  Thanks.

Allen,

Given the high duty cycle and hig output power I would recommend a lower switching frequency such as 300kHz.

R33=45.3K

Output cap values seem to be pretty low as shown.

L2=1.5uH is a good choice

R28 at 10K gives a max output current of around 5A with the 17mohms Rdson FET used.  If you would like to set at >8A limit would recommend changing R28 value to 14.7k.

Inductor DCR info is missing to verify R29 and C30.

Regards,

Mathew

LM27402 PWM Regulator Design_e2e.xlsmPlease use this as a design tool also. I think you need to use 100uF output with a esr of around 10mohms in this case to get good phase margin.

Hi Matthew,

Per your recommended simulation, I tried the following configuration, and I was unable to get this regulator to start-up.  I tried removing D3 without success.  I also tried removing the output capacitor (C32) without success.  Any other suggestions ?

Thanks,

~Allen

Here is the layout ....

Allen,

What is the condition of EN pin?

Regards,

Mathew

Hi Matthew,

I increased C33 to 0.1uF (startup capacitor) and added a voltage divider (200K & 56K) to the EN pin as shown below, and this did not fix the problem.  Any other ideas ?

Thanks,

~Allen

Allen,

I checked the simulation and please modify the compensation as shown.

Also please see Section 10 of the LM27402 datasheet for layout recommendation. Your layout does not look ideal. Inputs caps should be placed right across the FETS to get a close loop. On schematic there is no input caps but in layout it seems to be there but location and connections are far from ideal. Please see the DCR current sensing values that cane be implemented for 14A current limit.