LM25118: Design review request

Hello, 

Thanks for reaching out. Has this schematic already been double checked with the Excel Quick Start calculator https://www.ti.com/lit/zip/snvu065 ?
For the layout we recommend following the guidelines at page 35 of the datasheet, section 10.2 . Could you please elaborate a bit more on the issues you're seeing on this board? Under wich conditions are the mosfets overheating? If the output voltage dropped to 2 V it might be that the device is running in current limit. 

Kind regards,

EM

I'm assuming that the webench designer calculates the values using the same formula as that excel file, so in that case yes, I did. The board draws too much current (> 4.5 A), so my power supply  limits the voltage to around 5 volts to keep a constant current. At this voltage, the mosfets immediately start to melt the solder. I designed the board geometry as close as possible to a board generated by webench, Although page 35 section 10.2 does look slightly different.

how important is it that the forward voltage fot the diodes to be exact? I had to get some that were slightly different since they were out of stock.

I got this specific board working. the problem was insufficient grounding, and mosfet ringing, possibly burning them out. However, I created another version that is much smaller. I was wondering if there is anything in particular that needs big traces? the board that is generated by webench is very large. the new version I created has a much smaller footprint, and the lm25118 is getting hot. It starts out giving 12 volts, but over time goes down to 1.3 volts when a load is on, but never goes back to 12 when it is taken off.

Should I be using resistors for the mosfets? I don't see any under section 7.2 Functional Block Diagram on the datasheet for the LM25118. I guess the resistance Rt is what handles this.

Hello Josh,

What do you mean with using resistors for the MOSFETs? Do you mean in the gates? This will increase power dissipation.

For the circuits that need big traces, please have a look for hot loops, see here: 

https://e2e.ti.com/blogs_/b/powerhouse/posts/four-switch-buck-boost-layout-tip-no-1-identifying-the-critical-parts-for-layout
https://e2e.ti.com/blogs_/b/powerhouse/posts/four-switch-buck-boost-layout-tip-no-2-optimizing-hot-loops-in-the-power-stage
https://e2e.ti.com/blogs_/b/powerhouse/posts/four-switch-buck-boost-layout-tip-no-3-separating-differential-sense-lines-from-power-planes
https://e2e.ti.com/blogs_/b/powerhouse/posts/four-switch-buck-boost-layout-tip-no-4-routing-gate-drive-and-return-paths

Especially the hot loops (number 2) need wide traces.

Best regards,
Brigitte