LM25119 Webench generated design: Vin 30v to 40v, Vout 27v (max), Iout 12amp
Laid out the first board (4 layer) with signal on the outer layers and with 0805 passives to aid in debug. Board assembly house built first two and both have the same problem.
This design has provisions for adjustable output voltage, however I've removed the eePot and have fixed resistors.
The problem I'm having is output voltage is as expected, 27v, but when it is loaded with 30 ohm resistor the voltage sags to just above zero.
I'm looking for feedback on the board bringup procedure and if others have seen this kind of problem.
I've gone thru the components and they look correct - there could be an assembly error since this is first article. There could also be a layout or design error but I did a thorough check plot prior to fabbing the PCB.
Notes on Webench improvement suggeestion:: it would be good if Webench generated a 'dot' on intentional junctions and crossovers had no junction dot in the generated schematic. Also, the eagle cad generated output is a good start, however the *.sch and *.brd have consistency errors so I could only use the generated *.sch.
OK, so here's the board bringup procedure I'm following:
0) Go for it - connect 32v to the board set a current limit of 2amps, and apply power. Voltage output is 27v, however with 30 ohm resistor, the output voltage folds to near zero. Do the same on the second board. Same result. suspicions:
a) RT is set wrong and the switching frequency is way off
b) current sense resistor is wrong value (should be 0.008 ohms)
c) under voltage lockout is being activated.
1) verify design and layout against that generated by Webench
2) check component values as stuffed against BOM vs Webench generated, check part orientation of FETs
3) Share the design on TI e2e community in the hopes someone spots an error or has a helpful suggestion
4) measure voltages on UVLO, look at waveform on CS1, CS2
5) drink a beer...
I'd appreciate suggestions and I'll try to post what I find.