Hi -
I have an implementation of a TPS40400 converting from 18V down to 1V. In my first batch of prototypes, this power supply circuit has gone from working to failure and I can't figure out how to repair it. Replacing the IC repairs the problem, but I can't figure out what weakness in my implementation is slowly damaging the circuit.
Here is the schematic :
The register dump is the following : (apologies for the formatting)
00: XX 00 17 ff XX XX XX XX XX XX XX XX XX XX XX XX
10: 00 ff ff ff ff XX XX XX XX XX XX XX XX XX XX XX
20: 16 XX 00 XX XX 47 51 XX XX 00 XX XX XX XX XX XX
30: XX XX XX 13 XX 0b 0a XX 62 00 XX XX XX XX XX XX
40: 60 04 XX XX 39 04 14 04 XX XX 0f XX XX XX XX XX
50: c0 XX XX XX XX XX XX XX XX XX XX XX XX XX 6a 52
60: XX 2b XX XX XX XX XX XX XX XX XX XX XX XX XX XX
70: XX XX XX XX XX XX XX XX 02 02 00 00 XX 00 82 XX
80: XX XX XX XX XX XX XX XX 34 XX XX 90 0d XX XX XX
90: XX XX XX XX XX XX XX XX 11 XX XX XX XX XX XX XX
a0: 0c 50 XX XX 66 00 XX XX XX XX XX XX XX XX XX XX
b0: XX XX XX XX XX XX XX XX XX XX XX XX XX XX XX XX
c0: XX XX XX XX XX XX XX XX XX XX XX XX XX XX XX XX
d0: 00 XX XX XX 00 00 00 00 XX XX XX XX XX XX XX XX
e0: XX XX XX XX XX XX XX XX XX XX XX XX XX XX XX XX
f0: XX XX XX XX XX XX XX XX XX XX XX XX 15 XX XX XX
I have corrected the settings of the IOUT_CAL_GAIN (0x38) and the VOUT_SCALE_LOOP (0x29) registers, but would these being the factory defaults be enough to damage the device?
When the IC is working correctly there are good 18V pulses going to the input side of the coil. After failure the HDRV and LDRV lines are only putting out very weak (about 1V) gating pulses that don't produce much at the output side of the FET. The TPS40400 still responds to I2C queries though, so the device isn't completely dead. I can't figure out how to get the outputs active again though.
Thanks for any help and guidance you might be able to give.
Bill