Hello all,
I am experiencing troubleshooting with a TPS40422 which configuration is the following:
- Converts 5V0 to 1V0
- TPS40422 is powered with VDD = 5V0
- TPS40422 is configured in dual-phase mode
- TPS40422 is connected to PMBus, which is powered by an auxiliary voltage : 3V3_AUX (pull-up to 3V3_AUX are present on the SCL and SDA lines)
The regulator starts correctly when 5V0 power is established before 3V3_AUX. Nevertheless, I observed that sometimes it fails to start when 3V3_AUX is established before 5V0.
In this case, I observed the following behavior:
- BP6 output is measured to 2V8 (instead of 5V0)
- BP3 output is measured to 0V4 (instead of 3V3)
- TPS40422 is not responding to on PMBUS
- 1V0 power output is off
=> So, the regulator seems to be switched-off, probably due to internal LDOs failure.
I investigated two trails:
- My default LDO's decoupling solution uses 1uF on BP6 pin, and 100nF on BP3 pin. I tried to increase these values, but the error was still occurring (maybe it was a worst solution)
- I stopped the PMBus activity before 5V0 establishment, but there was no improvement. Additionally, I tried to reproduce the problem on the evaluation board (TPS40422EVM-091, modified to suit my needs in dual-phase mode, 1V0 output), but it doesn't appears on this EVM.
=> Does anyone have an idea on which element could stuck the internal LDOs at startup ?
=> Is there constraints on SDA/SCL, or CNTLx lines polarization before VDD establishment ?
Best regards,
XA.
