I am trying to troubleshoot a design where HDRV is not driving the high side FET. This is a 12V to 3.3V @ 5A rail.
We are hypothesizing that the bootstrap circuitry is failing and that HDRV is not being supplied with adequate voltage to turn on the high side FET. (Blown bootstrap diode?)
However, the bootstrap circuitry seems pretty fool proof. We have a 0.1uF bootstrap capacitor with no series resistor. I noticed that the internal 6V regulator has a ~100mA current limit. Given that we have normal component values, and the regulator is current limited, I don't see how we could have blown the internal bootstrap diode.
The 6V on pin BP6 looks good. I am working on getting some screen captures. Is there anything in particular that would be helpful for you to see?
In the meantime, do you think it's possible to damage the bootstrap diode? Is there a more common point of failure that we are overlooking? This only happens to ~1% of units and replacing the TPS40400 with a new IC resolves the issue.