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“The LTC4417 gate driver pulls down on G1, G2 and G3 with a strong P-channel source follower and a 2µA current source. When the clamp voltage is reached, the P-channel source follower is back biased, leaving the 2µA current source to hold G1, G2 and G3 at the clamp voltage. To minimize inrush current at start-up, the gate driver soft-starts the first input supply to connect VOUT, at a rate of around 5V/ms terminating when any channel disconnects or 32ms has elapsed. Once slew rate control has terminated, the gate driver quickly turns on and off external back-to-back P-channel MOSFETs as needed. A SHDN low to high transition or VOUT drooping below 0.7V reactivates soft-start.”
Root Cause: UPDATE: see Inrush current testing below
The root issue is that the chip is not soft-starting again when the input is switched on. Discharging the capacitors triggers the soft-start again. Our supplies have large series resistance due to the wires attached to them and thus the voltage drop caused by the inrush current (even for capacitors that are not fully charged) triggers an Under Voltage Fault, which recovers quickly once the output is switched off.
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I have still been able to get it stuck in the reset loop when the DC-DC input is at the low end of its voltage range, with the little bit of inrush current there is causing it to dip below the threshold. The DC-DC is regulated, so it will not be in this range, and the AUX will be monitored before we hit it with high currents. But overall, we just had too much current and too much resistance. So was all this testing worth it for a problem that we might not have actually encountered in the car? From a time perspective, maybe not, but from a learning perspective, definitely!
Main lessons learned:
Start testing out possible solutions immediately, and document what changes.
Always check your assumptions (I forgot the multimeter leads had resistance at one point)
Make sure that you know your board like the back of your hand, then you know where all the extra pieces for testing need to be put (I had to get creative with some of the components placement)
Make sure you’re using the right tools for the job - a clamp meter trying to measure inrush current on a 1ms timescale will not do anything when your inrush current only lasts for 8ms. Know your measurements timescales and the limitations of the equipment!
Sometimes seemingly complicated problems can be caused by simple effects → inrush current and ESR causing voltage droop = reset loop
Multimeter loading is a thing - multimeters will typically have 10M resistance between their input terminals. This can (and will) affect the signals that you measure!