I'm trying to troubleshoot a tps65261rhbr triple synchronous buck converter from 12V producing 1.8V, 3.3V and 5V.
The 5V hovers around 620mV with occasional spikes to 2.9V, but the calculations from the datasheet to select component values, and the simulation I've run from PSpice suggest that these components should work correctly. The voltage divider uses the exact resistance values specified in the datasheet for 5V.
Does anybody know what might cause this behavior in a SMPS?
My current best guess is that I'm tripping the over-current protection circuitry as the 5V comes online, causes a delay for several switching cycles, then tries again. I think the oscilloscope behavior seems to match this description from the datasheet:
The TPS65261, TPS65261-1 is protected from overload and over temperature fault conditions. The converter minimizes excessive output overvoltage transients by taking advantage of the power good comparator. When the output is overvoltage, the high-side MOSFET is turned off until the internal feedback voltage is lower than 105% of the 0.6V reference voltage. The TPS65261, TPS65261-1 implements both high-side MOSFET overload protection and bidirectional low-side MOSFET overload protection to avoid inductor current runaway. If the over current condition has lasted for more than the OC wait time (256 clock cycles), the converter will shut down and re-start after the hiccup time (8192 clock cycles)
However, I don't think the loads I have on the 5V should exceed the 2A that the SMPS supports on this power line. Currently the 5V has several loads: a raspberry pi CM3 VBat (700mA), a LDO driving a 4G cellular radio (maximum 500mA during transmission), a ZED-F9P GNSS circuit (130 mA), 4 relays, and some other assorted circuitry that should consume negligible current (e.g. USB switch). In total if everything was consuming the maximum current I believe it should be around 1.7A. So unless I've miscalculated somewhere, the only reason I could see enough current draw to trip the overcurrent would be a short circuit.
What I'm really looking for are suggestions for finding and proving root cause. I'm currently trying to rule out a short circuit, after that my next idea is to cut the 5v power trace and measure current over the gap and see if if I can power it with a bench power supply.