TPS51220A/about PGOOD low condition

Hi Yusuke-san,

PGood details description is:

The powergood function is activated after softstart has finished. If the output voltage becomes within ±5% of the target value, internal comparators detect power good state and the powergood signal becomes high after 1ms internal delay. If the output voltage goes outside of ±10% of the target value, the powergood signal becomes low after 1.5μs internal delay.

In your waveform, 

1, First, it looks not occur real OCP, OCP happens when the inductor current becomes larger than the overcurrent trip level, and then High-side will turn-off.

In an overcurrent condition, the current to the load exceeds the current to the output capacitor thus the output voltage tends to fall down, and it ultimately crosses the undervoltage protection threshold and shutdown. So If you make the output current a little bigger, I think you can see the real OCP and High-side will off.

2, At the point of the Pgood goes down, the output voltage should be below -10% of the target Vout.  After that, the Iout decrease cause Vout goes high to normal, and then after about 1ms, PGOOD goes high, so it match with the PGOOD function.

Frank Zhang