TPS546C20A: Output 5V 70A Debugging problem

User1932012,

The first thing that immediately comes to mind is to check whether you are losing your input supply.

With 0.9V @ 70A, the output power was only 63W, and the input power would be 70-75W, about 6A from a 12V source.

At 5V output, 26A is 130W of power, requiring about 11A of supply current.  With a 5V output, a sagging input voltage would trip output Under Voltage on the output before it tripped under-voltage lock-out on the input supply.

If the input supply is steady, check the ripple voltage between PVIN and AVIN (Pin 29)

The TPS546C20A has a pulse by pulse input current limit during the high-side FET by monitoring the drop from AVIN to SW during the on-time.  It is possible that the much longer on-time of the 5V output is creating sufficient ripple on PVIN that is not reflected on AVIN to trigger the high-side current limit, terminate the on-times early and cause an output under voltage protection fault as the high-side current limit restricts the duty cycle, reducing the output voltage.

User1932012,

Please let me know how this worked out for you.

I am going to mark this as resolved until I hear back from you.