LM25143-Q1: The load is 20A. After the power is cut off, the output drops and then rises sharply. What is going on?

Hello Peigen,

Thanks for the query.

The above phenomena can occur due to UVLO limit. The following diagram is referred to explain the phenomena.

The Voltage appearing at the input pin of the IC is effectively, (Vs - Iin*Rs).

When the Enable goes low, both the Vs and Iin starts decreasing and the Vin pin voltage starts falling.

As Vin pin falls, Vout falls too.

The point at which Vin goes below (Vin)'s UVLO limit, the Iin suddenly falls to zero.

This causes the Iin*Rs loss to become zero and hence, the Vin pin voltage can go above the UVLO limit once more which causes the IC to try to restart and try to bring up Vout voltage level once more.

This causes the sharp rise in Vout you observed.

But this does not persist long as again IR loss brings the Vin pin below the UVLO level, and the leftover energy left on the input side is completely used up finally shutting down the IC.

Hope this clarifies your doubt.

I am also attaching an application note explaining the same. You can refer to the section 3 about 'Hysteresis and UVLO Bouncing'.

slva769a.pdf

Also below is the datasheet's snap specifying the same.

Thanks and Regards, 
Sourish Nandy.