DRV8320: OCP trigger poin with differnt DC Bus

Part Number:DRV8320

Customer tried the OCP function with the DRV8320 without Motor loading.

1. VBUS=36V, The delay time between nFAULT pin and OCP trigger point of 52A is 2uS. With the time delay, the current goes up to 86.4A 

 2. VBUS=50V, The delay time between nFAULT pin and OCP trigger point of 52A is 3.2uS. With the time delay, the current goes up to 128A.

The delay time difference between 36V and 50V is 1.2uS (3.2uS-2uS).  How come? Is this time related to the bus voltage? Is this behavior able to be improved? Thanks.

DRV8320 setting:

OCP Setting: VOCP is set at 0.13V, (VDS pin is connected to GND thru 18KOhm)

Ext. MOSFET's Rds(on) = 0.0025R, so that the OCP trigger point is set 52A