[FAQ] LM74930-Q1: What is the function of diode on VS pin

Background
LM74800-Q1 and LM74930-Q1 are 70V (abs-max) rated controllers but can support transients above its rating because of its common-source topology capability. One example is the 200V unsuppressed load dump.With addition of some supporting components like D1,Rvs,Dz ; the controller can withstand higher voltages with OV pin and properly scaled Q1 FET. Only pin that is vulnerable to transients is the VS pin.

VS pin is protected by the Zener Dz and to minimize the power dissipation across the zener during clamp, a series resistor Rvs is used.
Reason
The value of Rvs is generally in the order of 5kΩ-10kΩ. During initial startup, the charge pump (VCAP-VVS) needs to charge from 0V to 12V and the device draws higher Iq mostly by the VS pin (as shown in the char graph). This higher Iq multiplied by the Rvs can cause voltage drop across Rvs causing VS voltage to be stuck at its POR threshold.

During startup, as the VCAP-VVS voltage reaches above charge pump UVLO, the HGATE starts to turn ON and the diode bypasses the Rvs by powering VS from the common source point enabling VS to charge above its POR value.

Startup behavior in common source topology:

Selected diode should be rated higher than Dz as the maximum drop across diode will be when EN is low and zener is active.