UCC27712: UCC27712 Rbias

Hello Yangbo,

I am an applications engineer supporting UCC27712. Before the internal logic circuitry is powered up, the low-side MOSFET cannot be activated. The voltage at the supply pins must at least be greater than the threshold voltage of the low-side MOSFET before it can be activated, regardless of the UVLO state. When the power supply is brought up with a fast slew rate, a large current will flow in the parasitic capacitances between the output and the supply/common pins. If the low-side MOSFET has not yet activated, there is no low-impedance discharge path for the accumulated output charge and a voltage will appear at the output pin proportional to the ratio of the parasitic capacitances. A resistor in series with the bias supply and VDD/HB pins limits the slew rate when the power supply comes up, ensuring that the voltage on the output cannot increase before the internal logic circuitry activates and the low-side MOSFET can begin clamping the output low.

Note that with large output load capacitance or small gate resistors, the load capacitance is typically several orders of magnitude larger than the parasitic capacitances and the total output voltage increase will be smaller. 

Regards,