LM74700-Q1: VCAP constantly oscillate

Hi Fred,

1. Under regulated conduction mode, the Ron of the FET is regulated to maintain the anode to cathode voltage as 20mV. To achieve this, the gate driver of LM74700-Q1 regulates the Vgs across the FET to achieve the required Ron for 20mV drop with a given load. Hence, gate and Vcap are not shorted together in regulated conduction mode, rather gate voltage is regulated to achieve 20mV forward drop.

2. When the device enters full conduction mode, the gate needs to be fully enhanced. Hence, the gate is shorted to the Vcap, so that Vgs of the FET is same as Vcap-Vanode. For such high Vgs, as the FET is fully enhanced the Ron does not vary much with the changing Vcap voltage. 

Below is the Ron vs Vgs curve of a typical power FET-

As you can see, at higher voltages the Ron of the FET is constant with respect to small changes in Vgs. Since because of this full enhancement, the Ron becomes constant, the Vcap oscillations do not reflect at the power path.

I hope this clarified your queries.

Thanks and regards,

Divyanshu

Hi  team,

the reason I ask if GM AMP's VDD =VCAP is because GM AMP is regulating the GATE under regulated conduction mode.

since the GATE must higher than Vanode, that's why I ask if the VDD of AMP is VCAP.  (VCAP>Vanode)

If yes, even they're not shorted, it will still affect GATE anyway under regulated conduction mode.

Thank your reply,

Regards,

Fred