Hi team,
I received a question about Cgd and Cgs for ACFET and BATFET from my customer.
1. The d/s states that "Cgs value should be 20 times or higher than Cgd" in the section of 8.4.3 System Power Selection. Could you tell me the reason why should Cgs value be 20 times or higher than Cgd? Is it to prevent self-turn-on?
To limit the adapter inrush current when ACFET is turned on to power system from adapter, the Cgs and Cgd external capacitor of ACFET must be carefully selected. The larger the Cgs and Cgd capacitance, the slower turn on of ACFET will be and less inrush current of adapter. However, if Cgs or Cgd is too large, the ACDRV-CMSRC voltage may still go low after the 20ms turn on time window is expired. To make sure ACFET will not be turned on when adapter is hot plugged in, the Cgs value should be 20 times or higher than Cgd. The most cost effective way to reduce adapter in-rush current is to minimize system total capacitance.
2. From the Fig.18 in the d/s, Cgs(C16) and Cgd(C17) are connected to ACFET, but only Cgs(C15) is connected to the BATFET. Could you tell me the reason why Cgd is not connected to the BATFET?
Regards,
Yamaguchi