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BQ24725A: Cgd and Cgs cap for ACFET and BATFET

Part Number: BQ24725A

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

  • Hey Yamaguchi-san,

    1. Yes, the excerpt you provided explains the purpose of Cgs > 20xCgd. Hotplugging a power supply can transiently pull up on the gate of the ACFET, partially or fully turning it on. 

    Takashi Yamaguchi said:
    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

    2. On the BATFET, we are only trying to add Cgs to have a turn-on/turn-off delay such that the BATFET and input protection FETs are not turned on at the same time.

    Regards,
    Joel H

  • Hi Joel-san,

    Thank you for the answer.

    I'd like to know more detail on the question #2.

    If adding the gate capacitance to adjust the FET turn-on / off time, I believe that connecting the cgs is enough. Why does the Cgd is connected to the ACFET?

    Regards,

    Yamaguchi

  • Hey Yamaguchi-san,

    I believe this capacitor is to help prevent the gate from being pulled up easily with a high dV/dt event on the input. 

    While the Cgs > 20xCgd requirement should be sufficient, it may be recommended to add extra Cgd capacitance to mitigate this effect further.

    Regards,

    Joel H