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BQ24780S: BQ24780S

user5779852
Prodigy 200 points
Part Number: BQ24780S

bq24780s datasheet recommend to place a 1000pF capacitor(PC8905) between  A/D DOCK IN pin and ACDRV pin ,and a  0.047uF capacitor(PC8922) between  CMSRC pin and ACDRV pin.

May i have your help to know why we should place these capacitor separately ,and how much is the loweset capacitor that we can use. 

  • 0
    TI__Guru**** 166006 points
    The capacitors are used to soft start the FET turn on in the hope of reducing inrush current to downstream capacitors at powerup. Without them, your input power supply may droop and false trigger ACDET, causing the FET to turn on and off several times as it charges those capacitors.
  • 0 in reply to Jeff F
    Prodigy 200 points

    Thanks for your anwser, and I want to know how to determine these capacitor's value. Is here any calculation method?

  • 0 in reply to user5779852
    TI__Guru**** 166006 points

    There is no easy calculation for determining the minimum capacitance. This is usually determined empirically based on the total capacitance on the SYS rail. However, the datasheet gives guidance on the maximum size:

    • Cgs should be 40× or higher than Cgd to avoid ACFET false turn on during adapter hot plug-in

    • Fully turn on ACFET within 20 ms, otherwise, charger IC will consider turn-on failure so the recommended 4kohm + Cgs time constant must be less than 20ms

    I recommend starting with the datasheet recommended values, measuring the startup waveform on the scope and, if successful startup with no pulsing, you can lower from there.

  • 0 in reply to Jeff F
    Prodigy 200 points

    Hi:

    From your anwser, Cgs should be 40x or higher than Cgd. 

    And I want to know the specific process about the ACFET false turn on.

    Besides ,I want to know how to determine the value of multiple 40 also.

  • 0 in reply to user5779852
    TI__Guru**** 166006 points


    The CGD and CGS capacitors form a capacitive divider.  If they are too close in size, they could allow a fast input voltage spike to pass through and turn on the ACFETs, causing a false turn on. 

  • 0 in reply to Jeff F
    Prodigy 200 points

    As I understand it，the way the capacitors to soft start the FET turn on is that delay the time of ACDRV through the 4kohm + Cgs.  If so, why we still need the Cgd,which is too small to make an impact

  • 0 in reply to Jeff F
    Prodigy 200 points

    As I understand it，the way the capacitors to soft start the FET turn on is that delay the time of ACDRV through the 4kohm + Cgs.  If so, why we still need the Cgd,which is too small to make an impact

  • 0 in reply to user5779852
    TI__Genius 10240 points
    Hi,

    The Qgs of a FET has Qgs1 (Vgs from 0 to Vgs-th) and Qgs2. The delay from the driver signal to FET start to switch is mainly determined by Qgs1 and the resistor. The FET current and voltage transition is mainly determined by Qgs2 + Qgd.