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BQ24780S: BQ24781 4-Cell 4.35V issue

Big Bill
Expert 1980 points
Part Number: BQ24780S
Other Parts Discussed in Thread: BQ24781,

Dear All:

we use the BQ24781 to design 4-cell charger system for Gaming application. the single cell voltage is 4.35v, so the battery pack is 17.4v

issue is as below.
 
remove the adapter  the voltage in input PIN is slowly falling until the input Pin voltage is equal to the battery voltage.
the battery voltage will pour back to the input Pin, the system think the adapter is not removed. so  we adjust the ACDET partial resistance

that    the lowest voltage is match 18.1v. but we use the 19.5V adapter, the output voltage allow 10% drop under overload time that it continues 10us,

that means the output voltage can drop 17.55V in this time。 it can't meet the lowest voltage  request.

can you  give the good advice  to solve this problem

by the way,  how much   the ACOK falling deglitch time  when  the ACDET detect  falling threshold voltage? I can't find the information in the datasheet. thanks！

  • 0
    Expert 1980 points
    Dear All:

    Help update! thanks
    ~
  • 0
    TI__Guru 56650 points

    Bill,

    If the input voltage only drops 10us, we can add a filter on ACDET pin to filter out this 10us glitch and keep ACDET setting at 18.1V.

  • 0 in reply to Wang5577
    Expert 1980 points
    Dear Wang li:

    if the input voltage drops from 2 to 300 us， do you have good advice to solve this problem？ thanks for your help~
  • 0 in reply to Big Bill
    TI__Guru 56650 points
    Bill,

    If we don't want to add too much ACDET capacitor for input filter, we can set a lower ACDET threshold (ACDET>2.4V with a little bit lower input; even a little bit lower than the battery full charge voltage). I would think the charger still works because:
    a. BQ24780S has an inductor current detection to prevent the battery reverse current.
    b. BQ24780S has a sleep comparator. Even the input voltage is low than the battery voltage, it should not give a big trouble.

    If we want ACOK is high until input voltage is higher than the battery full charge voltage with a low ACDET threshold setting. I would think we can use ACOK signal to add an extra hysteresis on ACDET pin.
  • 0 in reply to Wang5577
    Expert 1980 points
    Dear Wang Li:

    Thanks for your reply, may i pull down the ACOK pin by this condition that VCC-Vsrn< 170mv ？

    because the datasheet said pull up the ACOK pin as below condition VCC – VSRN > VVCC_SRN_FALL + VVCC_SRN_HYST

    and other question when into the Turbo boost mode ，the battery can boost to the vin. in theory the Vin voltage is 19V，

    in fact the VIN voltage is 17.8V， why is the Vin voltage different from the adapter output voltage ？
  • 0 in reply to Big Bill
    TI__Guru 56650 points

    Bill,

    You don't need pull down the ACOK when VCC-Vsrn<-20mV. The charger will automatically pull down ACOK if the VCC voltage is below V(vcc-srn_Fall) threshold.

    VCC to SRN COMPARATOR (VCC_SRN) Spec:

  • 0 in reply to Wang5577
    Expert 1980 points

    Dear Wangli:

       You are right , Thanks for your  advice.

       other question when into the Turbo boost mode ，the battery can boost to the vin. in theory the Vin voltage is 19V，

         in fact the VIN voltage is 17.8V， why is the Vin voltage different from the adapter output voltage  19V ？

       

  • 0 in reply to Big Bill
    TI__Guru 56650 points

    Bill,

    Do you mean VCC pin is 17.8V? Where do you measure the 19V?

  • 0 in reply to Wang5577
    Expert 1980 points

    Dear  Wangli :

         the adapter output voltage is 19v,   when into the  boost mode  we monitor the vin voltage is 17.8V. 

  • 0 in reply to Big Bill
    TI__Guru 56650 points
    Bill,

    This is the question I want to ask you: where do you measure the 19V? where do you measure the 17.8V? On which component? Or, on which cable?
    If adapter output is 19V and the charger input is 17.8V, that means your input cable take too much voltage drop.