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BQ24618 system power switch from battery to adapter undershoot issue

jun Wang70816
Prodigy 50 points

the 3.7V battery is running at full load with 2A current/ 3.4V,  while the 5V adapter plug in, the system output have undershoot, the voltage down to 2.8V, then go up to 5V.  from AC to battery, the system output doesn't have any undershoot issue. However, after some adjustment for  gate to source  of the ACFET , the battery to AC undershoot problem is almost solved, but, AC to battery have some undershoot under 3V. since our  system DC/DC works  above 3V, this undershoot cause system unstable.

Is there any way to solve the system power auto selection undershoot issue?

thanks ,

Jun

  • 0
    TI__Mastermind 31120 points

    Please let me know if my understanding is correct:

    1) Had issues with Vsys dropping below 3V when AC is applied (was running on battery but when switching to AC power the droop occurs).

    2) Adjusted AC FET drive to try and solve drop problem and almost got solved but now the transition from AC adapter power (unplugged) to Battery power has droop and cause system shutdown.

    *  First if the transition time is fairly quickly and you still have issues, then more system capacitance may be the answer.

    *  Turning the battery FET on slow and off fast helps eliminate shoot through current from the adapter to the battery but requires conduction through the BAT FET diode.  This works well if you have a full battery but as the battery discharges this large body diode voltage drop will cause the VSYS to droop.

    * One technique to use if you need a fast turn on and a slow turn off or the other way around is to use a diode in parallel with the RC on the gate to allow a lower impedance in one direction and not the other.

    To help you futher I will need your current schematic and some waveforms of the timing, Vin, Vsys, Vbat, BATDRV and ACDRIVE.

     

  • 0 in reply to Charles Mauney
    Prodigy 50 points

    yes, 1, and 2 are absolutely right.

    and also, we add a 330uF cap in the load, no help.

    Please provide the email address so I can send the sch and waveforms.

    thanks,

  • 0 in reply to jun Wang70816
    TI__Mastermind 31120 points

    I need to see the BATDRV signal in relation to Vsystem for the scope_54 plot.  Probably need to reduce C42 and or R38.  Another thing to try is put a schottky diode across R38 so it conducts when BATDRV goes low (turning on BATFET) and when BATDRV goes high, the diode does not change the turnoff time constant.

     In scope shot 55, is it a problem when the AC is removed that the system voltage spices to 3V for a very short time (<100th of a division)?

  • 0 in reply to Charles Mauney
    Prodigy 50 points

    add diode, increase the value of  Resistor and reduce the value of CAP, this solve the undershoot issue.

    thanks,