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BQ25790: use battery to power system load and body-diode parameter

Yishan Chen
Intellectual 1740 points
Part Number: BQ25790

Tool/software:

Hi Team,

Customer have question about when the system is power by the battery.

  • Could BQ25790 support 'use battery to power system' scenario? 
  • Could we control the BATFET to turn on when we need battery to power the system?
  • If we can't control BATFET in this scenario, could you please provide the specification of body diode for customer to judge how much current can draw by the system load?

Thank you,

Yishan Chen

  • 0
    TI__Guru**** 167096 points

    Hi Yishan,

    Regarding 1, yes.

    Regarding 2, no.

    Regarding 3, the body diode is rated the same as the FET's discharging current per below:

    Not that the FET is turned on for discharge on as long as V(BATP) > 2.4V-2.6V if no shipFET or 3.2V-3.4V with shipFET. 

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 1740 points

    Hi Jeff,

    thank you for the answer.

    For question 2, If we enable CE without the Vbus presents. Could we keep the BATFET on? What is the logic chain here?

    Thank you,

    Yishan Chen

  • 0 in reply to Yishan Chen
    TI__Guru**** 167096 points

    Hi Yishan,

    If charge is enabled with no battery and battery capacitance < 200uF, the charger sees BAT and SYS OVP.  The OVP causes the charger to turn off the converter for 30ms which results in SYS collapse.  With charge disabled, only current flow from to SYS to BAT is turned off.  Current from BAT to SYS is always available.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 1740 points

    Hi Jeff,

    Thank you for the response.

    Could you please explain the logic chain of the "1.5s"action you have mention in another E2E post:

    When charging is enabled, after the battery voltage falls below BATUVLO (~2.5), the charger attempts to regulate the BATP pin to a fixed voltage (5V for 2S battery) for ~1.5 s to allow the pack protector FETs time to close.  After that time, the charger resumes normal charging.  In your plots, because there is no battery and only the minimum capacitance at BAT, the battery voltage overshoots after the 1.5 s, causing SYS to overshoot, hitting both OVP thesholds and turning off the BATFET and the converter.  If your customer cannot add the additional capacitance to BAT, please recommend that they use the /CE pin to disable charge when no battery is attached.

    Is the action chain is like:

    Enable charger->no battery-> BAT UVLO-> Bring the BAT to 5V in 1.5s-> normal charging status-> BAT OVP-> System OVP-> turn off the converter for 30mS

    Thank you,

    Yishan Chen

  • 0 in reply to Yishan Chen
    TI__Guru**** 167096 points

    HI Yishan,

    If V(BAT)<BATUVLO~=2.5V:

    Reduced CV loop for BATP=BAT = #cells x 2.5V for 1.5s with 100mA current clamp

    Change to CC loop with 100mA trickle, pre-charge then fast charge until BATP = VREG

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 1740 points

    Hi Jeff,

    Thank you for the response.

    One more question is, if battery status is normal. How many seconds does it takes to let charger go in to normal charging status.

    Thank you

    Yishan Chen

  • +1 in reply to Yishan Chen
    TI__Guru**** 167096 points

    Hi Yishan,

    Most of the faults (e.g. TS fault) have a ~30ms deglitch before charge restarts after fault removal.  Auto recharge following termination has adjustable deglitch from 64ms to 2s.  

    Regards,

    Jeff