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BQ25790: VSYS drop by instantaneously large current drawing

Hideki Hayashi1
Expert 8380 points
Part Number: BQ25790

Hi team,

When a system load draw a large current momentarily, SYS pin voltage gets low. When the voltage is under VSYS_MIN, it will be regulated to VSYS_MIN. 

However how fast does BQ25790 reacts to the SYS voltage drop? customer didn't test it with BQ25790, but in current solution (competitor charger),  a DCDC to different load is caused to UVLO. So they want to know if it happens or not in advance.

Best regards,

Hayashi

  • 0
    TI__Mastermind 30705 points

    Hello Hayashi,

    It's currently the holidays in the US. We will get back to you in a week.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Expert 8380 points

    Hi Ethan,

    Happy new year. Is there any update?

    Regards,

    Hayashi

  • 0 in reply to Hideki Hayashi1
    TI__Guru**** 165886 points

    Hi Hayashi,

    The internal BATFET's body diode always clamps SYS no lower than a diode drop below battery voltage.  If the transient load results in IINDPM (input current clamped) or VINDPM (input voltage clamped) because the SYS load requires more power than the input can provide, the V(SYS) drops to V(BAT) as shown below:

    The transient response on SYS when above MINSYS is shown below:

    So, do you need the transient response when the charger is in MINSYS (battery below MINSYS) but does not enter DPM?  If you can provide test condition (VBUS, VBAT, MINSYS, ISYS, IINDPM, VINDPM) I can produce a transient plot.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Expert 8380 points

    Hi Jeff,

    I would like you to simulate a condition of VBAT<VSYSMIN as below

    VBUS = 5.0V, VBAT = 6.5V, VSYSMIN = 7.2V (2 cell system), ISYS = 0.6A -> 3A in 1ms, IINDPM = 1.5A, VINDPM = 4.0V

    Best regards,

    Hayashi

  • 0 in reply to Hideki Hayashi1
    TI__Guru**** 165886 points

    Hi Hayashi,

    Below are the transient plots.  I didn't know the charge current setting so I chose 1A.  dark blue Ch1 = V(BAT), pink Ch3=V(SYS), light blue Ch2 = IBAT or IBUS and green Ch4 = ISYS.  The rise time is a bit faster than 1ms.

    Charge enabled:

    charge disabled:

    There is a delay before IINDPM (input current) loop hands off the SYSMIN and charge current regulation loops.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Expert 8380 points

    Hi Jeff,

    I understand BQ25790 BGATE response is good to minimize VSYS drop. Customer tries in actual evaluation. When customer wants to add protection on the VSYS drop, what solution can be considered?

    Best regards,

    Hayashi

  • 0 in reply to Hideki Hayashi1
    TI__Guru**** 165886 points

    Hi Hayashi,

    I recommended either adding more low ESR capacitance on SYS, to hold up SYS longer. 

    Or adding an external Schottky diode in parallel with internal BATFET body diode from BAT to SYS to reduce drop across the body diode until the BATFET turns fully on.

    Or both.  

    Regards,

    Jeff