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BQ25570: Connection between VSTOR and VBAT to prevent cold-start delay

Petr Ledvina
Intellectual 480 points
Part Number: BQ25570

Hello,

I have design with BQ25570, using 4V solar panel and 2Ah LiPoly battery (with built in protection circuit).

I found problem when some instability caused charger to enter cold-start mode with almost full battery - it seems that VStor gets disconnected from battery and mostly discharged into VOut, then waiting until CStor (4.7uF) is charged above cold-start threshold. This happens quite infrequently (few hours), and happens only when there is some voltage on VIN (doesn't happen at night or in full sunlight) and is almost impossible to trigger intentionally. It seems that adding another 10nF capacitor to VStor (in parallel to 100nF and 4.7uF ) solves / reduces this problem.

It is possible that problem may be caused/exacerbated by small current flowing into VOUT (there is 3V UART interface to LTE modem connected to battery, modem 3V is about 0.1V higher than VCC and some current is flowing through 1k resistor and RX pin pullup). This seems to be fixed by disabling pullup, but I may install antiparallel diodes into UART lines to increase headroom.

The device can survive occasional loss of VCC, but from field deployment it seems that when this happens at dusk, BQ25570 won't recover till morning. I'd like to handle this situation.

Is is safe to connect VBAT to VSTOR using diode / diode + resistor / zener diode, so that VSTOR (and VBATOK) is restored even without VIN voltage?

What solution is preferred?

Thanks in advance,

Petr Ledvina

  • 0
    TI__Mastermind 20570 points

    Hello Petr,

    Typically we do not recommend to directly tie VBAT to VSTOR as the build in protections such as battery under-voltage will not work. 

  • 0
    TI__Guru**** 165786 points

    Hi Petr,

    There is a known high frequency noise issue on VSTOR which we thought the 100nF capacitor resolved.  It appears that the issue occurs at even higher frequency if the 10nF capacitor fixes it. 

    If you place diode the allows current flow from VBAT to VSTOR, the charger's undervoltage protection feature will be disabled.  This may be okay if the battery itself has undervoltage protection.

    Regards,

    Jeff   

  • 0 in reply to Jeff F
    Intellectual 480 points

    Hi Jeff,

    My PCB design isn't perfect, the 100nF is about 2mm from package but ground side is connected through multiple vias. I'll try to fix it in next version. Also disabling RX pin pullup (and thus avoiding current flowing into VOUT) seems to help.

    The 10nf is soldered between ground plane near pin 1 and C14.2, so it's almost as close as it can get (in 0603 size). 

    The important information for me is that it is safe to supply VSTOR externally, I was afraid that it may cause some other problem with BQ25570 algorithm ... 

    Regards,

    Petr 

  • 0 in reply to Petr Ledvina
    TI__Guru**** 165786 points

    Petr,

    We did find that the placement of that 100nF was critical.  It need to be as close as possible between VSTOR and VSS pins.

    Regards,

    Jeff