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BQ24070 charge termination with Timer disabled

Paul Slater
Prodigy 150 points
Other Parts Discussed in Thread: BQ24070

Hi, I am looking to use the BQ24070 to run a GSM application from VOUT and also charge a 10.2Ah battery @ 1A as a backup supply.  I am using the TI eval board for investigation.

If charging a flat battery, the maximum charge time of ~10hours may not be sufficient for full termination, so I disabled the timer by connecting TMR to VREF via 1k resistor.  The datasheet (page 7, note 9) states that connecting TMR to VREF will disable the safety timer and charge termination.  I presume that charge termination still occurs when the charge current has reduced to ISET/10, but it is the indication of charge termination that no longer works.  Please confirm.

My battery appears to charge OK, but I do not see charge termination indicated by STAT1 and STAT2 change of state. 

Thanks.

  • 0
    TI__Genius 11480 points

    Hi Slater,

    I am pretty sure that disabling the termination will allow the charge current to reduce indefinitely. You can check this by placing a mostly charged battery and watching for charge current to reduce below Ichg/10 on the EVM. If I can find time in lab  today, I will check and get back to you.

  • 0 in reply to David Wiest Jr
    Prodigy 150 points

    Hi David,

    Thanks for the quick reply.  I've watched the current on my PSU drop below the ISET/10 (100mA), down to around 60mA, so it looks as if it may continue down to zero eventually.

    I'm curious as to why the IC doesn't terminate charge (and indicate that it has done) when it sees that the current has reduced below the ISET/10 figure.  Why is termination only linked to having the timer enabled?

    With termination disabled, does charging still restart based on the V(RCH) threshold?

    Thanks

  • 0 in reply to Paul Slater
    TI__Genius 11480 points
    Slater,


    In normal operation, the charger will terminate at C/10. With termination disabled, the current will droop down to 0. In this mode, the charger will let the battery say it is "full", as full as it can be.


    It is a pretty common feature in linear chargers for Termination and Timer to be disabled together (You can look at the TTDM function of many of the linear chargers). Typically, this has been done to prevent chargers from disabling from a TS pin fault when a battery/thermistor has been removed. Unfortunately, I was not around when this device was defined, so I can't give you a solid answer as to why it is included here.


    Charging should still restart based on the V(RCH) threshold - I am pretty sure that is part of a different control loop in the device.
  • 0 in reply to David Wiest Jr
    Prodigy 150 points
    Thanks for the clarification.