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BQ24780S: Application of this part in order to monitor and limit battery discharge current.

Wes Stelter
Prodigy 10 points
Part Number: BQ24780S


We would like to understand how to:

1) Use PROCHOT_, IDCHG and ILIM in order to limit battery discharge current and associated behaviors in the limiting condition.

2) Can we use the BQ24780S as a battery discharge ammeter?

Thank you in advance. 

  • 0
    TI__Genius 11385 points

    Hi Wes,

    1. No, you cannot rely on BQ24780S to limit your battery discharge current. Most of the discharge limiting descriptions on the datasheet only applies when the device is in Hybrid Boost mode. On a typical BQ24780S design, battery current can always discharge through the body diode of the BATFET and there is nothing limiting it. You may, however, perform some current limiting using a separate battery protector/gauge/monitor.

    2. BQ24780S was designed to be a charger. A charger is a power delivery device. Recommend using a gauge/protector/monitor to monitor battery current.

    Thanks and I hope this helps,

    Peng

    *Please press "This Resolved My Issue" on the thread if my answer is satisfactory

  • 0
    Prodigy 40 points

    Thanks for replying.

    Our charger side is 19.5 +-1 V; Battery side can be as low as 12.5V.

    Do you think it would be concern for body diode with max 8V across voltage during charging stage?

  • 0 in reply to Evan Peng
    TI__Genius 11385 points

    Hi Evan,

    If you are talking about the body diode of the BATFET, then the answer is no concern if you are using a FET of high enough voltage rating.

    The body diode is similar to an actual diode with reverse blocking rating the same as the FET's voltage rating.

    Thanks and I hope this helps,

    Peng

    *Please press "This Resolved My Issue" on the thread if my answer is satisfactory

  • 0
    Prodigy 40 points

    Hi Pengzhao,

    For IDCHG pin on the IC. Can we get more clear understanding with the pin? If the charger doesn't limit the discharge current, what the purpose for IDHG pin there? 

  • 0 in reply to Evan Peng
    TI__Genius 11385 points

    Hi Evan,

    The battery discharge limit only applies when the device is in Hybrid Boost mode and that is through the ILIM pin, not IDCHG pin. For more info, you can see my colleague's response here:

    https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/764931/bq24780s-learn-function-with-discharge-current-regulation 

    IDCHG pin is an output pin for monitoring discharge limit. It is not an input to the charger.

    Thanks and I hope this helps,

    Peng

    *Please press "This Resolved My Issue" on the thread if my answer is satisfactory

  • 0 in reply to Pengzhao Zhu
    Prodigy 40 points

    Hi Pengzhao,

    Thanks for replying. Totally understand the IC charger doesn't support discharge limit under normal mode.

    What we want to know here is if it is possible to measure IDCHG pin to monitoring discharging current. Per pervious discussion, it sounds like it is not reliable under normal mode. Or I change my question in another way, what can IDCHG do in normal mode, even though normal mode doesn't support discharge current limit?

    Thanks

    Evan

  • 0 in reply to Evan Peng
    TI__Genius 11385 points

    Hi Evan,

    Yes, you can use IDCHG pin to monitor discharge current. Please see below from pin description section of the datasheet.



    You can use IDCHG in any condition. Be sure to set EN_LWPWR to "0".

    Thanks and I hope this helps,

    Peng

    *Please press "This Resolved My Issue" on the thread if my answer is satisfactory

  • 0 in reply to Pengzhao Zhu
    Prodigy 40 points

    Hi Peng,

    I follow your instruction and set up 0x12H to be 0x6109 (old value = 0xe109). Then measure IDCHG voltage across output cap, but the value = 0 mV when current is 1 A.

    Do you have any suggestion about what 's wrong with it?

    Thanks

    Evan

  • 0 in reply to Evan Peng
    TI__Genius 11385 points

    Hi Evan,

    You will need to set EN_IDCHG = '1' as well. 

    Thanks and I hope this helps,

    Peng

    *Please press "This Resolved My Issue" on the thread if my answer is satisfactory