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BQ77910 in two or three terminal application

Simone Freato
Prodigy 170 points

Other Parts Discussed in Thread: BQ77910A

Dear Sirs,

I'm developing a 10 cells battery pack with BQ77910. In the process a couple of questions came to my mind.

  1. I have to chose BQ77910 or BQ77910A but i can't see the differences beetween the two ICs. The datasheets are almost equal. I wonder if I'm missing something.
  2. I must use the series fet configuration. I have read SLUA612, but I can't understand how, if i put a flywheel diode like D11 in the application note, a current can flow into the IC and charge the filter capacitors on BAT pin
  3. I can't understand if CPCKN wakeup from shutdown is really working or not. If it is not is it still possible a two terminal configuration for a battery pack?
  4. In page 7, note 1 of the BQ77910A datasheet is explained that with series fet configuration the shutdown current is not predictible unless you use an external circuit to tie CPCKN to VSS. Can you give me the raccomended circuit?

Thanks for any help.

  • 0
    TI__Guru 54715 points

    1. The bq77910A prevents setting the RSVD1 bit (register 1 bit 1) which must not be set in the bq77910 as noted in the datasheet .

    2. The cells and their interconnections will have inductance which will cause the PACK+ voltage to rise when current flow stops. The flywheel diode will keep PACK- close to PACK+, but PACK+ will rise with respect to VSS of the part, so currents can flow as indicated in the application note.

    3. CPCKN does not wake up the part. It can temporarily force VREG on as described in the datasheet page 27. Some accessory circuit is needed to create a 2 terminal pack, circuits can vary greatly and are likely to have some side effect or reduce the features available. Options might include reset switches, magnetic switches actuated when in your charger or as complex as a microcontroller or timer. A simple solution may be to connect CHGST with a resistor divider to BAT so that it is always high. One circuit we have suggested is 1122.latched2wire.pdf which operates the part as if it were in the charger under all normal conditions. Since it will go to under voltage in the charger, it does not shut down from under voltage.

    4. Some circuit to keep CPCKN near VSS is needed. One solution is to tie CPCKN to VSS and use an external driver for the charge FET, that can be simple or complex.  A circuit which may be useful is 5226.bq77908A_sd_current.pdf. This keeps CPCKN a diode above the lower of PACK- or VSS to reduce the shutdown current.  The charge FET is still controlled by the bq77910A CHG output in both directions.  When CHG is high it drives through the T3 body diode.  When CHG is low T3 is on and CHG drives the power FET off. T4 must come on fast enough when PACK- rises to turn of T3 prevent driving current back into CHG.  With high impedance circuits you will need to optimize performance vs leakage and current drain.

  • 0 in reply to WM5295
    Prodigy 170 points

    Thanks a lot for your fast reply. Now I'm starting to understand a little more.

    1. Ok. So I can use BQ77910A or BQ77910 quite indifferently as long I don't make a wrong setting
    2. I was focusing in the wrong side of the switches. If the possible problem comes from the battery side inductance I feel quite reassured since it will be a very low inductance.
    3. Given that i think i will use an auxiliary contact to sense the charger presence. In my case it's surely easyer
    4. And if I don't tie CPCKN to VSS? What could happen? Couldn't I simply put a large value resistor (10M or such) from VSS to PACK- ?

    Thanks again for your help.

  • 0 in reply to Simone Freato
    TI__Guru 54715 points

    1. Yes

    2. Low inductance cells will make your design easier. Do test early and thoroughly. Short circuit may produce largest transients due to the high dI/dt.

    3. If you have the design flexibility for a charger presence contact this should allow the best range of features.

    4. If the FETs turn off leaving PACK- and CPCKN floating, CPCKN may rise to the 1 or 2 V range and the supply current to the 910A may increase from its normal operating current, even if shut down.  The exact values will vary part to part and are not characterized.  If shutdown current is a concern, some special circuit may be needed.  The datasheet suggests RLDRM_DET of 50K which will act through RDPCKN and the charge FET body diode to pull down the PACK-.  Even with the resistor, the CPCKN voltage and supply current can rise some.