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High quiescent currents on BQ77910A series FET topology

Dear all,

after implementing a BQ77910 circuit and going through all the chalenges in using the series topology, I discovered exteremly high quiescent currents.

my schematic is attached.

clearly I can see 2 areas to take care of:

  1. DSG pin with a voltage devider of 200K and 200K (R41 & R48). this discharges ~17.5 us and I can maybe live with it.
  2. CHGST pin has a terrible voltage divider of 15K and 1K (R34 & R33) which kills the batteries... it is ~2mA.

do you see any more points of leekage?

my solution is to increase the resistors of CHGST to 750K and 50K.
do you think it is too noisy for this pin?

can you think of a more steady power source to feed CHGST more than 0.9V it needs, other than P+?
do you think maybe connecting Vbat pin (pin 31) directly to CHGST or  through a resistor will work?

any suggestion will be welcomed.

best regards,

Schematic_B5.pdf
  • Those 2 are the appartent paths.  You could measure  voltage across resistors to look for other currents.

    For 1, Consider why R41 and R48 are these values.  Your FET will accept the maximuml gate drive voltage of DSG, so the voltage divider effect does not seem critical. The 100k parallel combination will discharge your FET gate during turn off.  This seems very large, but you worked with this before to find a workable solution. You might consider if you can get your desired performance with R41 as 100k and R48 as ~ 5M.

    For 2, Note from the datasheet the part will have best functionality when CHGST is switched by a charger.  The 1k pull down resistor on the EVM keeps CHGST low with dynamic currents, and the 15k series input resistor keeps the resistors from being damaged when the external CHGCTL terminal is connected to PACK+.  When keeping CHGST always high as in your design, you can change this circuit.  A higher resistance voltage divider as you propose is certainly suitable. You might use a zener at D4, remove R33 and adjust R34 for a suitable current.  While the datasheet does not show an input current, you should likely plan for 1-2 uA input current. and a few uA for bias of the zener over the range of your battery voltage.

    If topology changes are possible the datasheet allows CHGST to go to the BAT pin voltage, so a resistor between the 2 pins could work.

  • Dear WM5295,

    thanks for your rapid answers!

    1. the reason I used the 200K resistors is because of the high L*dI/dt, my system has.
      I had to reduce the voltage from the DSG pin.
    2. I think I will use the resistor divider, easier to change.

    thanks for your help