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BQ76930: DEVICE_XREADY is set to 1 in the balance state

BobLi
Prodigy 30 points
Part Number: BQ76930

we're using 2 pcs bq76930 for a 20 cells battery management system. but we meet a problem when doing the cells balance.

first, in no balance state, the system working normal.  when the cells meet a balance conditions, the host mcu will send the commond of balance.  we do the balance at 2 cells Intervals, for example, the 2nd is doing blance on, than the 3rd, 4th is off, and 5th is on(if it's meet the balance voltage). 

then we start the cell balance, a couple of minutes later，the DEVICE_XREADY is set  to 1. and the mosfet shut off  automatically. 

if we stop the cell balance, and clear the DEVICE_XREADY, it will recover the normal state.

thank you for help.

  • 0
    TI__Guru 54715 points

    Hi Bob,

    A couple causes of XREADY are loss of power and noise on power.  Look at the stability of the power pins (VC5X, BAT) during balancing.  If there is significant movement correct the cause.  Balance testing requires low impedance sources such as cells.  Testing with resistor dividers can cause the voltage to collapse. Also look for crosstalk from the current paths to the power pins.  The input filter resistance is typically large, 1k, but the filter capacitors are also large (typically 10 uF) to stabilize the supply pins. 

  • 0 in reply to WM5295
    Prodigy 30 points

    we have rechecked the the circuit, using 1k and 10uf, it can't resolved.
    and we check the battery input voltage, it's smoothly. it seems than the wave on cap2 come from inside of bq76930.

  • 0 in reply to BobLi
    TI__Guru 54715 points

    Hi Bob,

    There is not much information to tell what is wrong.  Check the voltages into the circuit to be sure they are stable.  Cells will have a low impedance and will not generally move quickly when balanced.  Operating balancing with a resistive cell simulator can be unpredictable as the circuit input voltages will move significantly.  Inspect the VC5X and BAT voltages which should also be stable.    

    Since balancing is causing the problem, check the capacitor connections compared to the data sheet.  Be sure the VCn inputs are not pulling on the supply pins.  VC6 and VC5B are filtered to VC5X, so there will be some movement.  CAP2 should move with any slight ripple on VC5X.  If it moves differently something is wrong in the circuit or the part could be damaged.