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Original question:

BQ77PL900 False UV Detection

BQ77PL900: BQ77PL900

Shailesh Kumar
Prodigy 40 points
Part Number: BQ77PL900

We have designed a Cell balancing system with the reference of Evaluation board of BQ77PL900. I have programmed Bq77pl900 chip over voltage at 4.15  with 200mv hysteresis and under voltage at  2.9 with 400mv hysteresis. I have also enabled Vout and reading it through a host controller. Vout is coming to be around 34.8 Volts also i have physically verified voltage at each cell . All are at around 3.5 v still on reading Status register of BQ77pl900 is returning me 0x0C which indicates that Over Voltage and Under Voltage both conditions is being generated by the chip as per the datasheet SLUS844B–JUNE 2008–REVISED JANUARY 2009

I am not able to get out of this condition even in host control mode using the recovery methods stated in Datasheet

My Questions are

1. Is it possible for BQ77PL900 to show both Over Voltage and Under Voltage Conditions at the same time?

2. What possible faults in the system can causing this situation?

3.How can we make BQ77pl900 to get out of this condition ?

  • 0
    TI__Guru 54715 points
    Hi Shailesh,
    1. Yes it is possible for simultaneous OV and UV.
    2. Possible causes of OV and UV include:
    Input pin not soldered
    Solder bridge between input pins
    Short at differential input filter capacitor
    Input filter resistor missing/not soldered
    Damage to inputs causing leakage into the IC and improper voltage.
    3. Check the voltage across each input filter resistor to check for unusual current flow. The voltage across each resistor should be very small. Voltage across 1 resistor indicates a flow of current to VSS. Voltage across 1 with the similar value and opposite polarity across the next indicates a differential short of the filter capacitor or input pins.
    You might also check the voltage at each pin on the IC rather than the board, it can show if a pin is not soldered if you don't push so hard that probing makes the connection.
    You might also check the configuration setting, although an improper connection will typically cause only an UV from the device checking the shorted input.