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BQ77905: bq7790503 stacked for 20S, not working

MI
Intellectual 560 points
Part Number: BQ77905

1. I have used Bq7790503 in stacked configuration for 20S application. Cell simulator is twenty 1ohm,10watt resistors in series.
3. Problem is in any voltage range of 54V-84V, voltage at DSG & CHG pin of lower most bq7790503 is always zero & main fets are always off thereby.
4. I have noted voltage for CTRD & CTRC for each also:
a) Lowest bq7790503 CTRD & CTRC voltage wrt to AVSS1 is 7.28V each.
b) second from below bq7790503 CTRD & CTRC voltage wrt to AVSS2 is 7.28V each.
c) third from below bq7790503 CTRD is 7.28V & CTRC voltage is 13.3V wrt to AVSS3.

5. I am not using LD feature, so I have permanently shorted lowest bq7790503 LD pin to gnd via bq7790503.


What could be the problem.

  • 0
    TI__Guru 54715 points
    I don't recognize a problem, but see you have more information in another post, will look at that.
  • 0 in reply to WM5295
    TI__Guru 54715 points
    Hi Deepak,
    Again the schematic looks normal. CTRC or CTRD at 7.28V or 13.3V is in the VCTR(DIS) range for a 54 - 84V battery with evenly distributed supply voltages. One cause could be an open wire on the top part. An open thermistor would also simultaneously set UTC and UTD.
  • 0 in reply to WM5295
    Intellectual 940 points
    I also have this problem,the lower device CHG PIN voltage is zero,What is the cause of the impact chg driver?
  • 0 in reply to user44411751
    TI__Guru 54715 points
    The CHG pin may be low due to various faults. The bq77905 data sheet figure 15 shows the driver off conditions. In a stacked implementation, checking the CTRC condition first may be helpful.
    When checking CTRC input level it can be helpful to measure CTRC with respect to the VDD of the lower IC since this is how the part determines its level. Even so, since the recommended RCTRC is 10M and many meters are 10M, be cautious of loading by the meter. The pin voltage may change due to the measurement, use a high impedance meter if necessary.
    Remember that if a fault has occurred, the recovery condition must be met for CHG to turn on again. See the fault descriptions, the summary of faults in table 3, and configuration details in table 2.
    Of course on a new board be sure to check that the pin and net are free of shorts and have not been overstressed during evaluation.
  • 0 in reply to WM5295
    Intellectual 940 points
    I have anther question,the current accuracy of BQ77905 is 20% in large current,30% in small current,the end customer think is too low.How to improve the current accuracy?I want to know how much is the temperature accuracy?
  • 0 in reply to user44411751
    TI__Guru 54715 points
    The current accuracy is set at production and cannot be adjusted by the end customer. It is expressed in percent, a difference/threshold. As the threshold gets small a small difference can become a large percentage. While typical accuracy will be within a few percent it will vary from part to part and with temperature and can statistically reach the datasheet limits.
    Accuracy of the temperature measurement is expressed as a ratio, actual temperature accuracy will also depend on the thermistor and external resistor used, those must be considered in a temperature accuracy calculation. The temperature threshold accuracy was tightened in a recent revision, be sure you are using the latest bq77905 data sheet.