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BQ76930: BQ76930

Part Number: BQ76930

Dear All:

   I  use the BQ76930 to monitor the 20S  cell  voltage . I use the BQ76930 ADC to show each cell voltage as below.

    

  I find the cell5  cell10 cell15 cell20 voltage is not correct. i use the oscilloscope to monitor  the abnormal cell  voltage  i find  the abnormal voltage drop 200mv  every. 2S as below 

 

the schematic as below 

 

Can you  tell me what cause the error result ?How can i modify the circuit to solve the problem? Thanks~

  • Hi Bill,

    You are seeing the thermistor load every 2 s.  Normally the TSn will draw current from the group power supply pin (VC5X, BAT)  when sampled causing a small drop in the power pin voltage.  For example at room temp, current is 3.3V/10k = 330 uA.  the 330 uA through the 1k power filter resistor is 330 mV.  The cell voltage should not change since it has separate filter resistor.  The VC5 to VC5X diode should not be forward biased enough to conduct.  As the temperature gets hotter the current increases and the diode may begin to conduct, you will see some effect in the cell voltage.  When the upper group samples you will see a rise in the VC5X voltage from current into the node from the upper thermistor, the 2 pulses will drift over time.

    When using a resistor divider, the power supply current will pull on the simulated cells and many cell voltages should change.

    In the schematic view above, CELL15 would normally be the top cell in the group, R108 and C56 would be the filter components for VC5 of the BQ76930.  A diode would connect from VC5 to VC5X.  C53 and C54 would presumably be the filter caps for VC5B and VC6, but these should connect to VC5X rather than VC5.  It is not clear if the capacitor connection would cause the variation observed, it seems more capacitance may help stabilize the VC5 node.  However if the power filter resistor is missing or improperly connected it may have a significant effect.