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BQ76930: questions about charge and discharge control circuit

FranMartin
Intellectual 600 points
Part Number: BQ76930
Other Parts Discussed in Thread: TIDA-00449,

Hello,
in the document '10s Battery Pack Monitoring, Balancing, and Comprehensive Protection, 50-A Discharge Reference Design' on the IT design 'TIDA-00449', on page 26, 'Figure 20. Sense Resistor and Protection FETs Schematic' shown the charge and discharge FETs as well as other components (I have added the schematic).


For a design with a requirement of a lower charge and discharge current, in which only two FETs were needed (one for the charge and another for the discharge). That is, removing Q15 and Q16:

1. Could L1, L2, L3 and L4 be eliminated? I understand that its function is to avoid a possible oscillation by having transistors in parallel, is it correct?

2.Could you explain why Q12, D14 and D17 are necessary?

3. Why Q12, D14 and D17 are not necessary to control the discharge?

Thank you.

Best regards,

Fran Martin

  • 0
    TI__Guru 54715 points
    Hi FranMartin,
    1. Yes, the need for the ferrites or small individual gate resistors depends on the FET design and board layout. With a single FET for charge or discharge they should not be needed.
    2. See the bq76930 data sheet figure 7-3
    3. For the discharge FET the source remains at approximately GND and the gate can safely swing from 12V to GND to control the FET. Discharge currents are high and the diode would not allow successful switching of the DSG FET. Control of the charge FET is different since when the discharge FET (Q13) is off the charge FET's (Q14) source and gate can be at PACK+, far above the driver. When Q14 is off the source can be pulled below GND and Q12 allows gate to follow so Q14 FET remains off.