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BQ76952: Driving capabilities of BQ76952

Star Xu
Genius 9645 points
Part Number: BQ76952

Hi

The customer design with BQ76952, when the battery rated voltage is 12V (4 cells) and 24V (8 cells), the charge and discharge drive level is only 10.35V, and the measured voltage on the bootstrap capacitor is 10.6V.

The bootstrap capacitor is 470nF, the diode D19 is a Schottky diode, and the measured voltage drop is 0.17V.

When the battery rated voltage is 48V (15 cells), the voltage on the bootstrap capacitor is 11.1V, and the charge and discharge drive level can reach 10.8V.

The queries as below:

1, What is the nominal charge and discharge drive current, and the drive ability?

2, Why is the bootstrap voltage of the 48V platform relatively high at 11.1V, while the driving voltages of 12V and 48V are relatively low at 10.6V? Is this a characteristic of the chip itself or is it related to the parameter design of the driving circuit?

3, On the 12V and 24V platforms, try adjusting the bootstrap capacitor to 1uF and 1.5uF. The level on the bootstrap capacitor remains unchanged at 10.6V. How can I adjust the parameters or adjust the circuit to increase the driving voltage?

Please help check it.
Thanks
Star

  • 0
    TI__Mastermind 34610 points

    Hello Star,

    Star Xu said:
    1, What is the nominal charge and discharge drive current, and the drive ability?

    Any load can cause the voltage to drop a little bit, that is normal. However, the FET drivers can provide around ~40-uA of DC current during normal operation to maintain a driving voltage of ~10-V. The higher the load on the FET drivers/charge-pump, the lower the voltage will be. I'd recommend to change the gate-source resistors from 5.1-MOhm to 10-MOhm.

    Now, during turn-off, the FET driver can sink ~70-mA initially. But its behavior is non-linear, so it is not constant during the whole turn-off/turn-on process.

    Star Xu said:
    2, Why is the bootstrap voltage of the 48V platform relatively high at 11.1V, while the driving voltages of 12V and 48V are relatively low at 10.6V? Is this a characteristic of the chip itself or is it related to the parameter design of the driving circuit?

    Most of the time the charge-pump voltage depends on the load of the charge-pump. Most of the load would be through the gate-source resistors of the FET circuitry. Now, the specs show that there can be variance on the FET driving voltage:

    Star Xu said:
    3, On the 12V and 24V platforms, try adjusting the bootstrap capacitor to 1uF and 1.5uF. The level on the bootstrap capacitor remains unchanged at 10.6V. How can I adjust the parameters or adjust the circuit to increase the driving voltage?

    Increasing the capacitor value would not change the driving voltage. The driving voltage is not something that can be increased. As I mentioned however, increasing the MOhm resistance between the gate-source of the FETs, will decrease the load current of the charge-pump, which may increase its voltage. 

    Something else to note, is that the DMM/Oscilloscope itself will add some load to the charge-pump.

    Best Regards,

    Luis Hernandez Salomon