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BQ76940: BQ76940: connection of +VBAT

Devayani Bodar
Intellectual 255 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ78350, BQ76200

Dear Team,

Please clarify that, what is use of series resistor in parallel with BQ76940, i.e. in series of pin-24 to pin-20 connection.

1. Why 1K resistor R106 is used?

2. Can we increase R106 resistor value to 100K or more for low power application.

3. Can we remove resistor R106 to make more power control during sleep mode?

Please refer attached photo.

Thanks & Regards,

Devayani Bodar

  • 0
    TI__Guru 54715 points

    Hi Devayani,

    R106 is the power filter resistor for the BAT pin.  The VC15 should also have a 1k typical input filter resistor.  The diode D1 is to limit the voltage of VC15 above the BAT pin.

    1. R106 is the power  filter resistor for the part's BAT pin.

    2. No, the part will require the same amount of power, the voltage drop across 100k will be large causing current to be drawn through D1 and altering the reported voltage of cell 15.

    3. No, R106 is required.  Without R106 supply current for BAT will be drawn through D1 and the input filter resistor causing a voltage drop and improper measurement on cell 15. 

  • 0 in reply to WM5295
    Intellectual 255 points

    Hi WM5295,

    Thanks for clarifications.

    Please clarify below questions;

    1. What is maximum resistor we can select fro +VBAT parallel resistor to diode?

    2. VC15 Cell voltage monitoring we are connecting directly from +VBAT  through 1K series resistor but my question is internal power supply section DC-DC regulators of BQ has separate power input pin i.e. VBAT pin. number 20, i am still not clear that if measurement of cell voltage and power pins are separate how it will effect performance of BQ reading cell voltage for cell 15.

    Let me know your view for the same.

    Thanks,

    Devayani bodar

  • 0 in reply to Devayani Bodar
    TI__Guru 54715 points

    Hi Devayani,

    1. The resistors are not really parallel with the diode.  Rf to BAT and Rc to VC15 for the BQ76940 both come from the top cell and have a maximum recommended value of 1k in the Recommended Operating Conditions section of the data sheet, we commonly suggest 1k as the value.

    2. There are several power pins on the BQ76940, see the pin map in the data sheet.  

    BAT provides power for the voltage measurement sections including the thermistor, and registers in the part.  

    VC5X and VC10X are also power pins but have only offset currents, see the Leakage and Offset Currents section of the Electrical Characteristics section of the data sheet.

    REGOUT is available for use externally and also powers the I/O circuit for the MCU interface and current measurement circuit. REGSRC is the current input for the REGOUT and FET driver outputs.

    VC15, VC10 and VC5 are sense inputs, but have extra bias current compared to other sense inputs, see the dICELL specification.

  • 0 in reply to WM5295
    Intellectual 255 points

    Dear Team,

    I agree with you but in datasheet nowhere it is mentioned that series resistor is required, even in application not also not mentioned anywhere.

    Refer application schematic diagram in datasheet

    Figure 15. bq76940 with bq78350 Companion Controller IC and bq76200 High-Side N-Channel FET Driver

    Only in evaluation board only it is there. 

    due to this we can not implement effective sleep current requirements in end application.

    Anyway , I am going to remove series resistor in my application , as per datasheet application note. I have also tested by removing series resistor and it is working fine without effecting cell voltage monitoring, all readings are as expected. there is no issues.

    Thanks for your support.

    Regards,

    Devayani bodar