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BQ76930: Is P channel Mosfet at Chg pin really mandatory?

Part Number: BQ76930
Other Parts Discussed in Thread: TIDA-00449

Hi,

1. Is P-channel mosfet is really mandatory?, If yes please recommend its specs?  Does: BSS84,215 P channel Mosfet work well ?

specs:

50V , 130mA , Vgth 800mV , Rdson : 10ohms


3. What could be the value Rgate shown in picture for N - channel mosfet?
4. Is Rdrain really necessary?, i don,t see such resistor in  any schematic present in datasheet?
5. Value of Rd 100 ohms ok?

Please look at the images affixed. Also Kindly have a look at my schematic ( focus on capacitors i doubt them), Please let me know if there are mistakes

Thankyou

  • Hi Mounish,

    1) Yes, the P-channel mosfet is needed. It prevents the voltage of the CHG pin from being pull below the Abs Max datasheet voltage. There is a full description of this circuit in the document BQ769x0 Family Top 10 Design Considerations (see Section 6 on FET Drive): https://www.ti.com/lit/an/slua749a/slua749a.pdf

    2) I think the device you mentioned should work. The devices I have seen on different reference designs are the ZXMP10A13FTA and the TP0610K-T1-GE3.

    3) 100k is the value used on the EVM and in multiple reference designs

    4) The drain resistor is discussed in the Top 10 Design Considerations document Section 9. 100 ohms is a good value and is used in the TIDA-00449 reference design. The EVM uses 200 ohms for this resistor.

    Some notes on the schematic:

    • RF1 and RF2 should be 1k instead of 100 ohms
    • The cell input capacitors (CC1, CC2, etc) can be reduced to 0.22uF to reduce measurement variation during cell balancing. Some of our reference designs use 0.22uF.
    • The ALERT pin should have a 510k pull-down resistor and a 470pF capacitor to GND very close to the device. The ALERT pin is also used as an input (for ALERT_OVRD fault) and it is very sensitive to noise. 
    • As discussed above, Rdrain on the REGSRC circuit should be included and the PCH FET on the CHG pin circuit should be used.

    Best regards,

    Matt