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BQ2970: Refearence Design「TI Designs: 50mA Wireless Charger Booster Pack」 of BQ29707

FRANK1
Expert 8555 points
Part Number: BQ2970
Other Parts Discussed in Thread: CSD85301Q2

Tool/software:

Hi team,

I help customer to post this question due to he can't post a new post to the E2E Design Support Forum.

This is a question about the circuit of the "TI Designs: 50mA Wireless Charger Booster Pack" using the BQ29707.

The battery protection circuit for the coin-shaped Li-Ion rechargeable battery of this circuit is "current Limit 100mA".

Question 1. Is it correct to understand that in order to achieve "current Limit 100mA", this circuit fine-tunes the constants of R18(221 Ω) and R19(2.21k Ω) compared to the circuit in the specification sheet, and adds "R20(0.82 Ω)"?

Q2.What is the constant calculation formula for R20 (= 0.82 Ω) that was added to achieve "current Limit 100mA"?

The FET is CSD85301Q2.

Thank you.

Regards,

Frank

  • +1
    TI__Mastermind 42060 points

    Hello Frank,

    FRANK1 said:
    Question 1. Is it correct to understand that in order to achieve "current Limit 100mA", this circuit fine-tunes the constants of R18(221 Ω) and R19(2.21k Ω) compared to the circuit in the specification sheet, and adds "R20(0.82 Ω)"?

    No. The only component added that can affect the threshold is the R20 resistor. The part makes protections based on the voltage drop between V- and Vss.

    FRANK1 said:
    Q2.What is the constant calculation formula for R20 (= 0.82 Ω) that was added to achieve "current Limit 100mA"?

    This is likely dependent on the MOSFET's used. These FETs will have a RDSON specified by its datasheet. So based on the voltage threshold, you can estimate what is the resistance needed to trigger the right protections.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    TI__Expert 8555 points

    Hi Luis,

    Thank you for your response.

    I understand Q1&Q2。

    One more question, why did you change the R18 (221 Ω) and R19 (2.21k Ω) circuits in the ・ "TI Designs: 50mA Wireless Charger Booster Pack"
    In the reference circuit in the specification sheet, R18=330 Ω, R19=2.2k Ω.

    Thanks.

    Regards,

    Frank

  • +1 in reply to FRANK1
    TI__Mastermind 42060 points

    Hello Frank,

    I am not really sure why that is the case, the people who worked on the reference design are no longer in the team. It may be that these were readily available resistors.

    Best Regards,

    Luis Hernandez Salomon