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Original question:

BQ25703A: Replace input and output current sense resistors for higher currents

BQ25703A: Changing Current Sense Resistor between SRP and SRN, 10mohm to 20mohm

Shinichi Inoue
Guru 21586 points
Part Number: BQ25703A

Dear Specialists,

My customer is considering BQ25703A and has questions.

I would be grateful if you could advise.

---

(1) The LSB of battery discharge current detection (ADCIBAT) is described as 256mA,

This value is defined when the detection resistance RSR is 10 mΩ.

If RSR is 20 mΩ, will the LSB be 128 mA?

(2) The RSNS_RSR register settings has 10mΩ and 20mΩ.

If the RSNS_RSR register is set to 10mΩ and the actual resistance value is 20mΩ, the LSB will be 128mA,

Or when RSNS_RSR is set to 20 mΩ, is the LSB change to 256mA?

(3) In addition, if IBAT_GAIN iresister is changed, will the LSB be 256mA?

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Mastermind 25800 points

    Hey Shinichi,

    For (1) That is correct. The LSB will be cut in half as the controller is trying to regulate the differential voltage across SRP and SRN.

    For (2) That is also correct.

    For (3) The gain determines the output voltage for a given V(SRP-SRN). This is the gain that is applied to that differential charging voltage (across the sense resistor) shown as an output voltage on the IBAT pin.

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    Guru 21586 points

    Hi Joel,

    Thank you for your reply.

    (1) I understand.

    (2) Could you please tell more detail,

    RSNS_RSR resister is set to 20 mΩ and sense resistance is 20mΩ, which is the LSB 256mA or 128mA?

    I assume 256mA.

    I think RSNS_RSR resister is difined the gain of SRP-SRN amplifier (20x or 10x), if so, 10mΩ is 20x and 20mΩ is 10x, is it correct?

    (3) I understand.

    I appreciate your great help and patience.

    I'm looking forward to waiting your response.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 21586 points

    Hi Joel,

    I am still looking forward to waiting your advise.

    ---

    (2) Could you please tell more detail,

    RSNS_RSR resister is set to 20 mΩ and sense resistance is 20mΩ, which is the LSB 256mA or 128mA?

    I assume 256mA.

    I think RSNS_RSR resister is defined the gain of SRP-SRN amplifier (20x or 10x), if so, 10mΩ is 20x and 20mΩ is 10x, is it correct?

    ---

    I appreciate your great help.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 25800 points

    Hey Shinichi-san,

    For (2)

    RSNS_RSR = 20mOhm and RSR = 20mOhm physically would keep the LSB as 64mA. 

    I want to clarify a statement from before. The 10mOhm LSB is 64mA, not 128mA. So all statements above are related to an original LSB of 64mV. 

    As far as the RSNS_RSR, it is meant to match the resistance value used. Being able to use 20mOhm allows for a more accurate 64mV step.

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    Guru 21586 points

    Hi Joel,

    Thank you for your reply.

    I could not explain enough, could you please let me know again.

    (2) asks about IDCHG, LSB is 256mA.

    RSNS_RSR resister is set to 20 mΩ and sense resistance is 20mΩ, which is the LSB 256mA or 128mA?

    Thinking from your comment, is it possible to judge 256mA?

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 25800 points

    Hey Shinichi,

    If the RSNS_RSR setting matches the external sense resistance value, the LSB remains the same at 256mA for IDCHG. The improvement however is the accuracy of these lower current measurements. 

    Now if you change the RSNS_RSR to 20mOhm, but the external resistance is 10mOhm, the LSB would effectively reduce to 128mA, albeit with less accuracy. 

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    Guru 21586 points

    Hi Joel,

    Thank you for your reply.

    I will share your suggestion with the customer.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi