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BQ24773: RSR resistance quesiton

Patrick Chen
Mastermind 8875 points
Part Number: BQ24773

Hello,

                  if customer would like to decrease RSR resistance from 10m ohm to 1m ohm. If setting charging current to 5A, do we need to change any FW setting by charging current? Customer would like to 5A charging current even decrease RSR. 

                  Does RSR has any minimum decrease value? Thank you. 

BR

Patrick

  • 0
    TI__Mastermind 20180 points

    10mOhm sense resistor is recommended. Any value less than 10mOhm will affect the current regulation accuracy. The battery charge/discharge current regulation is done by sensing the voltage difference between SRN and SRP. With 1mOhm sense resistor, the noise on the pin can potentially much higher than the actual voltage across the resistor. The regulation can be very in accurate.  

  • 0 in reply to Jing Zou
    TI__Mastermind 20180 points
    The accuracy in the spec is based on the voltage across the sense resistor. So when using 1mohm sense resistor for 5A current regulation, the accuary is around +/- 25%
  • 0 in reply to Jing Zou
    Mastermind 8875 points
    Hello Jing,

    So we don't change any FW while decrease the resistor, right? Thank you.

    BR
    Patrick
  • 0 in reply to Patrick Chen
    TI__Mastermind 20180 points

    Again, the IC senses the voltage across the sense resistor to estimate the current. All the charge current register current setting is based on 10mOhm resistor. For example,  ChargeCurrent() is set to 1024mA. Since this is based on 10mOhm sense resistor, to regulate current at 1024mA, the voltage difference between SRN and SRP should be around 10mV. When you set the same value in the ChargeCurrent() register with 1mOhm resistor, the IC will still look for SRN/SRP difference of 10mV. Then the actual current through the 1mOhm resistor will be 10mV/1mOhm = 10A. 

    Thus you will need to re-calculate the current setting of the ChargeCurrent() register based on the sense resistor value. 

    For 5A charge current, we do not recommend 1mOhm sense resistor since the accuracy will be affected.

    Please order an EVM for evaluation before implementing. 

  • 0 in reply to Jing Zou
    Mastermind 8875 points
    Hello,

    Could I ask one more question? Has any calculate formula of RSR resistor for charging current calculation? Thank you.

    BR
    Patrick
  • 0 in reply to Patrick Chen
    TI__Mastermind 20180 points

    It is basically V = I*R. The IC internal voltage threshold is set based on 10mohm sense  resistor, thus the current as shown in the register table 10 multiply by 10mohm. Then you can use the corresponding voltage divide by the new sense resistor value to obtain the current setting.

  • 0 in reply to Jing Zou
    Prodigy 20 points
    What if I want to change the sense resistor to reduce the power loss across the sense resistor , can i change it to value lower than 10 m ohm may be 5 m ohm. My application current will change between 500mA to 13A.
  • 0 in reply to user5093200
    TI__Guru 56650 points
    Yes. They can change bq24773's charge current sense resistor to 20mohm or 5mohm.