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BQ24171: BQ24171 Fast Charge Current Tolerance

George Peterson
Expert 3170 points
Part Number: BQ24171

Dear Team,

The customer is using the BQ24171 in a charging circuit and would like to verify the fast charge current tolerance.    The application requires a 1.5 Amp charge current.    We are suggesting sizing the Rsense resistor to create around a 40 mV at 1.5A (~25 milliohms).   

Does the +/-  4% @ 40 mV Charge Current Regulation Accuracy noted in the datasheet already account for the tolerances of Vref (+/- 1%) and standard resistors?   

In order to calculate a complete stack up of tolerances, do we need to account for the Rsense tolerance, Viset Resistor tolernaces, Vref tolerance, and the Charge Current Regulation Accuracy (+/-4% at 40 mV)?

Thank You,

George

  • 0
    TI__Guru**** 165766 points

    Hi George,

    The +/- 4% accounts for the KISET tolerance but not the RISET and RSENSE tolerances.

  • 0 in reply to Jeff F
    Prodigy 10 points

    Can you provide more information on KISET? After reviewing the datasheet, it appears you could use KISET to calculate charge current. Voltage on ISET x 5 = Charge Current. If I were to include the KISET tolerance to our calculations, the charge current tolerance would be close to +-10%. This is due to the voltage drop across Rsense being 16mV. 

  • 0 in reply to Joshua Grant
    TI__Guru**** 165766 points

    Hi Joshua,

    The equation is ICHARGE = VISET/(20*RSR).  The 20x is the gain of the sensed voltage as shown on the block diagram.  The sensed voltage of 20 x I(SRP-SRN) x RSR is comparing to the ISET voltage. For given RSR=10mohm, every 200mV represents 1A, so it is KISET=5A/V typical.

     

     

     

    For the accuracy, we can assume the comparator has a fixed offset ~1.5mV typical, then we can find that the accuracy is ~4% for 40mV, ~8% for 20mV and ~30% for 5mV, aligned with the accuracy number.  The error is primarily due to the amplifier offset.