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BQ24725A: Can I put the discharge BATFET before charge current sense resistor?

Howard Zou
Genius 12365 points
Part Number: BQ24725A
Other Parts Discussed in Thread: BQ24780S

Hi,

My customer would like to monitor the current when discharging from battery to system.

Then the current flowing direction is reversed and I'm not sure if our device can handle it.

In datasheet page1's schematic, the N-FET for discharging is at the left side of the sense resistor.

But in all our typical application, the N-FET for discharging are all at the right side of the sense resistor.

  • 0
    TI__Expert 5330 points

    Hello Howard,

    Your customer should not modify the placement of this sense resistor versus what is shown in the datasheet.  In addition to measuring battery current as a differential measurement, the SRN pin is used as a single-ended measurement of battery voltage.  Placing the sense resistor anywhere other than directly on the battery terminal is going to cause a misreading of this measurement.

    Regards,

    Steve   

  • 0 in reply to Steve Preissig
    TI__Genius 12365 points

    Steve,

    The datasheet itself is contradictory.

    Page 1 the N-FET is at the left side of sense resistor. Page 28, 35, 36 the N-FET is at the right side of the sense resistor.

  • 0 in reply to Howard Zou
    TI__Expert 5330 points

    Hello Howard,

    Thank you for pointing out this inconsistency in the datasheet.  I have let our systems team know.  As you say, on Page 1, the BATFET Source is attached to the SRP node on the sense resistor, whereas on all other pages it is attached to the SRN node on the sense resistor.

    Please follow the schematics of the EVM as these are generated from the design files of the EVM (http://www.ti.com/tool/bq24725aevm-710 )  This is the validated design. Here the BATFET Source is attached to the SRN node on the sense resistor, matching pages 28, 35 and 36.  It appears that there was an oversight in the simplified diagram on the first page. 

    Regards,

    Steve

  • 0 in reply to Steve Preissig
    TI__Genius 12365 points

    Steve,
    then how can I measure discharge current?

  • 0 in reply to Steve Preissig
    TI__Genius 12365 points
    Steve,
    then how can I measure discharge current?
  • 0 in reply to Howard Zou
    TI__Expert 5330 points
    Hi Howard,

    Are you intending to measure discharge current using the BQ24725A or externally? The BQ24725A does not provide a mechanism for measuring or limiting discharge current. Most customers would rely on a gas gauge IC in the battery pack to provide this measurement.

    If you want to measure discharge current with the charger device, then you should move to the newer device in the same product line which is the BQ24780s. This device is validated with the sense resistor in the position you are asking for, and it provides a pin, IDCHG where the discharge current is converted into a voltage that can be directly measured. This device also provides battery discharge current limiting via SMBUS register setting, which the BQ24725A does not.

    Regards,
    Steve