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BQ27Z746EVM-047: Low Current Measurement and Sense Resistor Range

Alex Candia
Prodigy 30 points
Part Number: BQ27Z746EVM-047
Other Parts Discussed in Thread: BQ27Z746, BQSTUDIO

Tool/software:

We are using the BQ27Z746 in a low-power application where typical current draw is around 500 µA. We understand that the default sense resistor is 1 mΩ, but we've seen application notes suggesting that increasing the sense resistor (e.g., to 50–100 mΩ) can improve resolution for low-current measurement. 

https://www.ti.com/lit/an/slua792/slua792.pdf   the enhanced fuel gauge application note

According to the BQ27Z746 datasheet, the SRP/SRN input range supports 1 mΩ to 20 mΩ.

Questions:

  1. Is it safe to increase the sense resistor above 20 mΩ (e.g., 50–100 mΩ) to improve current resolution in the sub-1 mA range?

  2. If we increase the sense resistor, will BQStudio be able to reliably measure and display currents as low as 500 µA or below?

  3. Are there alternative methods to improve accuracy for low current draws (e.g., <1 mA), such as:

  • 0
    TI__Mastermind 26845 points

    Hello Alex,

    This question has been assigned and will be reviewed.

    Thank you,
    Alan

  • 0 in reply to Alan Frias
    Prodigy 30 points

    Thank You Alan. 

    Is there any updates to this case? Thank you. 

  • 0 in reply to Alex Candia
    TI__Mastermind 26845 points

    Hello Alex,

    Alex Candia said:
    Is it safe to increase the sense resistor above 20 mΩ (e.g., 50–100 mΩ) to improve current resolution in the sub-1 mA range?

    Per the datasheet we do recommend staying with 1 to 20mΩ, however if the user would like to increase the sense resistor to increase the current resolution than the user would need to understand the input voltage range for SRN and SRP is "Vcc_cm-0.1 to Vcc_cm +0.1" meaning that which ever sense resistor value that is being used the current passing through the sense resistor must not past the acceptable input voltage for Vcc_cm-0.1 to Vcc_cm+0.1" 

    Alex Candia said:
    If we increase the sense resistor, will BQStudio be able to reliably measure and display currents as low as 500 µA or below?

    It would not be possible to make BQStudio do its calculation under 1mA

    Alex Candia said:
    Are there alternative methods to improve accuracy for low current draws (e.g., <1 mA), such as:

    The best methods would be based upon the application note you've attached.

    Thank you,
    Alan