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BQ79656EVM-021: BQ79656-Q1 Current sensing

Subhash Dhyani
Intellectual 300 points
Part Number: BQ79656EVM-021
Other Parts Discussed in Thread: BQ79656-Q1

Dears,

During BQ79656-Q1 data sheet study, we found that voltage limits of SRN and SRP Pins are -3V to 2.1V.

May I know the reason why there is only 0.3V (negative) margin on these PINs.

 Can you please tell the internal protections associated with these pins so that it will not damage during hot plugin/reverse recovery conditions.

 Can you please suggest suitable protection circuit (No leakage) so that there is no deviation in current measurement through these pins.

 Prompt response is highly appreciated.

 

Thanks, and regards

Subhash Dhyani

  • 0
    TI__Intellectual 2280 points

    HI Subhash,

    -0.3V is due to a typical ESD diode (Anode toward GND) for negative ESD protection. If the negative voltage higher than -0.3V then the diode could be forward biased and cause damage.

    Have you measured a -0.3V on the SRP or SRN pins? Because for BQ79656 CS it's a low side current sensing, ideally the shunt should be connect to the GND, that means they're the same potential.  As long as system design keep the parasitic inductance/resistance as small as possible between the GND and shunt, for example avoid long busbar between the GND and shunt, then the SRP/SRN voltage won't deviate from GND so much. 

  • 0 in reply to Ted Zhang
    Intellectual 300 points

    Hello  Ted Zhang,

    thanks for your reply,

    We have  to protect the  SRN SRP pins. So we have added 1.8V Zener diode (max SRN SRP Pin voltage limit  is 2.1V) on Both the pins.

    Can you please check and suggest if this modification is ok from your side. Here capacitor can be made not mount in final revision as it is not recommended from TI.

    Is  Zener diode  leakage current can hamper  for current measurement accuracy of device?

    TI_AFE_Discussion.pdf

    thanks and regards

    Subhash Dhyani

    TI_AFE_Discussion.pdf

  • 0 in reply to Subhash Dhyani
    TI__Genius 17370 points

    Subhash,

    I think this question would need to be tested on your side to confirm based on the diode used and any transient noise conditions in your system to evaluate any impact to accuracy. 

    Regards,

    Taylor