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BQ24170: Sense resistor value change for single fault condition simulation

SIDDHARTH ROY
Intellectual 510 points
Part Number: BQ24170

Hello,

I am using BQ24170 charger IC and associated circuit for charging a lithium Ion 3S2P battery pack with a charging current of 1.5A. The battery and charging circuit is part of a medical device. The design works perfectly so far. For 60601-1 medical device safety , we need to simulate a single fault condition i.e we need to short the current sense resistor connected across the SRP and SRN pins of the BQ24170. As per Design it is 10mohm on the PCB, but our plan is to short it( which means it will be close to  0ohm). We want to see what is behavior of the circuit in that scenario. We have 2A Fuse both at AC adaptor and battery power lines respectively, so that in case the current exceeds  this limit, the fuse will open up.

I would like to know what is the possible/expected behavior with this sense resistor value manipulation. Will there be any other protection from BQ24170 which can trigger?

As this is likely to be a destructive test, I want to know if the behavior can be predicted.

Attached is the schematic for reference. Just note Fuse F1 and F2 both are 2A( although one of them in the attached file is shown as 3A rated)

3377.Power Supply.pdf

Thanks,

Siddharth

  • 0
    TI__Mastermind 40845 points

    Dear Siddharth,

    The charge current is set by regulating voltage across the sense resistor. With a 0 ohm sense resistor, I believe (but have not tested) the charger would try to regulate the desired sense voltage by increasing the current accordingly. However, the BQ24170 has overcurrent protection; please see Section 9.3.13 of the datasheet, titled "Charge Overcurrent Protection." I have posted it below. The Electrical Characteristics table has the specification listed as "CHARGE OVERCURRENT COMPARATOR (CYCLE-BY-CYCLE)" and "HSFET OVERCURRENT COMPARATOR (CYCLE-BY-CYCLE)". 

    "The charger monitors top-side MOSFET current by high-side sense FET. When peak current exceeds MOSFET limit, the charger turns off the top-side MOSFET and keeps it off until the next cycle. The charger has a secondary cycle-to-cycle overcurrent protection. The charger monitors the charge current, and prevents the current from exceeding 160% of the programmed charge current. The high-side gate drive turns off when either overcurrent condition is detected, and automatically resumes when the current falls below the overcurrent threshold."

    I have not tested this, but I believe your battery fuse would blow first because of its 2A rating.

    Thanks,

    Mike Emanuel

    Please click "Resolved" if this answered your question.

  • 0 in reply to Michael Emanuel35
    Intellectual 510 points

    Hi Mike,

    My understanding of the design is also same as you. I also think that the 2A fuse on the battery power line likely to blow. Anyways, will let you know once I perform this. It will be an exiting experiment :).

    You can put this ticket into resolved state may be for now.

    Thanks,

    Siddharth