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BQ25970: Some questions about bq25970 application

Jacky Wang(QD)
Genius 9940 points
Part Number: BQ25970

Hi Team,

Our customer is evaluating the bq25970 in their projects, and during their test, they are having below questions about device's specification, can you help to check them?

1. VAC OVP response time:

In our datasheet we claimed the reaction time typical is 0.1us, but during customer's board test and our EVM ferification, we measured the time is up to 1.3us which is too long for their application. Is this the normal specification for our device?

2. OVP_GATE pin oscillation issue:

During customer's OVP test, they found that the OVP_GATE pin will be oscillation under below conditions, do you know what's the problem in here?

a. Set OVP threshold to 11V, OVP MOSFET will shutdown at ~10.94V.

b. When decreasing VAC voltage to ~10.8V, we will see OVP_GATE pin oscillation like below waveform.

c. When continue to decreasing VAC voltage to below 10.5V, it recovers to normal.

3. Battery temperature detection circuit application issue:

Currently customer is following our EVM circuit to bias the NTC from VOUT(battery voltage), but this method will introduce leakage current from battery during product power off situation. Do we have any other methods to bias the NTC without drawing current from battery?

Thanks a lot!

  • 0
    TI__Guru*** 143910 points

    Hi Jacky,

    Regarding VAC_OVP, the reaction time is measured from VAC trip point to SWx pin stop switching.  The scope shot above shows a part of the pulldown duration spec, which is a function FET gate capacitance.

    Regarding OVP_GATE ringing, the ringing is likely due to ringing on VAC around the trip point.  This will happen, especially if there are long inductive and/or resistive leads/traces from the power supply to the VAC pin, because the converter abruptly stops current flow, which is results in jump at VAC pin voltage.  In a real application, not a bench test setup, there will be lower inductance and there less of a chance of this ringing.  We will look into adding more hysteresis to our next device.

    Regarding TS pin pullup, it must be REGN not VBAT for the TS function to be accurate.  In charge and OTG modes, REGN is up.  In battery only mode, REGN only comes up if the ADC is enabled and measuring TS pin voltage.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Genius 11850 points

    Hi Jacky,

    To add on to Jeff's comment regarding question 1), Section 8.4.1.1 in the datasheet provides more details on how tvac_ovp is measured.

    tvac_ovp refers to the reaction time between VAC > VACOVP and OVPGATE starting to turn off. The actual time it takes to turn OVPGATE fully off depends on the FET gate capacitance and is not included in tvac_ovp.

    Best regards,

    Angelo

  • 0 in reply to Jeff F
    TI__Genius 9940 points

    Hi Jeff,

    Thanks for the comments, regarding to question 3, I think we need to revise our datasheet as we claimed that TS resistor divider should be connected to VOUT of the device:

    Thanks. 

  • 0 in reply to Jacky Wang(QD)
    TI__Intellectual 2320 points

    Hi Jacky,

    Thanks for you question. 

    For currently BQ25970, the reference and pull up voltage is VOUT.  We plan to change it to REGN to save the leakage current in our next generation device.   

    Alen

  • 0 in reply to Alen%2520Chen
    TI__Genius 9940 points

    Hi Alen,

    Thanks for the feedback, but I'm a little bit confuse here, as according to Jeff's comments above, we should connect to REGN for bq25970 instead of VOUT. Can you help to confirm if we can connect it to REGN for bq25970?

    Thanks.

  • 0 in reply to Jacky Wang(QD)
    TI__Intellectual 2320 points

    Hi Jacky,

    In most of charger we pull TS resistor network to REGN, but in BQ25970 please pull TS resistor network to Vout. 

    Typically, customer connect the NTC to gauge, and float TS pin.  Is there any special configuration which need to use TS pin?

    Alen

  • 0 in reply to Alen%2520Chen
    TI__Genius 9940 points

    Hi Alen,

    What's the design considerations for connecting to VOUT rather than REGN for bq25970? Is there any risk for connecting to REGN? We will try to check with customer if they can use gauge's NTC instead of bq25970's NTC circuit.

    Thanks.

  • 0 in reply to Jacky Wang(QD)
    TI__Intellectual 2320 points

    Hi Jacky,

    No specific reason. Some of customer think the leakage current is acceptable for there system.

    Alen