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BQ27510 BI/TOUT behaviour

Renato Duarte
Prodigy 40 points
Other Parts Discussed in Thread: BQ27510, BQ27510-G1, BQ27510-G2

Hi All,

I am having some trouble with BI/TOUT activity disturbing some sensitive analog parts near BQ27510, and I would like to know if did any of you have the same issue. It seems that BI/TOUT actually pulls-up (data sheet states that the pin is pulled LOW) every second the NTC network to check if there is a battery connected. But there is a 33nF capacitor connected at the BI/TOUT pin to GND, that cause a current inrush when the BI/TOUT is pulled up, since, with the battery connected, the termistor will put the network voltage value near zero. Is that correct? If yes, can I put a resistor at BI/TOUT pin to damp the current inrush into the capacitor? Will this resistor affect my battery temperature measurement? what is the maximum resistor value could be used in this case?

Thank you,

Renato

 

 

 

  • 0
    TI__Intellectual 2340 points

    Renato,

    Can you show us a schematic of your full thermistor implementation? The datasheet already has the way to do this but I want to verify if you have any variations to the application schematic of the datasheet before suggesting any adjustments.

  • 0 in reply to MikeV
    Prodigy 40 points

    Hi Mike.

    See below the BQ27510 implementation schematic. It's exact the same as in the datasheet. It seems that I have an old silicon or something, because the BQ27510-G1 datasheet says that BI/TOUT pin is a push-pull IO, and the BQ27510-G2 datasheet says it's an open drain IO. Anyway, the pin is never put in HI-Z condition. Instead, it is pulled up during the battery check test, causing a current inrush at C6 and at REG25 consequently.

  • 0 in reply to Renato Duarte
    TI__Intellectual 2340 points

    Renato,

    The BI/TOUT pin has dual function. The Battery Insert (BI) function is to go high impedance and detect a logic input to determine if battery is inserted or not. In a remoable pack the thermistor should be included in the battery pack.

    The Temperature Out (TOUT) function is to bias the thermistor network so that an analog voltage measurement is done through the TS pin.

    Both of these functions are repeated every second. So actually this pin is in Hi-Z when it is BI and it is pulled to VCC when it is TOUT.

  • 0 in reply to MikeV
    Prodigy 40 points

    Mike,

    But you should agree with me that this cause a current inrush at the 33nF capacitor, because the network termistor is near zero volts when the TOUT pin is pulled high. I can even see a bounce at the REG25 pin due to this. My question is: can I limit this current inrush without disturbing my temperature measurement? Maybe doing a NTC network recalc to maintain the same TS voltage as when the TOUT pin is directly connected to the 33nF capacitor? What maximun time constant I can use (or maximum resistor at TOUT pin can be used) to avoid temperature measurement errors?

    Renato

     

  • 0 in reply to Renato Duarte
    Prodigy 40 points

    Mike, any comments? It seems to be a good improvement opportunity to the component...

    Renato