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BQ76940: BOOT up of AFE

Rohan Patwardhan1
Intellectual 340 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ76920

Hi TI Team,

During AFE boot, we need to connect TS1 to C1 for a specified time. This connection is current limited using a 10k resistor. However, If TS1 to GND impedance is not 10K, can there be problems in booting up the AFE?

Regards,

Rohan

  • 0
    TI__Guru 54715 points

    Hi Rohan,

    Yes, there can be limits.  If the cell voltage is low and the thermistor is hot there may not be much of a rising edge on TS1.  It may be that you don't want your battery to start until it cools, or you might want it to wake up and tell the user it is hot and should not be used.  Design a circuit suitable for your system and operating conditions.  There is an application note  about boot switch alternatives in the BQ76940 product folder which may be helpful. 

  • 0 in reply to WM5295
    Intellectual 340 points

    Hi Expert,

    Can you specify the limits.

    Thanks

    Rohan

  • 0 in reply to Rohan Patwardhan1
    TI__Guru 54715 points

    Hi Rohan,

    I can't specify limits for the conditions in your design.  Look at the Vboot conditions in the BQ76920 data sheet, you must have a rising edge which exceeds the level.  Consider your thermistor and the temperature and resistance at which you want the pack to boot. Calculate the voltage and see if it is suitable or select a different resistor and add a limiting component (Zener) if the voltage becomes too high when cold.

  • 0 in reply to WM5295
    Intellectual 340 points

    Hi Expert,

    Since we are using an NTC for the 10k between boot pin and gnd, we have found a scenario where the 10k will be an open connection. This can happen if the AFE is powered up without connecting the temp sensor connector. Can there be any repurcussions for this case?

    Regards,

    Rohan

  • 0 in reply to Rohan Patwardhan1
    TI__Guru 54715 points

    Hi Rohan,

    The BQ76940 does not have internal thresholds for the temperature or to find an open condition.  The MCU would need to determine appropriate thresholds for distinguishing cold from open thermistor conditions.

    If the thermistor is missing and a capacitor is present on the TS1 pin, the capacitor can hold a voltage on the pin.  The voltage does not prevent shutdown, but may prevent re-start since you can't provide a rising edge on the pin if it is already high.  A high does not wake the part, it needs a rising edge detection which won't happen if there is already a high voltage on the pin. A large resistor across the capacitor will provide an identified disharge path for the capacitor if it is a concern in your system.  The resistor will be in parallel with the thermistor so it will alter the voltage vs temperature curve, more significantly at cold temperature where the thermistor voltage is high.