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Original question:

BQ34Z100-G1: BAT pin Capacitor C1

BQ34Z100-G1: BAT pin decoupling capacitor

user5014632
Intellectual 515 points
Part Number: BQ34Z100-G1

Hi,

I have a multi cell application using a 3300 pF capacitor on the BAT pin as advised on Fig. 16 of the datasheet.

I use the voltage() command to monitor battery voltage and trigger a low voltage disconnect via the host MCU. I have unexplained 200mV jumps on battery voltage measurements which trigger false detections. I use a 17.18:1 resistor divider on the BAT pin, so these 200mV reading jumps could be generated by ripples on the BAT pin as low as 10mV. I feel that an increased filtering on BAT pin could be beneficial, however I wonder why a 3300 pF is advised for multi cell application instead of the 0.1µF for single cell application.

Is it safe to use a 0.1µF on BAT pin for multi cell application?

A software filter could be an alternate solution, is there a way to have an average voltage information from the gauge, similar to the AverageCurrent() command?

Thank you for your insights for this use case.

Best regards,

Jérémie

  • 0
    TI__Mastermind 34775 points

    Hello Jeremie,

    Larger cap would help stabilize ripples on the cell voltage, but if the cap leakage is higher it can cause larger voltage error. I don't see how 10mV of ripple can cause 200mV error. You can try 0.1uF to see if it resolves your issue. I'm sure you probably did, but did you calibrate your battery voltage as well.

    Do you have a schematic you can share?

    The voltage is averaged, but not as AverageCurrent().

  • 0 in reply to Damian Lewis
    Intellectual 515 points

    Hi Damian,

    Here is the schematic of the voltage measurement part. The signal cde_BQ_BAT_1V_24V is used to switch between 4S and 8S battery configurations depending on the final product configuration.

    The gauge is calibrated and the voltage measurement are mostly good, the host MCU uses the voltage() command every 10 seconds. Once in a while (let say every 30 measurements) the voltage() command will return a voltage 200mV lower than the previous value, though no current is flowing in the battery, and no battery voltage change is recorded by external instruments. The next value is back to normal.

    I think a hardware or software low pass filter may be a  solution.

    Does this use case seem normal to you, or am I playing with the limits of the component?

    Looking forward to your insights,

    Best regards,

    Jérémie

  • 0 in reply to user5014632
    Intellectual 515 points

    The pasted schematic did not make it through, here it is:

  • 0 in reply to user5014632
    Intellectual 515 points

    Hi Damian,

    Any update on this voltage averaging topic? It would be nice to have more insight on how the gauge can report a 200mV jump on a rested battery, and how to get rid of these spurious measurements.

    Best regards,

    Jérémie

  • 0 in reply to user5014632
    TI__Mastermind 34775 points

    Hi Jeremie,

    Increasing your BAT cap might help with filtering. Are you seeing ripples on Vbatt or BAT (TP29)? Any idea what is causing the dip in voltage?