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BQ24780S: BQ24780S Query related

Max
Intellectual 840 points
Part Number: BQ24780S
Other Parts Discussed in Thread: BQ24800

Hi

We are using BQ24780S charger IC in our design.

In this we have used 10Kohm pull-up resistor and 42ohm series resistor in our SCLK and SDA lines. Also, we have calculated the total capacitance in SMBUS lines approximately 100pF, includes trace, cable, protection diode capacitance, board and charger IC pin capacitance.

We have done AC analysis for the SCLK and SDA lines and waveform is shown  below

The rise time is 4.54us which does not meet the SMBUS timing limit that is greater than 1us (as per datasheet)

The Fall time is 18.96 ns.

The setup time and hold time values of the SCLK and SDA is meeting as per requirements.

Whether the above rise time value is acceptable.

  • 0
    TI__Mastermind 26255 points

    Hi, Anandan,

    The rise time is unacceptable, which is too slow, not a square waveform. You may need to double check pull-up resistance or line capacitance. 

  • 0 in reply to Tiger Zhou
    Intellectual 840 points

    Hi Tiger Zhou

    Thanks for the response.

    In this we have used 2.2Kohm pull-up resistor and 42ohm series resistor in our SCLK and SDA lines. Also, we have calculated the total capacitance in SMBUS lines approximately 100pF, includes trace, cable, protection diode capacitance, board and charger IC pin capacitance.

    We have done AC analysis for the SCLK and SDA lines and waveform is shown  below

    The setup time is about 4.03us.

    The Hold time is about 119.276ns which does not meet the SMBUS timing limit.

    Whether the hold time is acceptable.

    The rise time is about 847.8ns

    The fall time is 12.27ns.

    Also whether it is right to have a different pull ups resistance on the SCLK and SDA lines?.

    Regards

    ANANDAN N

  • 0 in reply to Max
    TI__Mastermind 26255 points

    Hi, Anandan,

    I saw the waveform improved a lot by lowering the pull-up resistor.  Please conform to the datasheet specification on SMBus timing characteristics. See below.

  • 0 in reply to Tiger Zhou
    Intellectual 840 points

    Hi Tiger Zhou

    we are referring to timing characteristics of BQ24780s datasheet which differs with the information you shared.

    can you share the datasheet you referred to..

  • 0 in reply to Max
    TI__Mastermind 26255 points

    I am referring to BQ24800 datasheet--a new part in the same family.