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Original question:

TPS54550: The reason of oscillating output, when SSENA connected with TPS3808 and pull-up resistor.

TPS54550: pull-up resistor to SSENA pin

Hung Ching Hsu
Expert 3086 points
Part Number: TPS54550

Hi Team, 

TPS54550's datasheet says "Do not use external pull-up resistors".  

But now customer is connecting another IC to SSENA pin and using pull-up resistor. 

Sometimes SSENA will be "L" which cause TPS54550's stops work. 

Maybe this phenomenon is caused by abnormal of IC connected to SSENA pin, 

but is there any possibility that this phenomenon is caused by the pull-up resistor? 

If yes, could you explain why this happens?

Thanks.

Regards,

Jo

  • 0
    TI__Expert 5060 points

    Hi Jo,

    For your mentioned SSENA "L", I suspect that's pulled down by the another IC. For the pull-up resistors, if the voltage of another IC pin becomes 0, then it could be seen as a pull-down resistor. As you can see in datasheet, the internal current source is just 5uA, so if the external resistor value is not very large, the voltage 5uA*R cannot reach 0.5V.

    From my side, I would recommend customer to just follow our datasheet and not use external pull-up resistor. We can only guarantee the performance if the application follows our datasheet's recommendation.

    If customer wants to further check the root cause of this issue, I think the easy way is just to unpop the pull up resistor and see if the issue can be duplicated.

    Thanks,

    Andrew

  • 0 in reply to Andrew Xiong
    TI__Expert 3086 points

    Hi Andrew, 

    Thank you. 

    >For the pull-up resistors, if the voltage of another IC pin becomes 0, then it could be seen as a pull-down resistor. As you can see in datasheet, the internal current source is just 5uA, so if the external resistor value is not very large, the voltage 5uA*R cannot reach 0.5V.

    I couldn't get it well how can pull- up resistor becomes a pull-down resistor when the voltage of another IC pin becomes 0. 

    Could you help draw a picture to explain it? 

    For example, if 5V is connected to pull-up resistor, I think when the voltage of another IC pin becomes 0, 5V will applied to the pull-up resistor. So I think SSENA pin will just become 0V. 

    Thanks.

    Regards,

    Jo