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TPS1H100-Q1: Normally, Thermal PAD has no electrical connectivity to any of the HSD pins. However, Thermal PAD is shorted to Device GND after shorting Digital output to the Thermal PAD, Why?

Vikhyat Baxi
Prodigy 245 points
Part Number: TPS1H100-Q1

Goal : To provide thermal PAD heat dissipation path to metal enclosure.

Method : Provided Thermal PAD of HSD with electrical/thermal connection to metal enclosure. However, note that, metal enclosure is NOT connected to the 'Device ground'. The device ground is isolated from enclosure and device ground is also isolated from thermal PAD.

Fault Scenario: As a part fault mode, there is high probability that during installation digital output wire of HSD could short with enclosure,

Test Scenario: Hence we simulated similar scenario and connected Digital output of HSD to the enclosure. This resulted in shorting of enclosure with the device ground, which was isolated before conducting this test.

Piece of Puzzle :

1. Why shorting Digital output to Thermal PAD via metal enclosure resulted in Thermal PAD shorting to device ground of HSD ?

2. HSD is still functioning normally under normal operating condition.

3. What are the limitations of HSD Thermal PAD floating ?

  • 0
    TI__Genius 16095 points

    Hi Vikhyat,

    Thank you for reaching out on E2E!

    In regards to the TPS1H100, the device ground is internally shorted to the thermal pad.

    Best Regards,

    Elizabeth

    If this response has answered your question, please click the "This resolved my issue" button.

  • 0 in reply to Elizabeth Higgins
    Prodigy 245 points

    Hi Elizabeth

    I check TPS1H100 connectivity between thermal PAD and ground pin, it is open ckt. There is no electrical connectivity between thermal PAD and device ground pin.

    After conducting above mentioned test thermal PAD is shorted to device ground.

    Thanks

    Regards

    Vikhyat

  • 0 in reply to Vikhyat Baxi
    TI__Genius 16095 points

    Vikhyat,

    Could you elaborate on how you isolated device ground from the thermal pad?

    It would also be helpful if you could send a screenshot of your layout.

    Best Regards,

    Elizabeth

  • 0 in reply to Elizabeth Higgins
    Prodigy 245 points

    Hi,

    1. Thermal PAD is connected to metal enclosure.

    2. Metal enclosure is not connected to device ground.

    3. In HSD Thermal PAD is NOT connected to device ground ( as per DMM reading of standalone HSD chip).

    4. Before conducting above test HSD Thermal PAD was not connected to device ground.

    5. After conducting above test HSD thermal PAD is shorted to device ground.

    Thanks

    Regards

    Vikhyat

  • 0 in reply to Vikhyat Baxi
    TI__Mastermind 34935 points

    Vikhyat,

    The TPS1H100's power pad is internally shorted to ground. Are are you determining it is not connected to ground? Are you measuring the impedance? Also please share a screenshot of the layout as this will have a big factor on the results of your measurement. 

  • 0 in reply to Timothy Logan
    Prodigy 245 points

    Hi Timothy,

    Simply checking IC as received without mounting it on PCB.

    PAD is not connected to GND pin if you measure impedance through DMM. 

    Refer Datasheet also it gives option to leave PAD floating or connect to DGND.

    We chose NOT to connect to DGND and instead connect PAD to device metal case for better thermal heat dissipation under fault condition and prevent damage to the IC.

    As can be inferred we gave provision to connect PAD to either DGND net or Device metal case (EARTH net).

  • 0 in reply to Vikhyat Baxi
    Prodigy 245 points

    Also note that data sheet does not explicitly mention that PAD is connected to device ground of HSD.

    Datasheet also gives option to leave PAD floating or connected to DGND. which indicates HSD PAD is not connected to device ground and it matches our multi meter reading of IC not inserted into PCB.

     

  • 0 in reply to Vikhyat Baxi
    TI__Genius 16095 points

    Vikhyat,

    Sorry for the delay. We have verified with the design team and the TPS1H100 die is attached to the leadframe of the package. If there is no electrical isolation on the die-attach, the substrate of the device will be shorted to the thermal pad. The substrate of the device is ground which is why there is low impedance from the device ground to the thermal pad.

    Best Regards,

    Elizabeth

    If my reply answered your question, please click the "This resolved my issue" button. This can help others verify that a solution was found!