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TCA39306: Any risk without 200k resistor

Hale Tan
Intellectual 2860 points
Part Number: TCA39306

Hi team,

As the pictures shows, the customer ignore 200k resistor and they connected Vref2 and EN to VCC2 directly. Is there any risk in such a design? Will the device burn out?

  • 0
    Guru 260290 points

    This will create a low-impedance connection between Vref2 and Vref1. Not only might the current exceed the absolute maximum rating of 128 mA and damage the device, but the VCC1 supply might not be able to sink that current and float up, which can damage any device connected to it.

  • 0
    TI__Mastermind 27353 points

    Hi Hale,

    Ignoring the implementation of the 200kohm current limiting resistor may result in over-current issues which could cause permanent damage to the device if the absolute maximum current rating is exceeded (128mA). 

    The only component separating the Vref1 and Vref2 side is a passFET with the EN pin connected to the gate of the FET. If no 200kohm resistor is present, you can expect a voltage drop from VCC2 across a small RDS_ON of the FET into VCC1. You could run into several issues which Clemens and I have already mentioned. 

    The 200kohm resistor is also responsible for limiting the in-rush current on the EN pin, since the EN pin is the voltage present at the gate of each internal passFET of the device. 

    Regards,

    Tyler 

  • 0
    TI__Guru 53590 points

    Hi Hale,

    To add additional information, the image you show has Vcc1=Vcc2 if that is true then there wouldn't be a large concern for a short circuit unless the voltage mismatch/drift between the two rails was large. 

    The Enable pin voltage should not exceed Vref1+Vth if you are trying to do level translation though. 

    -Bobby