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LMC555: Input clamp diodes?

Gabor Szabo
Prodigy 20 points
Part Number: LMC555


Tool/software:

Hallo TI,

I would like to get information about the input structures of LMC555. I read on forums that it has some input clamp diodes, but the datasheet does not mention it, no input clamp limit in "absolute maximum ratings". (By the way the GND-0.3V ÷ VCC+0.3V rating for inputs is suspicious.)

The origin of the question is this circuit. The input is AC-coupled to TRIGGER#. When the input is LOW for long C3 will charge to VCC, then a LOW->HIGH transition will rise the voltage on TRIGGER# to around 2*VCC, which is not good. But if the IC input has internal clamp diode to VCC, we don't need to add an external diode.

Thank you in advance.

  • +1
    TI__Intellectual 2860 points

    Hi Gabor,

    1. As you've already stated, the voltage on the trigger pin cannot exceed V++0.3V (3.6V in this case) since that would be a violation of the absolute maximum ratings of the device. 
    2. You will need to add input protection externally. The device has internal protection for ESD, which is not designed for overvoltage protection. Please see our TIPL content on electrical overstress for guidance on selecting external protection diodes, which can be found here.

    Best Regards,

    Alex Curtis