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OPA172: Circuit protection in the absence of VCC

Part Number: OPA172

Thank you very much for your help.

I want to protect the circuit from the voltage applied to the output when VCC is not supplied to the OPA172.
Can you tell me the appropriate circuit configuration?

In normal operation, 12v is supplied to the vcc of the opA172, but when the main power supply fails and 12v is not supplied, the configuration is to use an external sub-battery to drive the emergency alarm circuit.
The sub-battery is assumed to be a coin cell battery such as cr2032.
In this case, the opa172 will be irrelevant, but 3v will be applied to the output line.
We want to make sure that the opa172 can be used without damage when the main power supply is restored.

By the way, I made some measurements with the evaluation board I created.
When there was no VCC, about 42mA was flowing from the sub-battery.
When I disconnected the output line of the OPA172, it dropped to about 3mA, which is the current consumption of the emergency alarm circuit.
Can you tell me about this phenomenon as well?

This may be a basic question about op-amps, but please help me.

  • Hi Fukasawa,

    this way?

    Kai

  • Hello,

    The method Kai recommends should help. 

    The likely reason there was higher current consumption was because the internal ESD diodes of the OPA172 were forward biased causing the current to flow. The current through the ESD diode should never exceed 10mA as stated in the Absolute Maximum Ratings Section of the datasheet. To test this, try measuring the current flowing into the output pin of the op amp during the test condition you stated. You could then add the diode Kai recommended to see if it solves your problem.

    For more information on protecting the device, I recommend watching our TI Precision Lab Videos on ESD and EOS.

    Thank you,

    Tim Claycomb