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opa2347 inputs voltage active up to 5v when VCC = 0V

Other Parts Discussed in Thread: OPA2347, LM293-EP

HI,

  1. The input level to this circuit is maximum 154V. The maximum input to op amp is approximately 5.1V. The op amp is used as a Schmitt trigger. I would like to know what happens in a situation when VCC_ISO to the op amp is turned off (standby mode) with 5.1V at the input of op amp. The input current is limited by series resistor (R3 or R9). Is there any possibility of op amp getting damaged? What may happen in such a standby mode with input still at 5.1V?

Actually i have the sch if needed.

regards,

Kamal

  • Hello Kamal,

    The OPA2347 has input ESD protection. The input cells consist of low-leakage diodes that are connected from each input to each supply rail. These diodes are off during normal circuit operation. However, if the power to the OPA2347 V+ pin is turned off becoming a high impedance, while a voltage is present at an amplifier input, it is possible to forward bias the diode connected from the input to the internal V+ rail.

    Under that condition current will flow through the diode. There will be about a 0.7 V diode drop from the +5.1 V level resulting in +4.4 V being applied to the V+ pin. The OPA2347 draws only 20 uA so if the current isn't limited to less than that, the op-amp becomes fully powered. It may be powered, but not in its linear operating range. The positive common-mode range is exceeded and the output could assume some unexpected level. The amplifier may be okay with arrangement as long as the maximum input current rating isn't exceeded. Sometimes there are other circuits connected to the V+ line and they too can draw current through the input. TI does not warranty products that have been placed in a use where the Absolute Maximum Specifications have been exceeded so it best to avoid any circuit where tha might happen.

    Here is a blog I wrote where some of the things I have described are illustrated:

    http://e2e.ti.com/blogs_/b/precisionhub/archive/2014/02/11/the-mystery-of-the-depleted-coin-cell

    Regards, Thomas

    PA- Linear Applications Engineering

  • Comment from my customer:
    I came across the part number LM293-EP, which is a dual differential comparator. It has an absolute maximum rating for input voltage of -0.3V to 36V on either input. As per my understanding this indicated that there is no ESD protection diode from either input to the V+ pin. Hence if the comparator is unpowered no voltage will be generated on V+ pin. But the datasheet does not mention about any ESD protection.
    Kindly let me know if my understanding is correct. Also it will be great if you could provide insights on the ESD protection that might be present in above mentioned part.