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OPA348 Output state when no VCC

Kelton Stefan
Prodigy 75 points
Other Parts Discussed in Thread: OPA348

So, I'm in a situation where I need to use a differential amplifier circuit to output 1.2 volts when the difference between two voltages is 57V. I've come up with an OpAmp circuit that seems to simulate correctly, but I'm concerned about what might happen if the inputs are "on" before VCC comes on. If the output of the OpAmp is floating, then the inverting input cold get a voltage of up to 72V. I was looking at the OPA348. Do I need to protect the inverting input with a zener perhaps?

  • 0
    TI__Guru* 93105 points

     

    Hello Kelton,

    The OPA348 has ESD steering diodes connected from each input to each supply rail. Normally these diodes are off when the amplifier is installed in a circuit with normal supply voltage applied. If the +3.3 V supply is not turned on, or is isolated by an on-off switch, that portion of the circuit probably looks like a high impedance. Then, if a high voltage is applied to an input current will flow through the 475 k resistor, through the ESD steering diode, and then to the OPA348 V+ pin.

    The OPA348 will become powered through the input resistor and internal ESD diode. It typically draws 45 uA of quiescent current. The voltage across the 475 k resistor would be about 21.3 V, and then the ESD diode would drop another 0.6 V, when the OPA348 has about 2.5 to 5.5 V applied to the V+ pin. The input would be at 24.4 to 27.4 V for that to happen. However, if the input voltage increases towards 72 V the V+ applied to the OPA348 would be high enough to damage it.

    Placing a zener at the inverting input to ground would clamp the voltage, but may cause other problems. The zener capacitance in conjunction with the feedback resistance will degrade the circuit's phase margin and stability could become a problem.

    Likely a better place to add a diode clamp would be at the OPA348 V+ pin. I suggest adding a 6 V transient voltage suppressor (TVS) at that pin. The TVS cathode connected to V+ and its anode connected to ground. If the V+ pin voltage attempts to rise above 6 V before the V+ supply is present, the TVS will turn on and clamp the voltage at about 6 V. The current through the input circuit and TVS diode will be limited to less than 150 uA when the diode is on. The ESD diode can handle 10 mA dc so the 150 uA will not be an issue.

    Once the supply comes up the TVS cathode will be forced to 3.3 V and it will turn off. The OPA348 will assume normal op amp operation and the inverting input, the summing node, will move very close to the common-mode voltage developed at the non-inverting input.

    Littelfuse, and other manufacturers, provide a wide variety of TVS devices. A low current, unidirectional TVS should be adequate for you application.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 75 points

    Thanks so much Thomas!

    Kelton