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OPA4197: Input Protection for OPA4197 and Devices Equivalent to OPA4323

Conor
Guru 12745 points
Part Number: OPA4197
Other Parts Discussed in Thread: OPA4323, , OPA192, OPA388, OPA320, OPA328, OPA322

Tool/software:

Hi,

I would like to inquire about the input protection behavior of the OPA4197 and possible alternatives to the OPA4323.

1. Behavior of OPA4197 when exposed to a sustained overvoltage condition (approx. 30V):

We are currently evaluating the OPA4197 as a replacement for the LTC6088. In our application, it is possible that a fault condition, such as a short on the sensor side, may result in a sustained overvoltage of approximately 30V being applied to the op amp input.

According to Figure 52 of the OPA4197 datasheet, it appears that the device includes internal clamp diodes to both supply rails.

The analog input circuit when using an OPA4197 instead of the LTC6088 is assumed to be as follows. I am wondering if the IC will be damaged if a steady voltage of about 30V is input due to a short on the sensor side. Am I correct in assuming that the clamp diode will function normally even in this case, and that the diode and IC will not be damaged? I don't think there will be a problem with the input current, but I would like to confirm this.

2. Devices equivalent to OPA4323 with internal clamp diodes similar to OPA4197:

We are also considering devices with similar specifications to the OPA4323 that include internal clamp diodes on the input pins. The ability to handle occasional or fault-level overvoltages with proper current-limiting resistors would greatly simplify our system design.

If you could recommend any devices with such characteristics, it would be greatly appreciated. If no suitable devices exist, we plan to consider adding external clamp circuitry.

Thanks,

Conor

  • 0
    TI__Mastermind 25998 points

    Conor,

    1. There are two important things to consider when protecting op amps with input steering diodes like the OPA192.
      1. You must limit the maximum input current to 10mA or less.  Typically I try to limit the current to 1mA for a safety margin.  The way you do this is that you place series limiting resistors and choose the value according to the maximum fault voltage.  For a 30V fault voltage and a 3.3V supply, you would calculate as follows:  R = (Vfault -0.7V - Vs)/1mA = (30V - 0.7V - 3.3V)/1mA = 26kΩ.  Your input already has resistance larger than this so that is good.
      2. You must also protect the power supply.  This may seem strange as the fault is on the input, but the fault voltage will be channeled through the input to the power supply.  Most power supplies are regulated with an low dropout regulator (LDO).  LDOs often cannot sink much (or any) current.  Also,  the may not respond quickly to transients.  Thus, when the fault happens on the input the diodes channel the fault to the supply and if the LDO cannot sink the current and maintain regulation the supply voltage will rise.  The rising supply voltage can damage the device or other devices on the same bus.  To fix this issue you should add a TVS diode to the supply.  The reverse standoff voltage should be equal to or greater than the power supply voltage.  Also, ideally the TVS breakdown should be lower than the absolute maximum supply voltage for the devices on that power supply bus.  In your case, the absolute maximum is 40V so the OPA192 is pretty safe against supply damage.  Nevertheless, I would add a TVS to the supply.  This is especially important if other devices on the same supply bus have a lower absolute maximum.
      3. Look at op amp precision labs section on electrical overstress for a detailed explanation of this protection scheme.
    2. For alternatives to OPA4323, the key characteristic is "zero-crossover".  This means that the device is rail-to-rail but does not have input crossover distortion.  If your main interest is other devices with zero-crossover, the OPA320, OPA328, OPA322, and OPA388.  Op Amp Input and Output Swing Limitations explains the benefits of zero-crossover devices.

    best regards, Art