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OPA197: Output lightning protection test with product unpowered

Part Number: OPA197

Hi,

I would like to perform an output lightning protection performance test on an unpowered circuit composed of an OPA197 followed by passive components and a TVS diode.

In order to setup my test, I would like to have further information than what I found in OPA197 datasheet.

I found this figure concerning its output impedance, which is applicable with VS = ±18V, but my test must be done with OPA197 not powered:

I tried to determine unpowered OPA197 output impedance by generating a DC voltage on its output and measuring its current, but the result seems to depend on output voltage applied to the amplifier.

So my first question is: can you provide me any information on OPA197 output impedance when it is not supplied?

 

In order to perform my test, I use a generator able to make a pulse of 70V during 1ms (this time takes into account the rising edge time of the pulse which lasts around 500µs). The voltage seen by OPA197 depends on its output impedance because of external components:

On this circuit, VOUT represents the pulse generator (Vout = 70V if the TVS diode is not present, else around 57V).

Could you tell me how much power OPA197 can dissipate in these conditions, or how much current can flow on its output?

Regards,

Kevin Marchenay

  • Hi Kevin,

    problematic is the case when Vout exceeds the rails. Then, the -input pin and the output pin of OPA197 are in danger. So, I would recommend to use an enhanced protection scheme:

    The protection diodes should be Schottkys.

    Also, you should increase C1:

    Kai

  • Hello Kevin,

    In addition to the extensive protection advice Kai provided here are some answers to your questions:

    I tried to determine unpowered OPA197 output impedance by generating a DC voltage on its output and measuring its current, but the result seems to depend on output voltage applied to the amplifier.

    So my first question is: can you provide me any information on OPA197 output impedance when it is not supplied?

    The open-loop Zo vs. frequency measurements that we perform on out Precision Amplifiers Op amps requires a sophisticated set-up, and has only been accomplished under powered conditions. This has not been done for the unpowered state and would be expensive for us to do this for a likely one-time request.

    The OPA197 output in the unpowered state looks into unbiased, off-state MOS output transistor structures, and ESD protection diodes to the supply rails. Changing the voltage applied to that pin will affect the leakage currents and the bias on the ESD diodes. Therefore, the off-state impedance will change with voltage level and that would be expected. We don't often characterize off state conditions because they usually aren't a concern.

    Could you tell me how much power OPA197 can dissipate in these conditions, or how much current can flow on its output?

    With the supply pins grounded as shown in your schematic a positive voltage applied to output pin would turn on the upper, output ESD diode that can be seen in datasheet Figure 52, "Equivalent Internal ESD Circuitry Relative to a Typical Circuit Application." If forward biased that diode will become the primary current path from the output pin through V+ which is grounded in the above schematic. The diode will clamp the voltage at a forward voltage around 0.7 V, dependent on the current through the diode. The OPA197 ESD diode continuous current is rated at 10 mA maximum so that would become the limiting factor.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Hi,

    Thank you both for your detailed answers. I am not the designer of the circuit so I cannot add an extensive protection to OPA197 output.

    I assume that the output voltage of OPA197 will always be around 0.7V in my test conditions, considering the answer of Thomas.

    I have now another issue: I must inject a minimal current of  in R1 of my first schematic to reach a test voltage VOUT = 70V, far from the limitation of 10mA you gave me.

    Considering that the limitation of 10mA is defined for continuous current, is it possible to have a higher current limit if VOUT is a pulse voltage of 1ms? Do you have a current vs pulse time measurement available for OPA197 ESD diode?

    Regards,

    Kévin

  • Hi Kevin,

    maybe I'm wrong, but I think that 57mA is not acceptable, even not for 1ms.

    Kai

  • Hi Kevin,

    We haven't any specific information about the OPA197 ESD diode maximum current vs. stress time, but we do know the diodes must be able to withstand the high current levels associated with out-of-circuit HBM and CDM ESD tests. So for a very short period of time such as that associated with an ESD event, on the order of hundreds of nanoseconds or microseconds, the current can surge to hundreds of milliamperes. However, even though the OPA197 must withstand and tolerate such testing no specific current maximums are implied or stated in the datasheet.

    I do know that the ESD diodes in our Precision Amplifiers Op amps are conservatively rated at 10 mA continuous. When I have cranked the diode forward current up on the bench to 100 mA none have failed, but note I couldn't tell you which Op amp was testing at the time. Using a maximum current of 57 mA and 0.7 V forward voltage results in a power dissipation of 40 mW. I suspect the OPA197 can safely dissipate that power level if the duration is kept short - milliseconds. Because that current exceeds the maximum of 10 mA TI cannot make any assurances about what might happen. Nonetheless, I do think the OPA197 will survive the test if you decide to try it.

    Regards, Thomas

    Precision Amplifiers Applications Engineering