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OPA454: OPA454 Output Current Limiting

Part Number: OPA454
Other Parts Discussed in Thread: STRIKE, REG1117, LM317HV, LM317

I am looking for some reference information or recommendations on how to best limit the current of OPA454.

I have an application where I want to do some voltage/current sweeps on some units using a signal generator, a series current sense resistor and a couple of scope probes similar to a curve tracer but in an embedded application.  My signal generator is only capable of signals between -5V and +5V and I want to use the OPA454 with a 10x gain that would get me into the +/-50V range. 

However, I would like to be able to set a maximum failsafe current limit on the output to a level less than 50mA as a safety check to prevent damage to my DUT.  My intended DUTs are actually going to be potentially damaged devices that I want to be able to be able to compare the IV curves of a known good device against a potentially compromised device that has suffered an ESD strike, or other malfunction.  I just don't want to create additional damage to a pin if it is unable to sink maximum 50mA output of the OPA454.

I have seen a variety of methods and information on how to create current limit circuits.  One method is to limit the current into the V+ and V- supply rails and there are many other methods using a variety of transistor and resistors to create a sense loop to control a pass transistor that could throttle the current flow of the output signal itself. 

In the datasheet there is an example of how to increase the output current with an external NPN and PNP transistor.  Is there a recommended method for reducing the output current for this device without creating a lot of signal distortion?  What other factors do I need to consider and problems I may need to avoid?  I haven't done a lot op amp designs more involved than a basic non-inverting buffer with some small amount of gain.  If you have any other thoughts and suggestions I would also appreciate those.

Thanks!

Jonathan

  • Hi Johnathan,

    Implementing an output current circuit will certainly alter the normal operation of an OPA454 op amp circuit and likely affect the otherwise low distortion should current limiting occur. We have not developed such circuits specifically for the OPA454, but there may be some ideas worth considering that could be applied to it.

    There is a Burr-Brown Applications Bulletin from May 1995, that may serve as a starting point. The bulletin titled, "An Easy Solution to Current Limiting an Op Amp" employs REG1117 voltage regulators as current source and sink circuits in the power supply lines. It remedies some issues mentioned in the article. The circuit was designed for lower voltage op amps (+/-15 V) and not the high-voltage OPA454 (+/-50 V). Nonetheless, it might be applicable with similar three-terminal regulators rated for high-voltage. The Applications bulletin can be viewed here:

    www.ti.com/.../sbva011.pdf

    The TI LM317 three terminal regulator an application where it is employed as a constant current source. It might be usable in place of the REG1117. Additionally, there is the LM317HV version which is good for supplies up to 60 V. The high-voltage version might be pressed into service in place of the REG1117 used in the bulletin circuit.

    If the three-terminal regulator is configured as a constant current source set for 50 mA maximum, but the OPA454 circuit draws less current, then the regulator must operate at this lower current. When two current sources of different values are connected in series the higher current one is forced to operate at the same level as the lower current source.

    The datasheet for the LM317HV can be found here:

    www.ti.com/.../lm317hv.pdf

    See Fig. 25 in particular.

    This method certainly would require some development to assure that it does what is needed.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • Hi Thomas,

    Thanks for this information and suggestions.  I will pursue this further.

    I do have one follow up question.  By setting the LM317HV to 50mA, the OPA454 output can source up to 50mA since it will be the "lower" current source.  However, as an example, could I not also set the LM317HV to a lower current level such as 10mA and force the OPA454 to operate less than 10mA?  I assume this is possible since the majority of the 50mA will be sourced to the load.

    Regards,

    Jonathan

  • Hi Johnathan,

    I did some simulations based on applying LM317 three-terminal regulators as constant current sources in series with the OPA454 power supply connections. The LM317 regulators were set for a constant current of approximately 50 mA. Some protection diodes were added around the regulators to keep them safe in a real circuit. The schematic and transient response results are shown below.

    The upper graph shows the results when the load resistance is 5000 Ohms, which results in 15 mA of OPA454 output current; 10 mA through the 5000 Ohm load, and 5 mA through the feedback and input resistor path. Vo1 is a sine wave with 10x the amplitude of VG1. Even thought he current sources are set for 50 mA they operate at a little more than 18 mA because the OPA454 only draws that much. It comes back to the series current sources behavior I mentioned in my previous post.

    The lower graph is with the load resistance reduced to 500 Ohms. If the output voltage were allowed to swing to +/-50 V, the peak current through the load would be +/-100 mA. However, in this case with the LM317 set up to source a maximum of 50 mA the current cannot reach 100 mA. The AM1 meter indicates a transient peak current of about 64.8 mA, but after the short peak the current drops to about 50 mA. The output voltage Vo1 is clipped due to the current limiting.

    The OPA454 cannot be forced to operate normally with a current less than its natural quiescent current level. If the LM317 current sources were set to a level less than the OPA454 quiescent current level of the op amp I would not expect it to function correctly.

    Here is my TINA Spice file if you would like to try some simulations:

    OPA454_dual sply_01.TSC

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Hi Thomas,

    Thanks for the information and the simulation data!