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OPA541 (TO-3) Replacement, OPA548T Evaluation

Brian Gosselin
Expert 4745 points
Other Parts Discussed in Thread: OPA541, OPA548, OPA549, OPA544, OPA547

Hi Team,

My customer was previously using our OPA541 (TO-3) package on one of their designs, however, this device got discontinued, so they are trying to replace it with something like our OPA548T.  The problem is that the OPA548T appears to have a problem with driving inductive loads.  It basically breaks out to spurious operations on the negative rail when driving 1kHz sine wave into a 1mH load and it likes to oscillate.  Note that the OPA541 was able to drive this load pretty easily.

Attached are two captures.  One shows the bench setup which has a heavy inductive load and simulation results which has distortion.  The other shows the real bench results which has even worse distortion.  They can use in-loop compensation or out-loop compensation or other techniques to make it better, but this is not what they would like to do.  They really need a device t work in their existing design without any modification related to the stability loop.  Do we have other options?

Key specs were +/- 24V rail, would like output to support rail - 4V, 1-2MHz unity gain bandwidth, and at least 0.5A output current.

Thanks in advance for your support.

Best Regards,
Brian Gosselin

  • 0
    TI__Guru 63205 points

    Hello Brian,

    These are the typical output responses from a power op-amp experiencing an output stage oscillation.  The solution is to use the snubber network recommended in the OPA548 product datasheet.  The 10Ohm + 10nF combination shown below has been successfully used in a few other applications experiencing similar issues.

  • 0 in reply to Collin Wells
    TI__Expert 4745 points
    Hi Collin,

    Thank you for the quick feedback. My customer already did this and it worked, however, this is not what they would like to do as this would require them to modify their existing design. The OPA541 in the TO-3 package did not have this problem. Is there a different solution we could recommend that would not require them to add this snubber network/modify their design?

    Best Regards,
    Brian Gosselin
  • 0 in reply to Brian Gosselin
    TI__Guru 63205 points
    Hi Brian,

    The OPA541 shares the same "all NPN" output stage structure as the OPA548 and is therefore also subject to output stage oscillations when the snubber network is not included. We don't provide firm guidance on acceptable inductive/capacitive load ranges that don't require the snubber and instead generally recommend that all applications with these products include the snubber circuit if they are interfacing with an inductive/capacitive load. The customer's load must have been within the acceptable range for the OPA541 but is not for the OPA548. Does the system operate over a wide temperature range? Some similar issues in the past were got worse at cold temperatures.

    Devices other than the OPA541 that fit the basic requirements of this system are the OPA544, OPA547, OPA548 and OPA549. These products will all require a similar output pin compensation network to produce a stable response. Since we don't provide information on the acceptable range of inductance we always recommend including the snubber when driving a reactive load.