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OPA548: Output disable/re-enable

William Brumby
Prodigy 10 points
Part Number: OPA548
Other Parts Discussed in Thread: OPA548

Hello,

I am using an OPA548 to drive 5Hz 1Apk signal into a 7.05ohm, 4.52mH RL load as shown. The E/S pin is on the output of a normally-open opto-relay for enable/disable control. After the part heats up a bit (tab of the package reaches around 104degC), the output begins to disable and re-enable in 15-30ms spurts around the peaks of both the positive and negative cycles. It continually gets worse until every cycle disables/re-enables as it gets hotter.

I verified that the E/S pin is toggling to disable/re-enable the output, and it is not being commanded to do so.

My question is if this part is damaged, or is this normal behavior? A 30ms dropout seems suspect for thermal shutdown, as the datasheet specs the output will not re-enable until die temp drops back to 140deg. If this part does appear to be damaged, could this be due to disconnecting the load with the output still enabled? Could this cause a large current spike that would damage the part? Or instability? Any help would be appreciated.

Thank you,

William

  • 0
    TI__Guru* 93105 points

    Hello William,

    The OPA548 thermal shutdown will occur if the junction temperature (Tj) exceeds about 160 C. Once shutdown occurs and output current ceases the device cools down. As the Tj drops below 140 C the OPA548 re-enables and will begin to operate. However, if the same thermal conditions exist the cycle will be repeated and thermal oscillation can occur. I have observed it on the bench occurring at the peaks of a waveform and it can be at a faster rate than we might expect.

    The OPA548 is designed to go into thermal shutdown to protect itself and normally won't be damaged by being exercised in that manner. However, it is a stress condition involving heating the Tj in excess of it Absolute Maximum Rating of 150 °C. Therefore, it is much better to provide an adequate thermal system for the OPA548 so that it isn't thermally overstressed and Tj remains below 150 C. I suggest you unload the OPA548 output and see if the op amp functions correctly when exercised as an op amp. If it is damaged it won't produce the correct output.

    One the easiest ways to damage the OPA548 is with an electrical overstress (EOS) condition at on one its pins. It is easy to unintentionally have such an occur at the output pin when driving a reactive load. Reactive loads can present a back EMF condition and attempt to force high current back into the output transistors. The datasheet Figure 38 Motor Drive Circuit shows how current steering power diodes are added from the output pin and each supply pin to help protect against back EMF. Today, we also recommend adding transient voltage suppressors (TVS) diodes to the supply pins in addition to the current steering diodes. They can provide a return path to ground for that current during a back EMF, or EOS event.

    Regards, Thomas

    Precision Amplifiers Applications Engineering