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OPA548: Thermal shutdown hysteresis

MC Terb
Prodigy 10 points
Part Number: OPA548

According to the OPA548 datasheet the device shutdowns at 160 degC and resets from shutdown once the junction returns to 140 degC.

I am driving a 8 ohm resistive load with a push pull configuration, at 8 V across the resistor to produce about 1 A.

See the schematic of one of the drivers:

At a heatsink temperature of about 70 degC, the voltage across the resistor drops momentarily and the E/S pin is pulled low momentarily. I will make a scope plot tomorrow, but it definitely drops out for less than a 100 msec.

I expected the junction to reach the thermal shutdown temperature, and then only after some **longer** time switch back on.

The TO-220 devices are mounted on a heatsink with a insulating thermal pad. I decreased the thermal resistance by changing the pads to Polycrylic Graphite Sheets, but the problem still occurred, only at a higher heatsink temperature (80 degC).

What I don't understand is how the device can shutdown and re-enable so quickly? Could this point to another problem?

Thanks,
Martin

  • 0
    TI__Expert 6275 points

    Refer to link for some good thermal analysis data:

    4. TI Precision Labs - Op Amps: Power and Temperature

    In addition with a 28V supply and 8V out you have a 20V drop across the op amp output.  At 1A out that is 20W dissipated inside of the op amp.

    Assume Ths=70C, Rojc = 2.5C/W and Rocs=1C/W this means TJ=(3.5C/W * 8) + 70C = 98C.  So I am suspicious of your heatsink temperature right at the part.  And also research the thermal impedance of your insulating thermal pad.  Best thermal interface is to float the heatsink and use thermal grease which is usually only 0.1C/W thermal impedance. I assumed in my calculation 1C/W for thermal washer.  Also be careful of mounting torque on TO-220 which can flex the steel tab and crack the epoxy attach from die to metal slug. See attached for recommend mounting torque.

    0576.OPA549 Mounting Recommendations.pdf

  • 0 in reply to Tim Green1
    TI__Expert 6275 points
    Correction in previous post TJ = (3.5C/W *20W) +70C = 140C. Again need to check all of your thermal impedances and true heatsink temperature right at the mounting of OPA548.
  • 0 in reply to Tim Green1
    TI__Expert 6275 points
    100ms is not unexpected for cooling back down once all of the power dissipation has been removed form the output stage. Running a part continuously into thermal shutdown is not recommended as it will decrease the life of the semiconductor. The cooler the junction stays the longer the part lives. To reduce power dissipation consider lowering your supplies as low as possible to deliver the 8V plus some headroom of about 3V for OPA548 - so say 12V supply.
  • 0 in reply to Tim Green1
    TI__Genius 11135 points
    Martin

    We haven't heard back from you so we assume you were able to resolve your issue. If not, post another reply below.

    Thanks
    Dennis