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LM2931 LDO Thermal Shutdown is greater than Junction Temperature

Kieran Dobbyn
Prodigy 120 points

Hello,

We are using the LM2931 and tests whereby we short the output to simulate a fault show that the case temperature rises above 150C and the device enters Thermal Shutdown with reduced input current.  This is as noted in the datasheet and is a common feature of linear regulators.  However, the datasheet also states the Max. Junction Temperature as 125C.  Therefore, when in Thermal Shutdown the device is exceeding the max. Junction Temperature.

Why is the Thermal Shutdown temperature not set below the max. Junction Temperature?

What is the impact of this on the device.  Does it affect reliability?

Some other forum posts suggest ensuring the device does not enter Thermal Shutdown.  This is not really a solution as it negates the Thermal Shutdown feature.  Why have a device with inbuilt Thermal Shutdown if it should not be used?

Our concern relates to the case temperature the device reaches when in Thermal Shutdown.  For our application, this high temperature is not acceptable.

Any comments or suggestions?

Regards,

Kieran

  • 0
    TI__Genius 13850 points

    Hello,

    Intend of the Thermal Shutdown protection mechanism is to prevent IC component to blow up or burn immediately when overheat condition occurs due to ambient temperature rise or short circuit of a device output. Exceeding this limit would permanently damage the device. This allows device to recovery from overheat condition after temperature cools down or short circuit condition ends.

    Maximum Junction Temperature is the limit at which IC is still operational. System level design should specify maximum ambient temperature and all devices should be designed so that those do not exceed Maximum Junction Temperature at that point. Operating at Maximum Junction Temperature or higher temperatures will reduce device expected operational life time significantly and is not acceptable.


    In a case that short circuit happens device Junction temperature will rise up very quickly. In thermal shutdown mode device cools down and starts up again and hits thermal shutdown if short circuit is present. In a system level there should be some sort of supervisory which shuts down power if there is short circuit and power rail fails.