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LM46002 - thermal shutdown not working

Kristijan
Prodigy 30 points
Other Parts Discussed in Thread: LM46002

Hi!


We are having issues with the thermal shutdown of our power supply with the LM46002. The schematic and other information about our power supply are described here.

We are artificially heating LM46002 with a heating gun and instead of shutting down it stays ON (LEDs blink instead of complete shutdown). Highest testing temperature was 200°C and aluminum heatsink was removed! In datasheet it is sad that shutdown temperature is 160°C and that it does not restart until temperature drops to 150°C.  In our case it restarts all the time, without any hysteresis!


Additionally, we had one chip destroyed probably because thermal shutdown is not working properly. The input voltage was set to 56V, with the switching frequency (deliberately increased)  540kHz (82k resistor) and output voltage 4.2V. After approximately 5 minutes chip just burst (you can see it on photo) without going to thermal shutdown!

Burst chip


  • 0
    TI__Guru 72035 points

    It looks like you have two issues.  Regarding the TSD, how did you measure the temperature ? Where is the heat-sink you mention ?

    It looks like an over-voltage event in you photo.  Be sure that the input voltage does not exceed the ratings, even for a transient.

    What load was applied when the damage occurred ?

     

  • 0 in reply to Frank De Stasi
    Prodigy 30 points

    1.We conducted our tests once more and concluded that thermal shutdown is working correctly. First we didn't measure the case temperature, but in new tests we measured it and the thermal shutdown was working properly. Sorry for inconvenience.

    2.  We are not sure it is overvoltage, when max. VIN that our laboratory power supply can give is 56V . There is no ringing on SW pin or VIN pin!) Maybe this is due the Ton and Toff parameters of internal transistors, but as we said before Webench tool is claiming that everything (fsw, VIN and VOUT) is inside specs. The applied load was 2A (loaded with LEDs and drivers!).

    3. Can you describe the operation of LM46002 when Tjuction exceeds 125°C and before it reaches 160°C (thermal shutdown). Is it possible that it works in some sort of a hiccup mode - our LEDs (load 2A) blink all the time.

    Sorry for all the hassle! We need to be sure for all possible operating modes (normal and irregular) before we can let it in production.

  • 0 in reply to Kristijan
    TI__Guru 72035 points

    It is possible that the device will not provide full load at at input of 56V at high ambient temperature.  In that case the junction will exceed the TSD threshold

    and the device will cycle on and off.  What is you ambient for this test?

    Regarding the damage, were there any transients during the test ?? Such as a load or EN cycle ?.  What was the ambient for this test ?

  • 0 in reply to Kristijan
    TI__Genius 11085 points
    When the junction temperature is above 125°C and below thermal shutdown temperature, the LM46002 will continue working as it should be. But the accuracy of the output voltage and current limit might shift due to over temperature. The life span of the IC will reduce as well. There's no hiccup mode based on over temperature other than thermal shutdown. When the LED is blinking, can you take scope shots of the output voltage and switch node to verify it is in ON/OFF mode or not? We do not recommend designing the LM46002 to operate above 125°C junction temperature due to reliability and life span concerns.
    Best, Yang