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REG103UA-5.0 is turning itself off

Randy Mitchell
Intellectual 330 points

REG103UA-5.0 SOIC package is turning itself off every 10 sec and restarting 0.5 sec later. The part is powering a TI/LOGIC SOM6748 DSP and the current draw is 160 to 240ma as Code Composer Starts, but the 5V supply keeps dropping out. It appears to be a thermal problem but the SOIC package doesn't require a heat sink (according to the spec sheet). Temp. probes show that the case of the part is only gettting to 54 degrees C and doesn't cycle as the part turns itself on and off -- and the part is only turned off for roughly 0.5 sec.  Placing a cooling fan above the part doesn't appear to make a difference. It seems like a thermal problem but might be a transient problem. Placing various value caps between Vout, GND and Pin 3 only appear to make things worse. Any thoughts as to what might cause the REG103UA-5.0 to drop out and restart itself every 10 sec or so?

  • 0
    TI__Expert 8890 points

    Hello Randy,

    You are probably correct that the part is over heating.

    I can't account for the thermal time constant that seems to be keeping the case temp at 54 degC average.  And I don't think that it is unusual for the junction temperature to heat more slowly than then it cools to cause the 10second-0.5second cycling.

    The SOIC package has a thermal resistance of 150 degC/Watt, junction temperature to ambient temperature.  The other packages offer better mechanisms for dissipating heat - in that they have large thermal tabs that can be soldered to the ground plane.

    The thermal equation to use is Tj = Ta + Pd*Theta, where Tj is the internal junction temperature, Ta is the ambient temperature, Pd is the power dissipated in the part and Theta is the thermal resistance.  By my calculation the part should be over heating if the output current is 240mA and the input voltage is more than 3.5V above the output voltage - this will cause the junction temperature to increase above the thermal limit and to shut down the part (for an ambient temperature Ta=25degC).

    I have attached a good app note discussing this issue.

    1376.Thermal Pkg Limits Scot Lester sbva027.pdf

    Regards

    Bill