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LM1085: Current issues for 5V regulator

Part Number: LM1085

I am using a LM1085IT-5.0/NOPB in a very simple circuit.  I have pin 3 connected to +24V and pin 1 connected to the ground of the 24V supply (without a load the output is 5V as expected).  When I connect a load of 3.34 ohms, the output drops to between 1V-1.5V.  My understanding is that this regulator is capable of 3A.  What am I overlooking?  Thanks

  • Hi Jeff,

    While you are correct that LM1085 is rated for up to 3 A, there are a couple things that could be limiting your output voltage. With 19 V of headroom (Vin - Vout) and a 1.5 A load you are attempting to dissipate 28.5 W. With a junction to ambient thermal resistance of 22.8 C/W, your junction temperature would be ~650 C above your ambient temperature at your attempted conditions well exceeding the 125 C junction temperature rating of LM1085. The reason you are not reaching the expected 5 V output is likely because you are triggering the thermal protection.

    It would be recommended to reduce the input voltage in order to reduce the power dissipating in the regulator.

    Very Respectfully,
    Ryan