This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2940-N: I am driving 300mA load from 12V sot223 package but the LDO starts dropping output voltage and heats up

Part Number: LM2940-N

I added capacitors on input and output but the voltage is stopping and output is not stable.How should I correct it?

  • Hi Riya,

    Any Linear Voltage Regulator dissipates power as a function of (VIN-VOUT)*ILOAD. So if your output is 10V:

    (12-10)*0.3=600mW of power dissipation. For the SOT-223 package:

    The thermal rise on the die would be RthetaJA which is 59.3C/W times the power dissipation of 600mW plus the ambient temperature:

    TJ = TA+Power Dissipated*RthetaJA or

    TJ= 25+0.6*59.3=60.6 which would be fine. 

    However, if VOUT is 5V, then this changes:

    The power dissipation is (12-5)*0.3 or 2.1W.

    So TJ=25+2.1*59.3=149.5C which is going to trigger thermal shutdown. 

    Note: I used TA as 25C but actually in the application this should be the PCB ambient which could be higher.  Changing to the TO-220 package would help a lot.

    I hope this helps.

  • My application is Vout=12V ,Vin =25V,Load current =500mA.The output voltage is dropping from 12V to 7V.The datasheet says output current can be up to 1A with stable output.But it is not happening in my case.

  • Hi Riya,

    The device is electrically capable of supporting 1A, but thermally it depends on the voltage drop across the device and the current as described above.

    For VIN=25V and VOUT=12V at 300mA, this is 3.9W. 12V to 7V at 300mA is 1.5W of power dissipation. 

    I think you are running into thermal shutdown.

    Does this help?