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

Riya Gupta
Prodigy 110 points
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?

  • 0
    TI__Mastermind 43630 points

    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.

  • 0 in reply to JCHK
    Prodigy 110 points

    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.

  • 0 in reply to Riya Gupta
    TI__Mastermind 43630 points

    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?