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LM340T to LP2950 heating up

Giuliano Messina
Prodigy 200 points
Other Parts Discussed in Thread: LP2950

Hello,

I'm attempting to use the 12V output of a LM340T (12V LDO) as the input for a LP2950 (5V LDO). Both have decoupling capacitor values as per datasheet recommendation, and they are being connected through a ~1ft RJ45 cable.

(The LP2950 goes to a AM26LS31CN)

My problem is that when they are connected, both LDO's begin to heat up.  I have tested with an ammeter and the demand is only 50mA (half the LP2950 max spec, and well below the LM340T's 1A max).

Any ideas as to what could cause this?  Thanks.

  • 0
    TI__Mastermind 22610 points

    "... Any ideas as to what could cause this? ..."

    Yes, this is most likey internal power dissipation.

          Pdiss = ((Vin -Vout) x Iout)

    For LP2980 the internal power dissipation in the package is ((12V-5V) x 0.05A) = 0.35W.

    You don't mention specific part numbers, or which package, but if the package thermal resistance (theta(j-a) ) is 160°C/W then the internal temperature will rise (160°C/W x 0.35W) = 56°C above the external ambient air. The case temperature will be close to that, so the package will feel warm to hot.

    Same concept applies to the LM340T.

  • 0 in reply to Donald Jones
    Prodigy 200 points

    Thank you for the response, it's a big help.

    How would you recommend combating this?  Perhaps a zener diode at the Vins for both LDO's to bring the input voltage closer to the output?