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LM317: Confirming details about power dissipation

Roland Melad
Intellectual 1185 points
Part Number: LM317

Hi,

We have a customer with a concern as below.

I'm using  LM317 for a 24 volt output. The input is at 35 volts. The load on the regulator is about 700 m.

1. what is the max operating temp of LM317 before the LM317 folds back and with this much heat dissipation.

2. is this beyond the limits of a TO-220 and should I consider a TO-3 instead?

Appreciate your help.

Roland

  • 0
    TI__Genius 14245 points

    Hi Roland,

    7.7 watts of power dissipation is a lot for any LDO in any package. On a standard JEDEC High-K PCB the TP-220  has a junction to ambient thermal resistance (Rja) of 23.5C/W so the junction temperature would rise ~181C above the ambient temperature. With a better PCB layout the Rja could be reduced by ~35% for this type of package (see this App Note for more details) but that would still result in an increase of ~118C above the ambient temperature making it difficult to keep it within the rated maximum junction temperature of 125C. 

    Have you considered using a switching regulator with higher efficiency or multiple LDOs in series to spread the power dissipation and limit the junction temperature?