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Linear vreg Thermal spreader

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Mosaic

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This is useful enough to bear repeating.
If u have a design and discover your v.reg is running too hot due to a large voltage drop, offloading the heaitng with a couple resistors in parallel can save the day.
I had a design that spec'd the D2 PAK size LM317 surface mount part. But due to limited PCB space it was downgraded to the smaller D-PAK case.
Eventually I discovered the unit was running at about a 72°C delta:arghh:.

No space for a heatsink...what to do?:confused:
The Vreg was dropping 8.4V @ 120mA.

I squeezed in a couple 39Ω , 0805 resistors in parallel and placed them 1mm away from the 1/4" spade terminal Vcc input for heat dissipation. I added a soldermask stop to permit thick solder on the 1mm PCB trace length. The resistors drop about 2.4V and have a 50°C delta. The LM317 also has a 50°C delta now.:) Without the 1/4" spade terminal the resistors have a 75°C delta.

The 50°C delta is useful as the module being designed might have to sit in subzero temps and not form condensation on the PCB. Sealed Enclosure Air volume is only 0.02L
 
I assume you mean the resistors are in series with the regulator input, not in parallel.
 
I assume you mean the resistors are in series with the regulator input, not in parallel.

If the load on the regulator varies, say +- 25% of its average load, you could put a resistor around (in parallel with) the regulator. That might help if the regulator is already operating close to its dropout voltage.
 
Actually, since my load is about 120mA, I did both series (20Ω net) and parallel (500Ω net) resistors to obtain a 35°C improvement in the V reg op temp.
 
i've used parallel resistance as an extreme last measure to quiet a noisy supply's switch element in few occasions
 
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