# LM317 temperature [duplicate]

I recently bought this LM317, and I connect it with resistances in order to get 4.2 V: LCSC LM317 clone product page

The problem is when I connect it with a 12 V 500 mA power supply it gets extremely hot, like you can touch it without getting burned.

I recheck the datasheet it says that it support 1.2 A and I only use more or less 500 mA max.

• Is this normal?
• Do I have to check my schematic again?
• It is the same thing with a bigger SMD LM317?
• 12V-4.2V=7.8V. 7.8V*0.5A=3.9W is dissipated by the LM317. So, thats why it becomes hot. You need quite some copper area on your PCB to cool that – Huisman May 16 '19 at 4:19
• Lol, I love how they even steal the example schematic pixel-by-pixel from the Fairchild LM317 datasheet. Mehdi, while every linear regulator has to dissipate the energy difference between high input and lower output voltage times current as heat, I wouldn't trust a single number from this data sheet – someone not even willing to draw their own schematic in a data sheet is probably also not doing proper testing and thermal qualification on their components. – Marcus Müller May 16 '19 at 7:53
• @MarcusMüller you propably right !! it get extremly hot it's scaring – Mehdi May 16 '19 at 13:38
• no, it's not, anything dissipating close to 4W in a package that size will get hot. Read Humpawumpa's answer, please. – Marcus Müller May 16 '19 at 13:39

The LM317 is a linear regulator that means, as mentioned in the comment, it has to dissipate the energy resulting from the voltage drop between its input and the regulated output:

$$P_\text{diss}=(V_\text{in}-V_\text{out})\cdot I_\text{out}$$

The datasheet doesn't list any maximum rating for power dissipation but a maximum junction temperature 150°C. So you'd have to check the thermal information of your type and try to calculate the required cooling.

Another option would be to use a switched regulator which have a much better efficiency.

• The LM317 does not dissipate the voltage difference. The correct way to say this is that it has a voltage drop across it's input and output terminals. – Bart May 16 '19 at 7:23
• thx for the input :) – po.pe May 16 '19 at 7:32
• i will do this thank you for the support – Mehdi May 16 '19 at 13:40

With Humpawumpa's answer in place, we know that every linear regulator will produce the power $$\P_\text{diss}\$$ as heat. In you case, that is up to 3.9 W.

# But how much hotter does your package get?

So the Changjiang data sheet claims a thermal resistance of 5°C/W, i.e. for every watt you dissipate, your device gets 5°C warmer, if the pins of the package are connected to an infinitely good cooler. If they are not, it will be warmer.

However, that claim is wrong: It's from the Fairchild LM317, from where also the schematic in the data sheet is stolen:

# Fairchild data sheet ca 2001, drawing probably 2e1 years older

Such low thermal resistances are achievable with large packages with large thermal contacts, such as the TO-220 package of the original LM317:

# Conclusion

So, someone simply copy-and-pasted together the datasheet of your version of the LM317. All bets are off – whoever sells these regulators clearly didn't have the intention to spend money on thermally qualifying their devices.

# Course of action

So, throw away your LM317. Instead of buying from sketchy Chinese sellers, selling sketchy chines clones of simple devices, buy from a reputable distributor selling original parts from TI, ST, On semiconductors or Analog Devices – these all actually test their chips.

Still, in your usage scenario, no matter which chip you'll buy, it'll get hot – use a switch-mode power supply instead if that bothers you.

• Just a comment- LCSC (distributor) and Changjiang (manufacturer) is pretty much the equivalent of buying from a Chinese Digikey or Mouser. In no way is that like buying stuff on eBay or Aliexpress where it could be counterfeit or remarked. – Spehro Pefhany May 16 '19 at 11:28
• I read the 5°C/W and $P_{MAX}$=20W and I started laughing. – StainlessSteelRat May 16 '19 at 11:57
• @SpehroPefhany didn't know that, and tbh, Changjiang's blatantly ripped-off datasheet doesn't make a strong case for that – Marcus Müller May 16 '19 at 13:16
• @MarcusMüller i will check the switch mode method instead, thank you very much – Mehdi May 16 '19 at 13:43
• is it a good practice to use switching regulator when i have a GSM/GPRS and RF in my PCB ? is it ok for the noise ? – Mehdi May 16 '19 at 14:23

A heat sink must be attached with paste and screw if you want to keep your IC safe.

• won't help. The junction-case thermal resistance is probably dominant here, and you can't remedy that with an external heat sink. Also, this is an SMD package that can't be screwed. – Marcus Müller May 16 '19 at 15:14