LM317A surface mount

[jai1980]

Hi! Dear.

Can someone please suggest me how to make calculation for heatsink ? In my application I am using LM317A with 12v input volt. 1A is going to be output current. And output volt is going to be 4v. Could you please explain me how much heat dissipation would take place ?

And what size of heat sink should I use to prevent my board getting damage ? Is LM317A good enough for my application or should I try some thing else ? As I thought of using some resistor in series between supply volt (12v) and Vin of LM317 . But I am not sure whether it would be of great help ? If so what would be the value of resistor would i need ?

Please help me as I am just begginer in the design engineering.


Many thanks.

Jainish.

 

[MikeMl]

It will not work. You are dissipating (12-4)*1 = 8W, way beyond the capacity of a surface mounted part!!!!

 

[crutschow]

To reduce the regulator power dissipation you could add a power resistor in series between the supply and the LM317 input. The LM317 needs at least 2V across it, so you could drop 6V @ 1A (6Ω) for a resistor dissipation of 6W (use at least a 10W resistor). This leaves 2W that the LM317 has to dissipate.

Edit: The maximum power dissipation in the regulator will occur when the LM317 equivalent resistance is equal to 6 ohms. The current at that point is 8V/12Ω = 2/3A and the dissipation is 2/3A * 4V = 2.67W. Thus interestingly, with the resistor the maximum LM317 power dissipation does not occur at maximum output current but at 2/3rds maximum.

 

[MikeMl]

To reduce the regulator power dissipation you could add a power resistor in series between the supply and the LM317 input. The LM317 needs at least 2V across it, so you could drop 6V @ 1A (6Ω) for a resistor dissipation of 6W (use at least a 10W resistor). This leaves 2W that the LM317 has to dissipate.

This will only work if the load on the LM317 is fairly constant, and never exceeds 1A, otherwise, the 317 regulator will drop out of regulation. This still dissipates 8W, just that less of it is in the LM317.

At 8W of dissipation, you should really be looking at a switch-mode buck converter.

 

[jai1980]

Hi! Mike and Carl.

Thanks very much for such a prompt response. The issue with me is I am trying to use this 12v supply for two reasons as with two LM317 I am trying to get 4 and 9.5v. It seems heat sink would be of good help and will work fine for 9.5v configuration. But I am really worried for 4 v configuration. As would I be able to use heatsink which can absorb this heat and of course it has to be not massively large I mean fairly complex and small if possible. Or should I add some resistor before the input of LM317 for getting 4v configuration.

If so what could be the value of my resistor ? Or can I use some series or parallel combination to drop around 6-7 Watts before it leaves around 2 watt for LM317 to dissipate.

Please guide me through.

Many thanks in advance.


Sincerely,
Jainish.

 

[MikeMl]

If you need 9.5V, use one regulator to get that, and then also use that as the input to the second regulator, which reduces 9.5V to 4V. Depending on the currents needed at 9.5V and 4V, this is likely a better solution.

 

[carmusic]

for the 4 v output i would use this
**broken link removed**

and in serie a low dropout regulator like this one LM2941
you would have only 1w dissipation at 1A for the lm2941 and few mw on the swithing regulator.
if you dont need a precise 4V you could replace lm2941 by 1 or 2 diode in series to drop output voltage near 4V