I've looked at many posts in this forum about lm317 and its applications but never found the answer to the following thought:

According to the datasheet, the lm317 is rated to provide up to 1.5A for a Vin-Vout <= 15V. Above 15V (approx) the maximum current that the lm317 can provide decreases.

So, if we have Vin-Vout = 15V and Iout = 1.5A ; the maximum power through the lm317 will be 15x1.5 = 22.5W.

For a thermal resistance calculation for the heatsink I've got the following standard inputs:

Ta = 25°C

Tjmax = 125°C

Rth(jc) = 4°C/W (datasheet)

Rth(heatsink) = to be calculated

With the data given above, the maximum thermal resistance must not exceed 4.44 °C/W = (125-25)/22.5 = Rth(jc) + Rth(heatsink)

**Therefore Rth(heatsink) must be under 0.44 °C/W. Is this value possible to have?**

The result is even worst if using lm350, which allows up to 3A instead of 1.5A. In this case, the power through the lm350 will be 15x3 = 45W, which leads to a total thermal resistance of 2.22 °C/W but with a Rthjc = 4 °C/W (according to datasheet),

**the minimum value for Rth(heatsink) will never be reached !**>> the only solution is to share the power with additional device (two lm350 in parallel or additinal power transistor).

**Why the lm350 is rated to provide 3A when Vin-Vout = 15 if it cannot support a single configuration layout with heatsink ?**

Thanks in advance for your support,

Efron