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Actually you can use a standard LM317. Max allowed difference between input and output voltage is 40V.
In your case the difference is only 48V-15V=33V
So there is no need to use the special high voltage type (LM317HV), just use standard LM317.
LM317 data sheet: **broken link removed**
If R1 is 237ohm 1%, then R2 should be 2.55kohm 1% for 15V output.
33V*300mA=9.9W
It is much higher than the power capability of LM317 alone, even using a heat sink regulation maybe affected by high chip teperature.
also as u know theinput voltage may be higher than 48V, therefore it must be concerned.
for your application there is 2 solutions:
1. analog regulator such as LM317 plus a power zener at input for voltage reduction. u can use several lower power zener in series also.
2. a better and efficient way but alittle more copicated method is using integrated switchers from several manufacturers like: www.powerint.com www.irf.com
or even if u want u can make your own SMPS, if so refer to: https://www.onsemi.com/pub/Collateral/HB206-D.PDF https://www.onsemi.com/pub/Collateral/SMPSRM-D.PDF
as i mentioned it is possible, but the power disspation is acceptable for your application?
again: it is better to use a zener at input to reduce power dissipation of regulator, obviousely power will be dissipated in zener so the zener must capable of dissipating it without damage. power zener are inexpensive and can be used in series for higer power.
You can also use a series resistor at the input of LM317.
Min Vin is Vout+3V=18V
Max voltage over resistor (or zener): 48V-18V=30V
Max resistor value @ 300mA max: 30V/300mA=100ohm
If you use a 80 ohm resistor you will get this dissipation in resistor @ 300mA: 300mA^2*80ohm=7.2W
The rest will be dissipated in LM317: 9.9W-7.2W=2.7W
You can use more resistors in series or parallel to spread the power dissapation.
Use a simple "pre-regulator" with a cheapest bipolar transistor ( e.g. 2N3055 or equiv.)+zener to reduce the voltage down to about 20VDC and then use LM7815. That's it.
GL
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