you can use ic like 34063 which are dc to dc converter. these devices consists of internal temperature compensated reference,comparator,controlled duty cycle oscillator,with active current limit circuit. you can make either step up or step down converter with minimum use of components..
but limitation is that it can operate from 3volt to 40 volt input.
so for your 48volt input search in for similar IC like these..
good luck.
There's a ton of existing solutions, 48VDC is a standard
telecom line voltage. Why mess with ICs when you can
buy a mass-market part that's had all the cost driven
out of it?
Do you suppose Googling '48V "DC-DC converter"' might
turn you up at least one hit? Wouldn't it be worth a try?