I need to have a regulated 5V from 48V power supply. I'm thinking of using LM117 regulator. However, the absolute maximum differential input/output voltage of LM117 is 40V only. So, is it possible to use two sequential LM117 ???
This could be done, but the power efficiency using linear regulators is very low. I suggest the TL2775HV-05 if output current is 1 amp or less.For higher output current, try TPS40200.
Regards,Ronald MichallickLinear Applications
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It is not possible to avoid the full +48V across the LM117 by simply putting two in series. At start-up, the first LM117 will have the full 48V across it in this case. If you can avoid the start-up problem, the next obsticle is what happens in current limit. If the first device enters current limit, then the output may fall to zero thus causing the differential input to output voltage to exceed absmax.
Despite this, TI does offer fairly large selection of LDO's and Linear Regulators that can handle 48V and above such as the LM117HV (high voltage version of the LM117).
As one more option, the TL783 linear regulator has a maximum input of 125V.
Why use a linear regulator when the voltage difference is so high?
What is output current requirement?
All the responses given are correct, but if you still want to use LM117 for final output you could use simple/crude voltage pre-regulator to bring the voltage down. This could be as simple as two resistors an NPN (or NMOS) transistor, and maybe a low value capacitor from base (gate) to gnd.
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