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LM317 specifications question

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Cicada:
Hi

I have a 48V POE source that I need to step down to a stable as possible low noise 30V. The 30V is used for a bias voltage of a avalanche photo diode (APD). The APD requires at most 2mA continuous current when activated. I will have a small capacitor near it.

My question
The LM317  have a Absolute Maximum rating of 40 between Vin and Vo  (https://www.ti.com/lit/ds/symlink/lm317a.pdf)
I will have Vi - Vo = 48 - 30 = 18V over the device.

What about the 48V to ground on the input terminal? Will that be a problem?
I cannot find anything that indicate 48V on the input would be a problem.


P.S.
I need to build this circuit in a hurry. So no time to source the LM317HV with abs. max. of Vi-Vo = 60V
I will also ensure the Minimum load current requirement of 10mA max is satisfied by adding a load resistor on the output.

Ian.M:
Depending on the risetime of the 48V source at power on, it may be a problem as there can be >36V from Vin to Vout as the small capacitor initially charges.  The fix is a resistor in series with Vin and a Zener direct from Vin to Vout, chosen to limit the transient voltage to <36V but not conduct in normal operation (i.e. >>(Vin-Vout) ).  Try 1K and a 24V Zener.

Cicada:

--- Quote from: Ian.M on February 22, 2024, 12:49:33 pm ---Depending on the risetime of the 48V source at power on, it may be a problem as there can be >36V from Vin to Vout as the small capacitor initially charges.

--- End quote ---

Good observation. I will implement the protection as prescribed.

Cicada:

I did notice now in section 5 of the document they give a input voltage limits for the different LM317 devices.

So they do limit the input voltage to 40V on the non-HV option devices. HV option can have up to 60V on input.

Taking into account that my current draw is so low I can maybe build some sort of a pre-regulator circuit.

Andy Chee:
For reference, I propose using the circuit depicted in Figure 10 of the pdf

https://ixys.com/Documents/AppNotes/IXAN0063.pdf

The problem of course is, not everyone has a depletion-mode MOSFET in their parts bins  ;)

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