I believe it might be a silly question but I'm not 100% sure about this, so I would like your opinion:

There are 2 LM317, both connected in parallel from the same source and they output respectively 5.1V and 4V.

Since the LM317s need 2.5V of headroom, I believe that 7.6V of common source will be fine for both of them since they both see enough voltage at the source.

I also believe that the following sentence IS NOT TRUE:

Since 2 LM317 are in parallel, the total headroom has to be 2.5V+2.5V (meaning a minimum of 9.1V at the source.)

This question derives from the fact that I don't know the reason for those 2.5V.

edit: Adding photo to visualize and as pointed out in the comments, I left 3V headroom instead of 2.5V --> in this example Vin is 8.1V,


  • 1
    \$\begingroup\$ Schematics are better than words, but your 2 beliefs are very likely correct. \$\endgroup\$
    – glen_geek
    Commented Sep 18, 2021 at 14:07
  • \$\begingroup\$ It needs only 2.5V headroom, since VI is common to both of the regulators. 3V may be better: "8.4.2 Operation With Low Input Voltage The device requires up to 3-V headroom (V I – V O ) to operate in regulation. The device may drop out and OUTPUT voltage will be INPUT voltage minus drop out voltage with less headroom." \$\endgroup\$
    – jay
    Commented Sep 18, 2021 at 14:12
  • 1
    \$\begingroup\$ Hi! Could you please add a schematic of the circuit to the original question? Usually a picture tells more than a 1000 words. If I understand correctly, you are correct, just as @glen_geek says. The 2.5 V is needed for the voltage regulation. \$\endgroup\$
    – user24368
    Commented Sep 18, 2021 at 14:13
  • \$\begingroup\$ Interestingly everyone talks here about 2.5V minimum voltage drop but checking with TI datasheet I see "Minimum recommended voltage headroom 3V". Doesn't change the essence of the question but may be good to qualify. LM317 may be produced by various manufacturers and the voltage headroom doesn't necessarily answer only "does it work" but also "how well it performs". ti.com/lit/ds/symlink/… \$\endgroup\$ Commented Sep 18, 2021 at 16:21

1 Answer 1


Since the outputs are independent 5.1 V and 4 V supplies then the LM317s know nothing about each other. Each device needs a supply that is 2.5 V higher than the output so one needs ≥ 7.6 V and the other ≥ 6.5 V. A 7.6 V supply meets the requirements of both.

This question derives from the fact that i don't know the reason of those 2.5V.

enter image description here

Figure 1. LM317 internal diagram. Image source: (my) LM317 constant-current power supply notes.

Note that between Input and Output there are transistors 4 and 5 and resistor 6.

  • To turn on Q5 base current will have to be supplied by Q4 so there will be a minimum of two base-emitter voltage drops (typically 0.7 × 2 = 1.4 V) caused by those alone.
  • The amplifier (3) will require a little headroom and R6 will drop a little more.

The end result is that the input must be 2.5 V higher than the output for the device to regulate properly.


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