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I'm looking to create the simple adjustable voltage regulator below

enter image description here

However, I am confused by the input-output voltage relationship. Looking at the LM317 data sheet (https://www.onsemi.com/pub/Collateral/LM317-D.PDF, pages 1,2 and 7 in particular), the Vref (the voltage across resistor R1) is specified as being a constant 1.25V.

Page 2 under characteristics of Vref states "Reference Voltage, 3.0 V ≤ VI−VO ≤ 40 V ... "

  • What is the input-output relationship in the LM317?
  • Can Vin be any voltage as long as Vout is between 1.25 - 37V but the difference between Vin and Vout (Vin - Vout) cannot be greater than 40V or less than 3V?
  • is Vin - 3 the maximum allowed Vout?
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    \$\begingroup\$ ELI5 doesn't work here. This is electrical engineering. We'll explain like we're expecting you to understand the basics of electronics, at least. That's fine, your "like I'm 5" is an understatement, you seem to have a grasp on what voltages are. \$\endgroup\$
    – Marcus Müller
    Commented Jan 1, 2020 at 23:55
  • \$\begingroup\$ Mechanical, are you more of a mechanical nerd, then electronic? If so, you might imagine that the LM317 requires an "over-pressure" of say 50 psi, in order to maintain a precision 300 psi at its output. In short, it wouldn't work well unless you provided at least 350 psi at the input, if you wanted the pressure regulator to function in maintaining 300 psi at its output. More would be okay. But you'd have to have that much, at least, for the system to maintain control. If the pressure at the input fell below 350 psi then you couldn't count on 300 psi at the output, anymore. Does that help? \$\endgroup\$
    – jonk
    Commented Jan 2, 2020 at 6:45

2 Answers 2

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The LM317 will raise or lower the output voltage until the voltage it sees on its Adj pin is the reference voltage (of 1.25 V).

It does that by dynamically adjusting the effective resistance between in- and output. Simply imagine an adjustable resistor between in- and output, and someone tuning that resistor so that the output is always at the same voltage.

It does that within it's physical limits: it can't drop less than 3 V (being a very, very, VERY old regulator), and the more it drops between in and output, the more power it converts to heat (in that resistance mentioned above).

At some point, the things simply breaks down and can't act as resistor anymore. 40 V (depending on the actual manufacturer of the LM317, that thing is so old, there are multiple masks out there) sounds realistic.

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If you see the voltage R divider on the output R2 / (R1+R2), well inside there is a 1.25 Band gap reference that compares Vadj and drives the output higher only if Voltage across R1 is < 1.25. Thus with V(R1) regulated to 1.25 and this divider relationship from output to ground, the driver amplifies this R Ratio to regulate the output voltage.

Due to the saturation voltages of NPN Darlingtons and 1.25V ref there is a spec for Vin-Vout dropout voltage which due to saturation (Rce) is dependent on current for each 3 terminal adjustable LDO.

Since FETs offer much lower RdsOn values than NPN transistor, Rce's they also offer thousands of different FET LDO's with much smaller dropout voltages and voltage and current limits.

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