# LM317 to step voltage up

I am sorry for questioning the "law of conservation of energy" first of all.

The expression for the output voltage of the LM317:

## Vout=1.25*(1+R2/R1)

contains no any relation to the input voltage. It doesn't care what's the input voltage. It only cares about the two resistors R1 and R2 (in my simulation). Increasing the value of R2 increased the output. But once I get the output equal to the input, further increasing the value of R2 doesn't increase the output voltage. What is the factor that doesn't let the output voltage go beyond the input voltage. I know it won't go (law of conservation of energy) but why?

I have attached the snippets of the output voltage for two different values for the R2 resistors.(I don't know whether those large valued resistors are available. Please bear with me for this simulation)

Edit: So its all the matter of 'Minimum Dropout voltage' (Vin-Vout). But going through this datasheet shows the difference of -0.3 which indicates higher output voltage.

• No, the -0.3V for the OnSemi part is not the recommended dropout voltage. That number is in the "Maximum Ratings" section and they are saying that if you ever allow the input voltage to be more than 0.3V below the output voltage then the device may be permanently damaged. Commented Sep 2, 2019 at 16:50

All the LM317 can possibly do is act like a (controlled) variable resistor between the input and output, it cannot create energy out of thin air.

And it can't even do that, being a bipolar part it will have some significant voltage drop at best with even moderate currents (some CMOS-output LDO regulators have a few mV dropout at low currents such as a few mA).

In the case of the LM317 (which is not an LDO = Low Drop Out) part, there is a Darlington transistor pair between input and output, and the drive can't go all the way to the input voltage.

See this "representative" schematic from this datasheet:

In general you should have 3V or more between input and output, as specified under the "line regulation" parameter in the datasheet.

• Ahh! I can see why there is this limitation. And I think the datasheet I have attached is wrong I guess. Thank you @Spehro Pefhany Sir. Commented Sep 2, 2019 at 16:49
• @JuneStar_2918 No, that data sheet is not wrong. Rather you are mistaken in only looking at the formula and ignoring the conditions the data sheet gives which bound when the formula is applicable. There may also be very slight difference between brands, both in the actual details and especially in how they are presented. Reading data sheets is a skill, generally when a manufacturer wants to provide something more tutorial in nature they create an Application Note or Reference Design, sometimes for novel parts there would be hints in a data sheet but not for something common like this. Commented Sep 2, 2019 at 17:38

An LM317 is defined by many parameters, not just the equation you provide. All of the required operating conditions must be satisfied, and one of the those is the "minimum dropout voltage". This is the minimum difference between the input and output voltage, where the input voltage must be greater than the output voltage. The datasheet for your specific LM317 will provide that number.

Edit: For the Texas Instrument's version of the LM317, the important parameter is in section 6.3 of the datasheet.

• I have edited my question based on you answer. Thank you. Commented Sep 2, 2019 at 16:45