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LM317 basics.

Contents


Introduction.

This is the basic calculations and some application hints for LM317 and similar 3-pin voltage regulators where the reference voltage is referred to the output voltage pin.
Much of this info applies to both positive and negative regulators. The pin-out for different regulators are different - see the datasheet.
These calculations being basic, some "finer" points like line- and load-regulation, temperature drift, etc. are ignored ( these calculations are essential for some applications though ).


LM317.

basic LM317 schematic.
Fig.1: Basic LM317 schematic.

C1 is an input decoupling capacitor. This is required for stability. A 100 nF ceramic located within some mm of the regulator normally works.
The rectifier electrolytic may be sufficient for stability with some circuits/PCB lay-outs, but you can buy a lot of 100 nF ceramics for the time it takes to test this over the relevant line- and load-range.

R1 and R2 set the output voltage:

Vout=Vref*(1+R2/R1).

To find R1 or R2:

R1=R2/(Vout/Vref-1).

R2=R1*(Vout/Vref-1).

Including Iadj in the calculations ( this is irrelevant for many applications ):

Vout=Vref*(1+R2/R1)+Iadj*R2.

R1=R2/((Vout-Iadj*R2)/Vref-1).

R2=(Vout-Vref)/(Iadj+Vref/R1).

LM317 have a minimum load current to maintain regulation. If the load current is less than this, the output voltage will rise.
The minimum load current is typically around 10 mA at maximum Vin-Vout at high temperature.
Most application manuals suggest to design for half this value, and it normally works well.
Designing for a minimum load current of 0 on Vout, the minimum load current for the regulator must flow in R1, setting a maximum value for R1:

R1<=Vref/Iomin.

A value of 240 Ω is suitable for most applications.

In general, R1 must be connected as close as possible to the LM317 output pin for good regulation.
The resistor Rw in fig. 1 is the wire resistance in the circuit and it can be seen that the voltage Io*Rw is added to the voltage across R1, causing the output voltage to drop as the load current is increased.
If you need to increase the regulator output resistance, use a "real" resistor for Rw.

C3 and C4 is the output decoupling capacitor. This is optional and can improve the regulator response for transient loads and/or reduce the regulator output noise.
I normally use a 100 nF ceramic for C3. This will reduce the regulator's sensitivity to HF pick-up from the output wiring without upsetting the regulators feedback loop.
C4 is a larger value capacitor ( 10 µF to 1000 µF ) to improve transient response and/or reduce output noise.
This capacitor MUST have a minimum series resistance to maintain regulator stability. In most cases, the ESR ( Resr in fig.1 ) of a general purpose electrolytic is sufficient.
If you - for any reason - decide to use a low-impedance electrolytic for C4, do make room for some additional series resistance in R3 or Resr locations.
The circuit with R3 will have better transient load response, but DC load regulation will be worse.
Typical values for R3/Resr is 0.5 Ω to 2 Ω, depending on the output voltage and current.
You may have to go to extremes ( like reading the datasheet and application manuals ) to get this right.

C2 improves the ripple rejection of the regulator by referring the regulator AC reference to GND rather than Vout.
A typical value is 10 µF which will improve the ripple rejection by 10 dB to 20 db depending on the output voltage and current.
In addition to increased ripple rejection, C2 ( together with a suitable C4 ) can reduce the regulator's output noise.
Increasing C2 beyond 10 µF does very little for ripple rejection or output noise, but can be used to reduce the regulators low-frequency output impedance.

D2 is required if C2 is used. It prevents C2 from discharging through the regulator's ADJ pin.

D1 must be included if there is any possibility that Vout can go above Vin.

D3 must be included if there is any possibility that Vout can be pulled below GND.

D1..D3 can be almost any general purpose 1 A rectifier diode. 1N4001 is generally a sensible choice.

This type of voltage regulator does not have a maximum input voltage specification as it has no ground connection.
The maximum voltage between the VI and VO pins is specified. As the output capacitance normally is discharged at power-on, the maximum input voltage is the maximum VI-VO voltage.
In some applications where Vin has a reasonable limited current, it may be possible to operate the regulators at higher voltages by replacing D1 with a zener-diode with a sufficient power dissipation capability.

Do calculate how much heatsink you require for a given application. Small clip-on heatsinks are insufficient in many cases.
The thermal characteristics between manufacturers / types can vary widely. A TO-220 cased LM317 comes with a junction-to-case thermal resistance between 1 °C/W and 5 °C/W.


Downloads.

Spread-sheet LM317 calculations.
Calculates component values, output voltage including component tolerances, output noise, and basic thermal parameters.


Appendix A: LM317 output noise.

Some people have the idea that the LM317 series are very low-noise regulators compared to e.g. LM7xxx type regulators.
This may - or may not - be true. It depends on the application.
Comparing datasheet values is difficult ( to put it mildly ) as the specifications are given under different operation conditions.
Noise and ripple rejection can be specified at different load currents - both are load current dependent.
Noise can be specified at different bandwidths - requiring a little math to compare them ( assuming it is pure thermal noise of course ).
The only way to find the best regulator for a given application is to measure the performance in that application.
Despite this, modern 3-terminal regulators are excellent value for money and overall they are difficult to match with a discrete design. Don't bother with one ( discrete design, that is ) unless you have very special requirements.
The table below is an attempt to compare datasheet values for LM3x7 and LM7xxx regulators. Measurements do not exactly agree.
When measured like here, LM3x7 regulators have a 4 dB..11 dB better noise performance than LM7xxx regulators. However, I have seen applications where the opposite is true.

The data used for following calculation are from the datasheets listed in "References".

The LM317 output noise is specified as 0.003% * Vout in a 10 Hz to 10 kHz bandwidth.
To find the output voltage noise, simply multiply 0.003% with the output voltage.
With C2 in circuit, the LM317 AC voltage gain is one, so the output voltage noise is 0.003% * 1.25 V.

The LM78xx output voltage noise is specified as V in a 10 Hz to 100 kHz bandwidth.

Measurements are from a real rectifier/regulator circuit, C2=10 µF, C4=100 µF. Ripple voltage is in all cases below wide-band noise.

Table A1: Output noise of some regulators.
RegulatorOutput
voltage
Output voltage noise
10 Hz to 10 kHz (1)
Output voltage noise
10 Hz to 100 kHz (1)
Output voltage
noise density
Output voltage noise
20 kHz BW
Measured output
voltage noise
22 Hz to 22 kHz
at 50 mA load (2)
Measured output
voltage noise
22 Hz to 22 kHz
at 100 mA load (2)
LM317, no C25 V150 µV 1.5 µV/√Hz210 µV  
LM317, C2=10 µF5 V38 µV 380 nV/√Hz53 µV  
LM78055 V 40 µV130 nV/√Hz18 µV  
LM337, no C2-5 V150 µV 1.5 µV/√Hz210 µV  
LM337, C2=10 µF-5 V38 µV 380 nV/√Hz53 µV  
LM7905-5 V 125 µV400 nV/√Hz56 µV  
LM317, no C212 V360 µV 3.6 µV/√Hz510 µV  
LM317, C2=10 µF12 V38 µV 380 nV/√Hz53 µV  
LM781212 V 300 µV240 nV/√Hz34 µV  
LM337, no C2-12 V360 µV 3.6 µV/√Hz510 µV  
LM337, C2=10 µF-12 V38 µV 380 nV/√Hz53 µV  
LM7912-12 V 75 µV950 nV/√Hz130 µV  
LM317, no C215 V450 µV 4.5 µV/√Hz640 µV  
LM317, C2=10 µF15 V38 µV 380 nV/√Hz53 µV33 µV33 µV
LM781515 V 90 µV280 nV/√Hz40 µV52 µV57 µV
LM337, no C2-15 V450 µV 4.5 µV/√Hz640 µV  
LM337, C2=10 µF-15 V38 µV 380 nV/√Hz53 µV35 µV36 µV
LM7915-15 V 380 µV1.2 µV/√Hz170 µV120 µV130 µV

(1)Datasheet value.
(2)Noise measurement filter with 20 kHz noise-bandwidth.

[5] are some actual noise measurements on LM3x7 regulators.


References.

[1] Texas Instruments: LM117, LM317-N datasheet, SNVS774P - May 2004 – Revised October 2015
[2] Texas Instruments: LM340, LM340A, LM7805, LM7812, LM7815 datasheet, SNOSBT0L – February 2000 – Revised September 2016
[3] Texas Instruments: LM137, LM337-N datasheet, SNVS778E - May 1999 – Revised January 2016
[4] Texas Instruments: LM7905, LM7912, LM7915 datasheet, SNOSBQ7C - June 1999 – Revised May 2013
[5] TNT Audio: "Simple Voltage Regulators"
Some measurements on LM3x7 noise performance.

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