LM317 Voltage Regulator


The LM317 is a popular three terminals adjustable voltage regulator.

The LM317 series of adjustable 3-terminal positive voltage regulators is capable of supplying in excess of 1.5A over a 1.2V to 37V output range. They are exceptionally easy to use and require only two external resistors to set the output voltage. Further, both line and load regulation are better than standard fixed regulators. Also, the LM317 is packaged in standard transistor packages which are easily mounted and handled.

In addition to higher performance than fixed regulators, the LM317 series offers full overload protection available only in IC's. Included on the chip are current limit, thermal overload protection and safe area protection. All overload protection circuitry remains fully functional even if the adjustment terminal is disconnected.

Normally, no capacitors are needed unless the device is situated more than 6 inches from the input filter capacitors in which case an input bypass is needed. An optional output capacitor can be added to improve transient response. The adjustment terminal can be bypassed to achieve very high ripple rejection ratios which are difficult to achieve with standard 3-terminal regulators.
 


Besides replacing fixed regulators, the LM317 is useful in a wide variety of other applications. Since the regulator is "floating" and sees only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input to output differential is not exceeded, i.e., avoid short-circuiting the output.

Also, it makes an especially simple adjustable switching regulator, a programmable output regulator, or by connecting a fixed resistor between the adjustment pin and output, the LM317 can be used as a precision current regulator. Supplies with electronic shutdown can be achieved by clamping the adjustment terminal to ground which programs the output to 1.2V where most loads draw little current.

 



The output voltage from the LM317 can be set using two resistors (R1 and R2).

The resistors values can be selected according to the following formula:


 VOUT = 1.25 * ( 1 + R2/R1 ) 

 

Using common stock resistors, you can use this refrence 
chart to quickly work out he likly output voltage: 

  

R2\R1 150Ω 180Ω 220Ω 240Ω 270Ω 330Ω 370Ω 390Ω 470Ω
68Ω 1.82V 1.72V 1.64V 1.60V 1.56V 1.51V 1.48V 1.47V 1.43V
82Ω 1.93V 1.82V 1.72V 1.68V 1.63V 1.56V 1.53V 1.51V 1.47V
100Ω 2.08V 1.94V 1.82V 1.77V 1.71V 1.63V 1.59V 1.57V 1.52V
120Ω 2.25V 2.08V 1.93V 1.88V 1.81V 1.70V 1.66V 1.63V 1.57V
150Ω 2.50V 2.29V 2.10V 2.03V 1.94V 1.82V 1.76V 1.73V 1.65V
180Ω 2.75V 2.50V 2.27V 2.19V 2.08V 1.93V 1.86V 1.83V 1.73V
220Ω 3.08V 2.78V 2.50V 2.40V 2.27V 2.08V 1.99V 1.96V 1.84V
240Ω 3.25V 2.92V 2.61V 2.50V 2.36V 2.16V 2.06V 2.02V 1.89V
270Ω 3.50V 3.13V 2.78V 2.66V 2.50V 2.27V 2.16V 2.12V 1.97V
330Ω 4.00V 3.54V 3.13V 2.97V 2.78V 2.50V 2.36V 2.31V 2.13V
370Ω 4.33V 3.82V 3.35V 3.18V 2.96V 2.65V 2.50V 2.44V 2.23V
390Ω 4.50V 3.96V 3.47V 3.28V 3.06V 2.73V 2.57V 2.50V 2.29V
470Ω 5.17V 4.51V 3.92V 3.70V 3.43V 3.03V 2.84V 2.76V 2.50V
560Ω 5.92V 5.14V 4.43V 4.17V 3.84V 3.37V 3.14V 3.04V 2.74V
680Ω 6.92 5.97V 5.11V 4.79V 4.40V 3.83V 3.55V 3.43V 3.06V
820Ω 8.08V 6.94V 5.91V 5.52V 5.05V 4.36V 4.02V 3.88V 3.43V
1000Ω 9.58V 8.19V 6.93V 6.46V 5.88V 5.04V 4.63V 4.46V 3.91V
1200Ω 11.25V 9.58V 8.07V 7.50V 6.81V 5.80V 5.30V 5.10V 4.44V
1500Ω 13.75 11.67V 9.77V 9.06V 8.19V 6.93V 6.32V 6.06V 5.24V
1800Ω 16.25V 13.75V 11.48V 10.63V 9.58V 8.07V 7.33V 7.02V 6.04V
2200Ω 19.58V 16.53V 13.75V 12.71V 11.44V 9.58V 8.68V 8.30V 7.10V
2700Ω 23.75V 20.00V 16.59V 15.31V 13.75V 11.48V 10.37V 9.90V 8.43V
3300Ω 28.75V 24.17V 20.00V 18.44V 16.53V 13.75V 12.40V 11.83V 10.03V



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