LM317

LM317

The LM317 is a popular adjustable positive linear voltage regulator. It was designed by Robert C Dobkin in 1976 while he worked at National Semiconductor.

The LM337 is the negative complement to the LM317, which regulates voltages below a reference. It was designed by Robert "Bob" Pease, who also worked for National Semiconductor.

Specifications[edit]

LM317 pinout showing its constant voltage reference.
SymbolParameterValueUnit
VoutOutput voltage range1.25  – 37V
Vin – VoutVoltage differential3  – 40V
TJOperating junction temperature range0 – 125°C [1]
IO(MAX)Maximum output current1.5A
IL(MIN)Minimum load current3.5 mA typical, 12 mA maximum[1]
PDPower dissipationInternally LimitedW [1]
RθJAThermal resistance, Junction to ambient80°C/W [1]
RθJCThermal resistance, Junction to case5°C/W [1]

Without a heat sink with an ambient temperature at 50 ⁰C such as on a hot summer day inside a box, a maximum power dissipation of (TJ-TA)/RθJA = ((125-50)/80) = 0.98 W can be permitted. (A piece of shiny sheet metal of aluminium with the dimensions 6 x 6 cm and 1.5 mm thick, results in a thermal resistance that permits 4.7 W of heat dissipation[2][3]).

In a constant voltage mode with an input voltage source at VIN at 34 V and a desired output voltage of 5 V, the maximum output current will be PMAX / (VI-VO) = 0.98 / (34-5) = 32 mA.

For a constant current mode with an input voltage source at VIN at 12 V and a forward voltage drop of VF=3.6 V, the maximum output current will be PMAX / (VI - VF) = 0.98 / (12-3.6) = 117 mA.

Operation[edit]

Main article: Linear regulator

As linear regulators, the LM317 and LM337 are used in DC to DC converter applications.

Linear regulators inherently waste power; the power dissipated is the current passed multiplied by the voltage difference between input and output. A LM317 commonly requires a heat sink to prevent the operating temperature from rising too high. For large voltage differences, the power lost as heat can ultimately be greater than that provided to the circuit. This is the tradeoff for using linear regulators, which are a simple way to provide a stable voltage with few additional components. The alternative is to use a switching voltage regulator, which is usually more efficient, but has a larger footprint and requires a larger number of associated components.

In packages with a heat-dissipating mounting tab, such as TO-220, the tab is connected internally to the output pin which may make it necessary to electrically isolate the tab or the heat sink from other parts of the application circuit. Failure to do this may cause the circuit to short.

Voltage regulator[edit]

Schematic of LM317 in a typical voltage regulator configuration, including decoupling capacitors to address input noise and output transients.

The LM317 has three pins: INput, OUTput, and ADJustment. Internally the device has a bandgap voltage reference which produces a stable reference voltage of Vref= 1.25 V followed by a feedback-stabilized amplifier with a relatively high output current capacity. How the adjustment pin is connected determines the output voltage as follows.

If the adjustment pin is connected to ground the output pin delivers a regulated voltage of 1.25 V at currents up to the maximum. Higher regulated voltages are obtained by connecting the adjustment pin to a resistive voltage divider between the output and ground. Then

Vout = Vref (1 + RL/RH)

Vref is the difference in voltage between the OUT pin and the ADJ pin.[4] Vref is typically 1.25 V during normal operation.

Because some quiescent current flows from the adjustment pin of the device, an error term is added:

Vout = Vref (1 + RL/RH) + IQRL

To make the output more stable, the device is designed to keep the quiescent current at or below 100µA, making it possible to ignore the error term in nearly all practical cases.[5]

Current regulator[edit]

A constant current source circuit constructed with LM317

The device can be configured to regulate the current to a load, rather than the voltage, by replacing the low-side resistor of the divider with the load itself. The output current is that resulting from dropping the reference voltage across the resistor. Ideally, this is:

Iout = Vref/RH

Accounting for quiescent current, this becomes:

Iout = (Vref/RH) + IQ

LM317 can also be used to design various other circuits like 0 V to 30 V regulator circuit, adjustable regulator circuit with improved ripple rejection, precision current limiter circuit, tracking pre-regulator circuit, 1.25 V to 20 V regulator circuit with minimum program current, adjustable multiple on-card regulators with single control, battery charger circuit, 50 mA constant current battery charger circuit, slow turn-on 15 V regulator circuit, ac voltage regulator circuit, current-limited 6 V charger circuit, adjustable 4 V regulator circuit, high-current adjustable regulator circuit and many more. [4]

Compared to 78xx/79xx[edit]

The LM317 is an adjustable analogue to the popular 78xx fixed regulators. Like the LM317, each of the 78xx regulators is designed to adjust the output voltage until it is some fixed voltage above the adjustment pin (which in this case is labelled "ground").

The mechanism used is similar enough that a voltage divider can be used in the same way as with the LM317 and the output follows the same formula, using the regulator's fixed voltage for Vref (e.g. 5 V for 7805). However, the 78xx device's quiescent current is substantially higher and less stable. Because of this, the error term in the formula cannot be ignored and the value of the low-side resistor becomes more critical.[6] More stable adjustments can be made by providing a reference voltage that is less sensitive than a resistive divider to current fluctuations, such as a diode drop or a voltage buffer. The LM317 is designed to compensate for these fluctuations internally, making such measures unnecessary.

The LM337 relates in the same way to the fixed 79xx regulators.

Second sources from Eastern Bloc[edit]

The LM317 has an East European equivalent, the B3170V, which was manufactured in the German Democratic Republic (East Germany) by HFO (part of Kombinat Mikroelektronik Erfurt). Also, in USSR was manufactured and most popular ICs K142EN12A and KR142EN12A. These ICs are functional analogues of the LM317

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