LM317

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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 (negative complement to the LM317) regulates voltages below, rather than above, the reference. It was designed by Robert "Bob" Pease.

Video LM317

Specifications

Without a heat sink with an ambient temperature at 50 ?C such as on a hot summer day inside a box, a maximum power dissipation of (T_J-T_A)/R_?JA = ((125-50)/80) = 0.98 W can be permitted. (A piece of shiny sheet metal of Aluminum 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).

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

For a constant current mode with an input voltage source at V_IN at 12 V and a forward voltage drop of V_F=3.6 V, the maximum output current will be P_MAX / (V_I - V_F) = 0.98 / (12-3.6) = 112 mA.

Maps LM317

Operation

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. In use an LM317 commonly requires a heat sink to prevent the operating temperature rising too high. For large voltage differences, the energy lost as heat can ultimately be greater than that provided to the circuit. This is the trade-off 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

The LM317 has three pins: INput, OUTput, and ADJustment. Internally the device has a bandgap voltage reference which produces a stable reference voltage of V_ref= 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

V_out = V_ref (1 + R_L/R_H)

V_ref is the difference in voltage between the OUT pin and the ADJ pin. V_ref 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:

V_out = V_ref (1 + R_L/R_H) + I_QR_L

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.

Current regulator

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:

I_out = V_ref/R_H

Accounting for quiescent current, this becomes:

I_out = (V_ref/R_H) + I_Q

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.

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Compared to 78xx/79xx

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 labeled "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 V_ref (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. 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.

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See also

• Bandgap voltage reference
• Brokaw bandgap reference
• List of LM-series integrated circuits

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References

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External links

• LM317 Circuit Schematics and Pinouts
• Online calculator to pick resistors for LM317 circuit

Band-Gap

• The Design of Band-Gap Reference Circuits: Trials and Tribulations - Robert Pease, National Semiconductor (shows LM317 design in Figure 4: LM117)
• LM317 Bandgap Voltage Reference Example (ECE 327) - Brief explanation of the temperature-independent bandgap reference circuit within the LM317.

Datasheets / Databooks

• Voltage Regulator Databook (Historical 1980), National Semiconductor
• LM317 (positive), LM350 (3 Amp), Texas Instruments (TI acquired National Semiconductor)
• LM317 (positive), LM350 (3 Amp), ON Semiconductor
• LM317 (positive), STMicroelectronics
• LM337 (negative), Texas Instruments

Source of the article : Wikipedia