+

Over Temp &

Over Current

Protection

Input

Adj.

Output

1.25V

LM317L

SLCS144E – JULY 2004 – REVISED OCTOBER 2014

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8 Detailed Description

8.1 Overview

The LM317L device is a 100-mA linear regulator with high voltage tolerance up to 35 V. The device has a

feedback voltage that is relative to the output instead of ground. This ungrounded design allows the LM317L

device to have superior line and load regulation. This design also allows the LM317L device to be used as a

current source or current sink using a single resistor. Any output voltage from 1.25 to 32 V can be obtained by

using two resistors. The bias current of the device, up to 2.5 mA, flows to the output; this current must be used

by the load or the feedback resistors. The power dissipation is the product of pass-element voltage and current,

which is calculated as shown in Equation 1.

(1)

The application heat sink must be able to absorb the power calculated in Equation 1.

In addition to higher performance than fixed regulators, this regulator offers full overload protection, available

only in integrated circuits. Included on the chip are current-limiting and thermal-overload protection. All overload-

protection circuitry remains fully functional even when ADJUSTMENT is disconnected. Normally, no capacitors

are needed unless the device is situated far from the input filter capacitors, in which case an input bypass is

needed. An optional output capacitor can be added to improve transient response. ADJUSTMENT can be

bypassed to achieve very high ripple rejection, which is difficult to achieve with standard three-terminal

regulators.

In addition to replacing fixed regulators, the LM317L regulator 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. Its primary

application is that of a programmable output regulator, but by connecting a fixed resistor between ADJUSTMENT

and OUTPUT, this device can be used as a precision current regulator. Supplies with electronic shutdown can be

achieved by clamping ADJUSTMENT to ground, programming the output to 1.25 V, where most loads draw little

current.

The LM317LC device is characterized for operation over the virtual junction temperature range of 0°C to 125°C.

The LM317LI device is characterized for operation over the virtual junction temperature range of –40°C to 125°C.

8.2 Functional Block Diagram

6

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