LM317

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7

APPLICATIONS INFORMATION

Basic Circuit Operation

The LM317 is a 3–terminal floating regulator. In

operation, the LM317 develops and maintains a nominal

terminals. This reference voltage is converted to a

The regulated output voltage is given by:

Vout + Vref 1 )

R2

R1

) IAdj R2

represents an error term in the equation, the LM317 was

constant. To do this, all quiescent operating current is

returned to the output terminal. This imposes the

requirement for a minimum load current. If the load current

is less than this minimum, the output voltage will rise.

Since the LM317 is a floating regulator, it is only the

voltage differential across the circuit which is important to

performance, and operation at high voltages with respect to

ground is possible.

Figure 17. Basic Circuit Configuration

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LM317

Load Regulation

The LM317 is capable of providing extremely good load

regulation, but a few precautions are needed to obtain

maximum

performance.

For

best

performance,

the

the regulator as possible to minimize line drops which

effectively appear in series with the reference, thereby

near the load ground to provide remote ground sensing and

improve load regulation.

External Capacitors

A 0.1

µF disc or 1.0 µF tantalum input bypass capacitor

impedance.

The adjustment terminal may be bypassed to ground to

ripple from being amplified as the output voltage is

increased. A 10

µF capacitor should improve ripple

rejection about 15 dB at 120 Hz in a 10 V application.

Although the LM317 is stable with no output capacitance,

like any feedback circuit, certain values of external

capacitance can cause excessive ringing. An output

aluminum electrolytic capacitor on the output swamps this

effect and insures stability.

Protection Diodes

When external capacitors are used with any IC regulator

it is sometimes necessary to add protection diodes to prevent

the capacitors from discharging through low current points

into the regulator.

Figure 18 shows the LM317 with the recommended

protection diodes for output voltages in excess of 25 V or

discharging through the IC during an output short circuit.

discharging through the IC during an input short circuit.

Figure 18. Voltage Regulator with Protection Diodes

1N4002

LM317

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1N4002

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