Application Hints

In operation, the LM317L develops a nominal 1.25V refer-

ence voltage, V

terminal. The reference voltage is impressed across pro-

gram resistor R1 and, since the voltage is constant, a con-

stant current I

giving an output voltage of

Since the 100µA current from the adjustment terminal repre-

sents an error term, the LM317L was designed to minimize

I

To do this, all quiescent operating current is returned to the

output establishing a minimum load current requirement. If

there is insufficient load on the output, the output will rise.

EXTERNAL CAPACITORS

An input bypass capacitor is recommended in case the

regulator is more than 6 inches away from the usual large

filter capacitor. A 0.1µF disc or 1µF solid tantalum on the

input is suitable input bypassing for almost all applications.

The device is more sensitive to the absence of input bypass-

ing when adjustment or output capacitors are used, but the

above values will eliminate the possibility of problems.

The adjustment terminal can be bypassed to ground on the

LM317L to improve ripple rejection and noise. This bypass

capacitor prevents ripple and noise from being amplified as

the output voltage is increased. With a 10µF bypass capaci-

tor 80dB ripple rejection is obtainable at any output level.

Increases over 10µF do not appreciably improve the ripple

rejection at frequencies above 120Hz. If the bypass capaci-

tor is used, it is sometimes necessary to include protection

diodes to prevent the capacitor from discharging through

internal low current paths and damaging the device.

In general, the best type of capacitors to use is solid tanta-

lum. Solid tantalum capacitors have low impedance even at

high frequencies. Depending upon capacitor construction, it

takes about 25µF in aluminum electrolytic to equal 1µF solid

tantalum at high frequencies. Ceramic capacitors are also

good at high frequencies; but some types have a large

decrease in capacitance at frequencies around 0.5MHz. For

this reason, a 0.01µF disc may seem to work better than a

0.1µF disc as a bypass.

Although the LM317L is stable with no output capacitors, like

any feedback circuit, certain values of external capacitance

can cause excessive ringing. This occurs with values be-

tween 500pF and 5000pF. A 1µF solid tantalum (or 25µF

aluminum electrolytic) on the output swamps this effect and

insures stability.

LOAD REGULATION

The LM317L is capable of providing extremely good load

regulation but a few precautions are needed to obtain maxi-

mum performance. The current set resistor connected be-

tween the adjustment terminal and the output terminal (usu-

ally 240

Ω) should be tied directly to the output of the

regulator rather than near the load. This eliminates line

drops from appearing effectively in series with the reference

and degrading regulation. For example, a 15V regulator with

0.05

Ω resistance between the regulator and load will have a

load regulation due to line resistance of 0.05

Ω xI

resistor is connected near the load the effective line resis-

tance will be 0.05

Ω (1 + R2/R1) or in this case, 11.5 times

worse.

Figure 2 shows the effect of resistance between the regula-

tor and 240

Ω set resistor.

With the TO-92 package, it is easy to minimize the resis-

tance from the case to the set resistor, by using two separate

leads to the output pin. The ground of R2 can be returned

near the ground of the load to provide remote ground sens-

ing and improve load regulation.

THERMAL REGULATION

When power is dissipated in an IC, a temperature gradient

occurs across the IC chip affecting the individual IC circuit

components. With an IC regulator, this gradient can be es-

pecially severe since power dissipation is large. Thermal

regulation is the effect of these temperature gradients on

output voltage (in percentage output change) per watt of

power change in a specified time. Thermal regulation error is

independent of electrical regulation or temperature coeffi-

cient, and occurs within 5ms to 50ms after a change in

power dissipation. Thermal regulation depends on IC layout

as well as electrical design. The thermal regulation of a

voltage regulator is defined as the percentage change of

V

applied. The LM317L specification is 0.2%/W, maximum.

In the Thermal Regulation curve at the bottom of the Typical

Performance Characteristics page, a typical LM317L’s out-

put changes only 7mV (or 0.07% of V

1W pulse is applied for 10ms. This performance is thus well

inside the specification limit of 0.2%/W x 1W = 0.2% maxi-

00906407

FIGURE 1.

00906408

FIGURE 2. Regulator with Line Resistance

in Output Lead

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