LM317LIBP datasheet(6/17 Pages) NSC

Teilenummer	LM317LIBP

Bauteilbeschribung	3-Terminal Adjustable Regulator

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Application Hints

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

ence voltage, V

REF, between the output and adjustment ter-

minal. The reference voltage is impressed across program

resistor R1 and, since the voltage is constant, a constant

current I

1 then flows through the output set resistor R2, giv-

ing an output voltage of

Since the 100µA current from the adjustment terminal repre-

sents an error term, the LM317L was designed to minimize

I

ADJ and make it very constant with line and load changes.

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 regu-

lator 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 de-

vice is more sensitive to the absence of input bypassing

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 80 dB ripple rejection is obtainable at any output level. In-

creases over 10µF do not appreciably improve the ripple re-

jection at frequencies above 120Hz. If the bypass capacitor

is used, it is sometimes necessary to include protection di-

odes to prevent the capacitor from discharging through inter-

nal 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 de-

crease 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 500 pF and 5000 pF. 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

L. If the set

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 espe-

cially severe since power dissipation is large. Thermal regu-

lation 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 inde-

pendent of electrical regulation or temperature coefficient,

and occurs within 5ms to 50ms after a change in power dis-

sipation. Thermal regulation depends on IC layout as well as

electrical design. The thermal regulation of a voltage regula-

tor is defined as the percentage change of V

OUT, per watt,

within the first 10ms after a step of power is 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

OUT = −10V) when a

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

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

mum. When the 1W pulse is ended, the thermal regulation

again showsa7mV change as the gradients across the

LM317L chip die out. Note that the load regulation error of

about 14 mV (0.14%) is additional to the thermal regulation

error.

DS009064-7

FIGURE 1.

DS009064-8

FIGURE 2. Regulator with Line Resistance

in Output Lead

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