LM317L-N

SNVS775J – MARCH 2000 – REVISED MARCH 2013

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APPLICATION HINTS

adjustment terminal. The reference voltage is impressed across program resistor R1 and, since the voltage is

(1)

Since the 100

μA current from the adjustment terminal represents an error term, the LM317L-N was designed to

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 0.1

μF 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 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-N to improve ripple rejection and noise. This

bypass capacitor prevents ripple and noise from being amplified as the output voltage is increased. With a 10

μF

bypass capacitor 80dB ripple rejection is obtainable at any output level. Increases over 10

μF do not appreciably

improve the ripple rejection at frequencies above 120Hz. If the bypass capacitor 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 tantalum. Solid tantalum capacitors have low impedance

even at high frequencies. Depending upon capacitor construction, it takes about 25

μF in aluminum electrolytic to

equal 1

μF 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 0.01

μF disc may

seem to work better than a 0.1

μF disc as a bypass.

Although the LM317L-N is stable with no output capacitors, like any feedback circuit, certain values of external

capacitance can cause excessive ringing. This occurs with values between 500pF and 5000pF. A 1

μF solid

tantalum (or 25

μF aluminum electrolytic) on the output swamps this effect and insures stability.

LOAD REGULATION

The LM317L-N is capable of providing extremely good load regulation but a few precautions are needed to

obtain maximum performance. The current set resistor connected between the adjustment terminal and the

output terminal (usually 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

0.05

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

Figure 17 shows the effect of resistance between the regulator and 240

Ω set resistor.

With the TO-92 package, it is easy to minimize the resistance 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 sensing and improve load regulation.

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