LM117, LM317-N

SNVS774L – MAY 2004 – REVISED FEBRUARY 2011

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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 0.5 MHz. For this reason, 0.01

μF disc may

seem to work better than a 0.1

μF disc as a bypass.

Although the LM117 is stable with no output capacitors, like any feedback circuit, certain values of external

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

μF solid

tantalum (or 25

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

the load capacitance larger than 10

μF will merely improve the loop stability and output impedance.

Load Regulation

The LM117 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 (case) 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 26 shows the effect of resistance between the regulator and 240

Ω set resistor.

Figure 26. Regulator with Line Resistance in Output Lead

With the TO-3 package, it is easy to minimize the resistance from the case to the set resistor, by using two

separate leads to the case. However, with the TO package, care should be taken to minimize the wire length of

the output lead. The ground of R2 can be returned near the ground of the load to provide remote ground sensing

and improve load regulation.

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. Most 10

μF capacitors have

low enough internal series resistance to deliver 20A spikes when shorted. Although the surge is short, there is

enough energy to damage parts of the IC.

When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge

into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage

is able to sustain 15A surge with no problem. This is not true of other types of positive regulators. For output

capacitors of 25

μF or less, there is no need to use diodes.

The bypass capacitor on the adjustment terminal can discharge through a low current junction. Discharge occurs

when either the input, or the output, is shorted. Internal to the LM117 is a 50

Ω resistor which limits the peak

discharge current. No protection is needed for output voltages of 25V or less and 10

μF capacitance. Figure 27

shows an LM117 with protection diodes included for use with outputs greater than 25V and high values of output

capacitance.

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