LM117, LM317A, LM317-N

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SNVS774O – MAY 2004 – REVISED JANUARY 2014

Figure 23. 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-39 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 24

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

capacitance.

D1 protects against C1

D2 protects against C2

Figure 24. Regulator with Protection Diodes

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