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LM317L-N

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SNVS775K – MARCH 2000 – REVISED APRIL 2016

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Device Functional Modes (continued)

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

that is able to sustain a 2-A surge with no problem. This is not true of other types of positive regulators. For

output capacitors of 25

μF or less, the ballast resistors and output structure of the LM317L-N limit the peak

current to a low enough level so that there is no need to use a protection diode.

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

when either the input or output is shorted. Internal to the LM317L-N is a 50-

Ω resistor which limits the peak

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

μF capacitance. Figure 15

shows an LM317L-N with protection diodes included for use with outputs greater than 25 V and high values of

output capacitance.

D1 protects against C1

D2 protects against C2

Figure 15. Regulator With Protection Diodes

7.4.3 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 20-A 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

that is able to sustain 15-A 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 LM317L-N is a 50-

Ω resistor which limits the peak

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

μF capacitance. Figure 16

shows an LM317L-N with protection diodes included for use with outputs greater than 25 V and high values of

output capacitance.