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 10µF capacitors have low enough in-

ternal series resistance to deliver 20A spikes when shorted.

Although the surge is short, there is enough energy to dam-

age 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 of the regu-

lator, and the rate of decrease of V

charge path is through a large junction that is able to sustain

a 2A surge with no problem. This is not true of other types of

positive regulators. For output capacitors of 25 µF or less,

the LM317L’s ballast resistors and output structure 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 dis-

charge through a low current junction. Discharge occurs

when

either the input or output is shorted. Internal to the

LM317L is a 50

Ω resistor which limits the peak discharge

current. No protection is needed for output voltages of 25V

or less and 10µF capacitance.

Figure 3 shows an LM317L

with protection diodes included for use with outputs greater

than 25V and high values of output capacitance.

LM317L micro SMD Light Sensitivity

Exposing the LM317L micro SMD package to bright sunlight

may cause the V

of fluorescent lighting the output is not affected. The LM317

micro SMD does not sustain permanent damage from light

exposure. Removing the light source will cause LM317L’s

V

DS009064-9

D1 protects against C1

D2 protects against C2

FIGURE 3. Regulator with Protection Diodes

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