LM317L Datasheet, PDF (7/17 Pages) Texas Instruments – 3-TERMINAL ADJUSTABLE REGULATOR

Application Hints (Continued)
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_IN. In the LM317L, this dis-
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 50Ω 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.
DS009064-9
D1 protects against C1
D2 protects against C2
FIGURE 3. Regulator with Protection Diodes
LM317L micro SMD Light Sensitivity
Exposing the LM317L micro SMD package to bright sunlight
may cause the V_REF to drop. In a normal office environment
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_REF to recover to the proper value.
7
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