LM317L
chip die out. Note that the load regulation error of about 14mV
(0.14%) is additional to the thermal regulation error.
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 internal
series resistance to deliver 20A spikes when shorted. Al-
though 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 of the regulator, and
the rate of decrease of V
IN
. In the LM317L, this discharge 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.
shows an LM317L with protection
diodes included for use with outputs greater than 25V and
high values of output capacitance.
906409
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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