LM117/LM317A/LM317
Application Hints
In operation, the LM117 develops a nominal 1.25V reference
voltage, V
REF
, between the output and adjustment terminal.
The reference voltage is impressed across program resistor
R1 and, since the voltage is constant, a constant current I
1
then flows through the output set resistor R2, giving an output
voltage of
aluminum electrolytic) on the output swamps this effect and
insures stability. Any increase of the load capacitance larger
than 10
μF
will merely improve the loop stability and output
impedance.
LOAD REGULATION
The LM117 is capable of providing extremely good load reg-
ulation but a few precautions are needed to obtain maximum
performance. The current set resistor connected between the
adjustment terminal and the output terminal (usually 240Ω)
should be tied directly to the output (case) of the regulator
rather than near the load. This eliminates line drops from ap-
pearing effectively in series with the reference and degrading
regulation. For example, a 15V regulator with 0.05Ω resis-
tance between the regulator and load will have a load regu-
lation due to line resistance of 0.05Ω
×
I
L
. If the set resistor is
connected near the load the effective line resistance will be
0.05Ω
(1
+ R2/R1) or in this case, 11.5 times worse.
shows the effect of resistance between the regulator
and 240Ω set resistor.
(1)
906305
FIGURE 1.
Since the 100μA current from the adjustment terminal repre-
sents an error term, the LM117 was designed to minimize
I
ADJ
and make it very constant with line and load changes. To
do this, all quiescent operating current is returned to the out-
put establishing a minimum load current requirement. If there
is insufficient load on the output, the output will rise.
EXTERNAL CAPACITORS
An input bypass capacitor is recommended. A 0.1μF disc or
1μF solid tantalum on the input is suitable input bypassing for
almost all applications. The device is more sensitive to the
absence of input bypassing when adjustment or output ca-
pacitors are used but the above values will eliminate the
possibility of problems.
The adjustment terminal can be bypassed to ground on the
LM117 to improve ripple rejection. This bypass capacitor pre-
vents ripple from being amplified as the output voltage is
increased. With a 10
μF
bypass capacitor 80dB ripple rejec-
tion is obtainable at any output level. Increases over 10
μF
do
not appreciably improve the ripple rejection at frequencies
above 120Hz. If the bypass capacitor is used, it is sometimes
necessary to include protection diodes to prevent the capac-
itor from discharging through internal low current paths and
damaging the device.
In general, the best type of capacitors to use is solid tantalum.
Solid tantalum capacitors have low impedance even at high
frequencies. Depending upon capacitor construction, it takes
about 25
μF
in aluminum electrolytic to equal 1μF solid tan-
talum at high frequencies. Ceramic capacitors are also good
at high frequencies; but some types have a large decrease in
capacitance at frequencies around 0.5 MHz. For this reason,
0.01
μF
disc may seem to work better than a 0.1
μF
disc as
a bypass.
Although the LM117 is stable with no output capacitors, like
any feedback circuit, certain values of external capacitance
can cause excessive ringing. This occurs with values be-
tween 500 pF and 5000 pF. A 1
μF
solid tantalum (or 25
μF
www.national.com
8
906306
FIGURE 2. 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. 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 LM117, this discharge path
is through a large junction that 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 dis-
charge through a low current junction. Discharge occurs when
either
the input, or the output, is shorted. Internal to the LM117
is a 50Ω resistor which limits the peak discharge current. No
protection is needed for output voltages of 25V or less and 10
μF
capacitance.
shows an LM117 with protection
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