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
can cause excessive ringing. This occurs with values be-
tween 500 pF and 5000 pF. A 1µF solid tantalum (or 25µF
aluminum electrolytic) on the output swamps this effect and
insures stability. Any increase of the load capacitance larger
than 10µF will merely improve the loop stability and output
impedance.
LOAD REGULATION
The LM117 is capable of providing extremely good load
regulation but a few precautions are needed to obtain maxi-
mum performance. The current set resistor connected be-
tween the adjustment terminal and the output terminal (usu-
ally 240Ω) should be tied directly to the output (case) of the
regulator rather than near the load. This eliminates line
drops from appearing effectively in series with the reference
and degrading regulation. For example, a 15V regulator with
0.05Ω resistance between the regulator and load will have a
load regulation due to line resistance of 0.05Ω x I
L
. If the set
resistor is connected near the load the effective line resis-
tance will be 0.05Ω (1 + R2/R1) or in this case, 11.5 times
worse.
shows the effect of resistance between the regula-
tor and 240Ω set resistor.
00906305
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
output 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
capacitors 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
prevents ripple from being amplified as the output voltage is
increased. With a 10µF bypass capacitor 80dB ripple rejec-
tion is obtainable at any output level. Increases over 10µF do
not appreciably improve the ripple rejection at frequencies
above 120Hz. If the bypass capacitor is used, it is some-
times necessary to include protection diodes to prevent the
capacitor from discharging through internal low current paths
and damaging the device.
In general, the best type of capacitors to use is solid tanta-
lum. Solid tantalum capacitors have low impedance even at
high frequencies. Depending upon capacitor construction, it
takes about 25µF in aluminum electrolytic to equal 1µF solid
tantalum at high frequencies. Ceramic capacitors are also
good at high frequencies; but some types have a large
decrease in capacitance at frequencies around 0.5MHz. For
this reason, 0.01µF disc may seem to work better than a
0.1µF disc as a bypass.
Although the LM117 is stable with no output capacitors, like
any feedback circuit, certain values of external capacitance
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00906306
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 10µF capacitors have low enough
internal series resistance to deliver 20A 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 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 25µF 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
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