LM317L
Application Hints
In operation, the LM317L develops a nominal 1.25V refer-
ence voltage, V
REF
, between the output and adjustment ter-
minal. 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, giv-
ing an output voltage of
tween 500 pF and 5000 pF. A 1µF solid tantalum (or 25µF
aluminum electrolytic) on the output swamps this effect and
insures stability.
Load Regulation
The LM317L 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 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.
Since the 100µA current from the adjustment terminal repre-
sents an error term, the LM317L 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.
Figure 2
shows the effect of resistance between the regula-
tor and 240Ω set resistor.
With the TO-92 package, it is easy to minimize the resis-
tance from the case to the set resistor, by using two separate
leads to the output pin. The ground of R2 can be returned
near the ground of the load to provide remote ground sens-
ing and improve load regulation.
DS009064-7
FIGURE 1.
External Capacitors
An input bypass capacitor is recommended in case the regu-
lator is more than 6 inches away from the usual large filter
capacitor. A 0.1µF disc or 1µF solid tantalum on the input is
suitable input bypassing for almost all applications. The de-
vice 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
LM317L to improve ripple rejection and noise. This bypass
capacitor prevents ripple and noise from being amplified as
the output voltage is increased. With a 10µF bypass capaci-
tor 80 dB ripple rejection is obtainable at any output level. In-
creases over 10µF do not appreciably improve the ripple re-
jection at frequencies above 120Hz. If the bypass capacitor
is used, it is sometimes necessary to include protection di-
odes to prevent the capacitor from discharging through inter-
nal 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 de-
crease in capacitance at frequencies around 0.5MHz. For
this reason, a 0.01µF disc may seem to work better than a
0.1µF disc as a bypass.
Although the LM317L is stable with no output capacitors, like
any feedback circuit, certain values of external capacitance
can cause excessive ringing. This occurs with values be-
DS009064-8
FIGURE 2. Regulator with Line Resistance
in Output Lead
Thermal Regulation
When power is dissipated in an IC, a temperature gradient
occurs across the IC chip affecting the individual IC circuit
components. With an IC regulator, this gradient can be espe-
cially severe since power dissipation is large. Thermal regu-
lation is the effect of these temperature gradients on output
voltage (in percentage output change) per watt of power
change in a specified time. Thermal regulation error is inde-
pendent of electrical regulation or temperature coefficient,
and occurs within 5ms to 50ms after a change in power dis-
sipation. Thermal regulation depends on IC layout as well as
electrical design. The thermal regulation of a voltage regula-
tor is defined as the percentage change of V
OUT
, per watt,
within the first 10ms after a step of power is applied. The
LM317L specification is 0.2%/W, maximum.
In the Thermal Regulation curve at the bottom of the Typical
Performance Characteristics page, a typical LM317L’s out-
put changes only 7mV (or 0.07% of V
OUT
= −10V) when a
1W pulse is applied for 10 ms. This performance is thus well
inside the specification limit of 0.2%/W x 1W = 0.2% maxi-
mum. When the 1W pulse is ended, the thermal regulation
again shows a 7 mV change as the gradients across the
LM317L chip die out. Note that the load regulation error of
about 14 mV (0.14%) is additional to the thermal regulation
error.
6
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