LM317L MDA National Semiconductor, LM317L MDA Datasheet - Page 6

• Manufacturer Part Number: LM317L MDA
• Description: 3-Terminal Adjustable Regulator
• Manufacturer: National Semiconductor

www.national.com
Application Hints
In operation, the LM317L develops a nominal 1.25V refer-
ence voltage, V
terminal. The reference voltage is impressed across pro-
gram resistor R1 and, since the voltage is constant, a con-
stant current I
giving an output voltage of
Since the 100µA current from the adjustment terminal repre-
sents an error term, the LM317L was designed to minimize
I
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 in case the
regulator 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 device is more sensitive to the absence of input bypass-
ing 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 80dB ripple rejection is obtainable at any output level.
Increases over 10µF do not appreciably improve the ripple
rejection at frequencies above 120Hz. If the bypass capaci-
tor is used, it is sometimes 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, a 0.01µF disc may seem to work better than a
0.1µF disc as a bypass.
ADJ
and make it very constant with line and load changes.
1
then flows through the output set resistor R2,
REF
, between the output and adjustment
FIGURE 1.
00906407
6
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-
tween 500pF and 5000pF. 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
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.
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.
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 es-
pecially severe since power dissipation is large. Thermal
regulation 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
independent of electrical regulation or temperature coeffi-
cient, and occurs within 5ms to 50ms after a change in
power dissipation. Thermal regulation depends on IC layout
as well as electrical design. The thermal regulation of a
voltage regulator is defined as the percentage change of
V
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
1W pulse is applied for 10ms. This performance is thus well
inside the specification limit of 0.2%/W x 1W = 0.2% maxi-
OUT
, per watt, within the first 10ms after a step of power is
FIGURE 2. Regulator with Line Resistance
in Output Lead
OUT
= −10V) when a
00906408
L
. If the set