LM317M
APPLICATIONS INFORMATION
Basic Circuit Operation
External Capacitors
The LM317M is a three−terminal floating regulator. In
operation, the LM317M develops and maintains a nominal
A 0.1 mF disc or 1.0 mF tantalum input bypass capacitor
(C ) is recommended to reduce the sensitivity to input line
in
1.25 V reference (V ) between its output and adjustment
impedance.
ref
terminals. This reference voltage is converted to a
The adjustment terminal may be bypassed to ground to
programming current (I
) by R (see Figure 19), and this
improve ripple rejection. This capacitor (C ) prevents
PROG
1
Adj
constant current flows through R to ground. The regulated
output voltage is given by:
ripple from being amplified as the output voltage is
increased. A 10 mF capacitor should improve ripple
rejection about 15 dB at 120 Hz in a 10 V application.
Although the LM317M is stable with no output
capacitance, like any feedback circuit, certain values of
external capacitance can cause excessive ringing. An output
2
R
R
2
1
ref ǒ1 ) Ǔ) I
V
+ V
R
out
Adj 2
Since the current from the terminal (I ) represents an
Adj
capacitance (C ) in the form of a 1.0 mF tantalum or 25 mF
O
error term in the equation, the LM317M was designed to
aluminum electrolytic capacitor on the output swamps this
effect and insures stability.
control I to less than 100 mA and keep it constant. To do
Adj
this, all quiescent operating current is returned to the output
terminal. This imposes the requirement for a minimum load
current. If the load current is less than this minimum, the
output voltage will rise.
Since the LM317M is a floating regulator, it is only the
voltage differential across the circuit which is important to
performance, and operation at high voltages with respect to
ground is possible.
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.
Figure 20 shows the LM317M with the recommended
protection diodes for output voltages in excess of 25 V or
high capacitance values (C > 25 mF, C > 5.0 mF). Diode
O
Adj
D prevents C from discharging thru the IC during an input
1
O
V
V
in
out
short circuit. Diode D protects against capacitor C
2
Adj
LM317M
+
discharging through the IC during an output short circuit.
The combination of diodes D and D prevents C from
R
1
1
2
Adj
V
ref
discharging through the IC during an input short circuit.
Adjust
I
PROG
V
out
D
1
I
Adj
R
2
1N4002
V
in
V
ref
= 1.25 V Typical
V
out
V
out
LM317M
+
Figure 19. Basic Circuit Configuration
Load Regulation
C
in
R
1
C
O
D
2
Adjust
1N4002
The LM317M is capable of providing extremely good
load regulation, but a few precautions are needed to obtain
maximum performance. For best performance, the
R
2
C
Adj
programming resistor (R ) should be connected as close to
1
the regulator as possible to minimize line drops which
effectively appear in series with the reference, thereby
Figure 20. Voltage Regulator with
Protection Diodes
degrading regulation. The ground end of R can be returned
2
near the load ground to provide remote ground sensing and
improve load regulation.
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