LM317, NCV317
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7
APPLICATIONS INFORMATION
Basic Circuit Operation
The LM317 is a 3−terminal floating regulator. In
operation, the LM317 develops and maintains a nominal
1.25 V reference (V
ref
) between its output and adjustment
terminals. This reference voltage is converted to a
programming current (I
PROG
) by R
1
(see Figure 17), and this
constant current flows through R
2
to ground.
The regulated output voltage is given by:
V
out
+V
ref
ǒ
1 )
R
2
R
1
Ǔ
)I
Adj
R
2
Since the current from the adjustment terminal (I
Adj
)
represents an error term in the equation, the LM317 was
designed to control I
Adj
to less than 100 mA and keep it
constant. To do 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 LM317 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.
Figure 19. Basic Circuit Configuration
+
V
ref
Adjust
V
in
V
out
LM317
R
1
I
PROG
V
out
R
2
I
Adj
V
ref
= 1.25 V Typical
V
out
Load Regulation
The LM317 is capable of providing extremely good load
regulation, but a few precautions are needed to obtain
maximum performance. For best performance, the
programming resistor (R
1
) should be connected as close to
the regulator as possible to minimize line drops which
effectively appear in series with the reference, thereby
degrading regulation. The ground end of R
2
can be returned
near the load ground to provide remote ground sensing and
improve load regulation.
External Capacitors
A 0.1 mF disc or 1.0 mF tantalum input bypass capacitor
(C
in
) is recommended to reduce the sensitivity to input line
impedance.
The adjustment terminal may be bypassed to ground to
improve ripple rejection. This capacitor (C
Adj
) prevents
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 LM317 is stable with no output capacitance,
like any feedback circuit, certain values of external
capacitance can cause excessive ringing. An output
capacitance (C
O
) in the form of a 1.0 mF tantalum or 25 mF
aluminum electrolytic capacitor on the output swamps this
effect and insures stability.
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 18 shows the LM317 with the recommended
protection diodes for output voltages in excess of 25 V or
high capacitance values (C
O
> 25 mF, C
Adj
> 10 mF). Diode
D
1
prevents C
O
from discharging thru the IC during an input
short circuit. Diode D
2
protects against capacitor C
Adj
discharging through the IC during an output short circuit.
The combination of diodes D
1
and D
2
prevents C
Adj
from
discharging through the IC during an input short circuit.
Figure 20. Voltage Regulator with Protection Diodes
D
1
V
in
C
in
1N4002
LM317
V
out
R
1
+
C
O
D
2
R
2
C
Adj
1N4002
Adjust
