LM317L
ΔV
out , OUTPUT
VOLTAGE DEVIATION (V)
Δ
Vout , OUTPUT VOLTAGE
DEVIATION (V)
1.5
1.0
0.5
0
−0.5
−1.0
−1.5
1.0
0.5
0
0
10
20
t, TIME (ms)
V
in
30
40
V
out
= 10 V
I
L
= 50 mA
T
J
= 25°C
C
L
= 1.0
mF;
C
Adj
= 10
mF
0.3
0.2
0.1
0
−0.1
−0.2
−0.3
100
50
0
0
10
20
t, TIME (ms)
I
L
30
40
C
L
= 0.3
mF;
C
Adj
= 10
mF
V
in
= 15 V
V
out
= 10 V
I
NL
= 50 mA
T
J
= 25°C
C
L
= 1
mF;
C
Adj
= 10
mF
Δ
V in , INPUT
VOTLAGE CHANGE (V)
Figure 15. Line Transient Response
I L , LOAD
CURRENT (mA)
C
L
= 0;
Without C
Adj
Figure 16. Load Transient Response
APPLICATIONS INFORMATION
Basic Circuit Operation
Load Regulation
The LM317L is a 3−terminal floating regulator. In
operation, the LM317L 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 13), and this
constant current flows through R
2
to ground. The regulated
output voltage is given by:
V
out
= V
ref
(1 +
R
2
) + I
Adj
R
2
R
1
The LM317L is capable of providing extremely good load
regulation, but a few precautions are needed to obtain
maximum performance. For best performance, the
programming resistor (R1) 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 R2 can be returned
near the load ground to provide remote ground sensing and
improve load regulation.
External Capacitors
Since the current from the adjustment terminal (I
Adj
)
represents an error term in the equation, the LM317L 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 LM317L 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.
V
in
LM317L
V
out
+
R
1
Adjust
V
ref
I
PROG
V
out
I
Adj
R
2
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 LM317L 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.
V
ref
= 1.25 V Typical
Figure 17. Basic Circuit Configuration
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