LM337
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7
APPLICATIONS INFORMATION
Basic Circuit Operation
The LM337 is a 3−terminal floating regulator. In
operation, the LM337 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
from 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 into the adjustment terminal (I
Adj
)
represents an error term in the equation, the LM337 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 LM337 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 17. Basic Circuit Configuration
+
-
V
out
C
O
R
2
I
PROG
R
1
Adjust
V
in
LM337
V
out
+
V
ref
= -1.25 V Typical
V
ref
V
out
I
Adj
Load Regulation
The LM337 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 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.
An output capacitance (C
O
) in the form of a 1.0 mF
tantalum or 10 mF aluminum electrolytic capacitor is
required for stability. Using the classical tantalum or
aluminum electrolytic capacitor types with non−reduced
ESR (Equivalent Series Resistance) value is necessary.
Low−ESR or similar capacitor types with reduced ESR
value and ceramic capacitors can cause instability or
continuous oscillations in the application.
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 LM337 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
the discharging through the IC during an input short circuit.
Figure 18. Voltage Regulator with Protection Diodes
+
-
+
C
in
-V
in
R
2
C
Adj
+
+
1N4002
LM337
V
out
V
out
V
in
D1
1N4002
R
1
D
2
C
O
Adjust
V
out
