LM337
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 (Vref) between its output and adjustment terminals.
This reference voltage is converted to a programming current
(IPROG) by R1 (see Figure 17), and this constant current flows
through R2 from ground.
The regulated output voltage is given by:
V out
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
A 1.0
µF
tantalum input bypass capacitor (Cin) is
recommended to reduce the sensitivity to input line
impedance.
The adjustment terminal may be bypassed to ground to
improve ripple rejection. This capacitor (CAdj) prevents ripple
from being amplified as the output voltage is increased. A
10
µF
capacitor should improve ripple rejection about 15 dB
at 120 Hz in a 10 V application.
An output capacitance (CO) in the form of a 1.0
µF
tantalum or 10
µF
aluminum electrolytic capacitor is required
for 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 LM337 with the recommended
protection diodes for output voltages in excess of –25 V or
high capacitance values (CO > 25
µF,
CAdj > 10
µF).
Diode D1
prevents CO from discharging thru the IC during an input
short circuit. Diode D2 protects against capacitor CAdj
discharging through the IC during an output short circuit. The
combination of diodes D1 and D2 prevents CAdj from the
discharging through the IC during an input short circuit.
Figure 18. Voltage Regulator with Protection Diodes
+ Vout
+
CO
Adjust
–Vin
LM337
Vin
D1
1N4002
Vout
R1
D2
1N4002
+
Vref 1
)
R2
)
IAdj R2
R
1
Since the current into the adjustment terminal (IAdj)
represents an error term in the equation, the LM337 was
designed to control IAdj to less than 100
µA
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
+ Vout
IAdj
R2
IPROG
+
CO
Adjust
Vin
LM337
Vref
R1
– Vout
Vout
Vref = –1.25 V Typical
R2
+
Cin
+
CAdj
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 (R1) should be connected as close to
the regulator as possible to minimize line drops which
effectively appear in series with the reference, thereby
– Vout
Figure 19. D2PAK Thermal Resistance and Maximum
Power Dissipation versus P.C.B. Copper Length
PD(max) for TA = +50°C
70
60
50
40
R
θJA
30
0
5.0
10
15
20
Free Air
Mounted
Vertically
3.0
2.0 oz. Copper
L
PD, MAXIMUM POWER DISSIPATION (W)
80
R
θ
JA, THERMAL RESISTANCE
JUNCTION-TO-AIR (
°
C/W)
3.5
2.5
2.0
1.5
1.0
Minimum
Size Pad
L, LENGTH OF COPPER (mm)
MOTOROLA ANALOG IC DEVICE DATA
ÎÎÎÎ
ÎÎÎÎ
ÎÎÎÎ
ÎÎÎÎ
L
25
30
7
ONSEMI [ ON SEMICONDUCTOR ]
