LM337

7

MOTOROLA ANALOG IC DEVICE DATA

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:

Vout + Vref 1 )

R

2

R

1

) I

Adj

R

2

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

CO

R2

IPROG

R1

Adjust

Vin

LM337

Vout

+

Vref = –1.25 V Typical

Vref

Vout

IAdj

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

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

+

–

+

Cin

–Vin

R2

CAdj

+

+

1N4002

LM337

Vout

Vout

Vin

D1

1N4002

R1

D2

CO

Adjust

Vout

Figure 19. D2PAK Thermal Resistance and Maximum

Power Dissipation versus P.C.B. Copper Length

30

40

50

60

70

80

Minimum

Size Pad

2.0 oz. Copper

L

L

Free Air

Mounted

Vertically

R

θJA

1.0

1.5

2.0

2.5

3.0

3.5

010

20

30

25

15

5.0

L, LENGTH OF COPPER (mm)

PD(max) for TA = +50°C