LM337M

7

MOTOROLA ANALOG IC DEVICE DATA

APPLICATIONS INFORMATION

Basic Circuit Operation

The LM337M is a three–terminal floating regulator. In

operation, the LM337M 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 to ground. The regulated

output voltage is given by:

R2

Vout = Vref (1 +

) + IAdj R2

R1

Since the current into the adjustment terminal (IAdj)

represents an error term in the equation, the LM337M 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 LM337M 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.

+

–

Vout

CO

R2

IPRONG

R1

IAdj

Adjust

Vin

LM337M

Vout

+

Vref = –1.25 V Typically

Vref

Figure 17. Basic Circuit Configuration

Load Regulation

The LM337M 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 LM337M 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

discharging through the IC during an input short circuit.

+

–

+

Cin

–Vin

R2

CAdj

+

+

1N4002

LM337M

Vout

Vout

Vin

D1

1N4002

R1

D2

CO

Adjust

Figure 18. Voltage Regulator with

Protection Diodes