7

137afb

LT137A/LM137

LT337A/LM337

Output Voltage

The output voltage is determined by two external resis-

tors, R1 and R2 (see Figure 1). The exact formula for the

output voltage is:

VV

R

R

IR

OUT

REF

ADJ

=+

⎛

⎝⎜

⎞

⎠⎟

+

()

1

2

1

2

Current. In most applications, the second term is small

more critical applications, the exact formula should be

R

VV

V

R

I

OUT

REF

REF

ADJ

2

1

=

+

–

Smaller values of R1 and R2 will reduce the influence

drain on the regulator will be increased. Typical values

for R1 are between 100Ω and 300Ω, giving 12.5mA and

4.2mA no-load current respectively. There is an additional

consideration in selecting R1, the minimum load current

specification of the regulator. The operating current of the

LT137A flows from input to output. If this current is not

absorbed by the load, the output of the regulator will rise

above the regulated value. The current drawn by R1 and

R2 is normally high enough to absorb the current, but

care must be taken in no-load situations where R1 and

R2 have high values.

The maximum value for the operating current, which must

be absorbed, is 5mA for the LT137A. If input-output volt-

age differential is less than 10V, the operating current that

must be absorbed drops to 3mA.

APPLICATIONS INFORMATION

EXAMPLES:

1. A precision 10V regulator to supply up to 1A load

current.

b. Calculate R2 =

VV

V

R

I

VV

V

μ

OUT

REF

REF

ADJ

–

–.

.

1

10

1 25

125

100

65

+

=

+

Ω

A

A

= 696 4

.

Ω

Use R2 = 698Ω

2. A 15V regulator to run off batteries and supply

a. To minimize battery drain, select R1 as high as

possible

R

V

mA

1

125

3

417

==

.

ΩΩ

,use 402 ,1%

b. The high value for R1 will exaggerate the error due

be used.

R

VV

V

R

I

VV

V

OUT

REF

REF

ADJ

2

1

15

1 25

125

402

=

+

=

+

–

–.

.

Ω

6

65

4331

μA

=Ω

Use R2 = 4320Ω

Capacitors and Protection Diodes

An output capacitor, C3, is required to provide proper fre-

quency compensation of the regulator feedback loop. A 1μF

or larger solid tantalum capacitor is generally sufficient for

this purpose if the 1MHz impedance of the capacitor is 2Ω

or less. High Q capacitors, such as Mylar, are not recom-

mended because they tend to reduce the phase margin at

light load currents. Aluminum electrolytic capacitors may

also be used, but the minimum value should be 10μF to

ensure a low impedance at 1MHz. The output capacitor

should be located within a few inches of the regulator to

keep lead impedance to a minimum. The following caution

should be noted: if the output voltage is greater than 6V

and an output capacitor greater than 20μF has been used,

it is possible to damage the regulator if the input voltage

Figure 1

LT137A

ADJ

R2

137A F01

+

R1

C3

1μF

+

C2

5μF

+

C1

10μF