LT137A/LM137

LT337A/LM337

8

137afb

becomes shorted, due to the output capacitor discharging

into the regulator. This can be prevented by using the diode

D1 (see Figure 2) between the input and the output.

The input capacitor, C2, is only required if the regulator is

more than 4 inches from the raw supply filter capacitor.

Bypassing the Adjustment Pin

The adjustment pin of the LT137A may be bypassed with a

capacitor to ground, C1, to reduce output ripple, noise, and

impedance. These parameters scale directly with output

voltage if the adjustment pin is not bypassed. A bypass

capacitor reduces ripple, noise, and impedance to that of

a 1.25V regulator. In a 15V regulator, for example, these

parameters are improved by 15V/1.25V = 12 to 1. This

improvement holds only for those frequencies where the

impedance of the bypass capacitor is less than R1. Ten

microfarads is generally sufficient for 60Hz power line

applications where the ripple frequency is 120Hz since

least as high as the output voltage of the regulator. Values

larger than 10μF may be used, but if the output is larger

than 25V, a diode, D2, should be added between the output

and adjustment pins (see Figure 2).

Proper Connection of Divider Resistors

The LT137A has an excellent load regulation specification

of 0.5% and is measured at a point 1/8" from the bottom

of the package. To prevent degradation of load regulation,

the resistors which set output voltage, R1 and R2, must be

connected as shown in Figure 3. Note that the positive side

of the load has a true force and sense (Kelvin) connection,

but the negative side of the load does not.

R1 should be connected directly to the output lead of the

regulator, as close as possible to the specified point 1/8"

from the case. R2 should be connected to the positive

side of the load separately from the positive (ground)

connection to the raw supply. With this arrangement, load

regulation is degraded only by the resistance between the

regulator output pin and the load. If R1 is connected to

the load, regulation will be degraded.

LT137A

ADJ

R2

*D1 PROTECTS THE REGULATOR FROM INPUT SHORTS TO GROUND. IT IS

**D2 PROTECTS THE ADJUST PIN OF THE REGULATOR FROM OUTPUT SHORTS

D1*

1N4002

D2**

1N4002

137A F02

+

R1

C3

+

C1

+

C2

Figure 2

LT137A

ADJ

R2

CONNECT R1

DIRECTLY TO

REGULATOR PIN

LEAD RESISTANCE HERE

DEGRADES LOAD REGULATION.

MINIMIZE THE LENGTH OF

THIS LEAD.

LEAD RESISTANCE HERE DOES

NOT AFFECT LOAD REGULATION

137A F03

R1

LOAD

Figure 3

APPLICATIONS INFORMATION