LT138A/LT338A

LM138/LM338

7

138afd

For more information www.linear.com/LT138A

applicaTions inForMaTion

Table 1. 0.5% and 1% Standard Resistance Values

1.00

1.47

2.15

3.16

4.64

6.81

1.02

1.50

2.21

3.24

4.75

6.98

1.05

1.54

2.26

3.32

4.87

7.15

1.07

1.58

2.32

3.40

4.99

7.32

1.10

1.62

2.37

3.48

5.11

7.50

1.13

1.65

2.43

3.57

5.23

7.68

1.15

1.69

2.49

3.65

5.36

7.87

1.18

1.74

2.55

3.74

5.49

8.06

1.21

1.78

2.61

3.83

5.62

8.25

1.24

1.82

2.67

3.92

5.76

8.45

1.27

1.87

2.74

4.02

5.90

8.66

1.30

1.91

2.80

4.12

6.04

8.87

1.33

1.96

2.87

4.22

6.19

9.09

1.37

2.00

2.94

4.32

6.34

9.31

1.40

2.05

3.01

4.42

6.49

9.53

1.43

2.10

3.09

4.53

6.65

9.76

Standard resistance values are obtained from the Decade Table by

multiplying by multiples of 10. As an example, 1.21 can represent 1.21Ω,

12.1Ω, 121Ω, 1.21k etc.

Bypass Capacitors

Input bypassing using a 1µF tantalum or 25µF electrolytic

is recommended when the input filter capacitors are more

than 5 inches from the device. Improved ripple rejection

(80dB) can be accomplished by adding a 10µF capacitor

from the ADJ pin to ground. Increasing the size of the

capacitor to 20µF will help ripple rejection at low output

voltage since the reactance of this capacitor should be

small compared to the voltage setting resistor, R2. For

improved AC transient response and to prevent the pos-

sibility of oscillation due to unknown reactive load, a 1µF

capacitor is also recommended at the output. Because of

their low impedance at high frequencies, the best type of

capacitor to use is solid tantalum.

Protection Diodes

The LT138A/LT338A do not require a protection diode

from the adjustment terminal to the output (see Figure 2).

Improved internal circuitry eliminates the need for this

diode when the adjustment pin is bypassed with a capaci-

tor to improve ripple rejection.

If a very large output capacitor is used, such as a 100µF

shown in Figure 2, the regulator could be damaged or

destroyed if the input is accidentally shorted to ground

or crowbarred, due to the output capacitor discharging

into the output terminal of the regulator. To prevent this, a

diode D1 as shown, is recommended to safely discharge

the capacitor.

Figure 2

LT338A

D1

1N4002

R1

NOT

NEEDED

138A/338A F02

ADJ

R2

10μF

100μF

+

Load Regulation

Because the LT138A is a three-terminal device, it is not

possible to provide true remote load sensing. Load regula-

tion will be limited by the resistance of the wire connecting

the regulator to the load. The data sheet specification for

load regulation is measured at the bottom of the package.

Negative side sensing is a true Kelvin connection, with the

bottom of the output divider returned to the negative side

of the load. Although it may not be immediately obvious,

best load regulation is obtained when the top of the resis-

tor divider, R1, is connected

directly to the case not to the

load. This is illustrated in Figure 3. If R1 were connected

to the load, the effective resistance between the regulator

and the load would be:

R2+R1

R1

⎛

⎝

⎜

⎞

⎠

translates to 4mV/ft at 1A load current, so it is important

to keep the positive lead between regulator and load as

short as possible, and use large wire or PC board traces.