6

LT138A/LT338A

LM138/LM338

138afb

For example, using 2% resistors and

±4% tolerance for

5V regulator design would be 4.66V

approximately

±7%. If the same example were used for a

15V regulator, the expected tolerance would be

±8%.With

these results most applications required some method of

trimming, usually a trim pot. This solution is both expen-

sive and not conductive to volume production.

One of the enhancements of Linear Technology’s adjust-

able regulators over existing devices is the tightened

sive 1% or 2% film resistors to be used for R1 and R2 to

set the output voltage within an acceptable tolerance.

With a guaranteed 1% reference, a 5V power supply

design, using

±2% resistors, would have a worst-case

manufacturing tolerance of

±4%. If 1% resistors are used,

the tolerance will drop to

±2.5%. A plot of the worst-case

output voltage tolerance as a function of resistor tolerance

is shown on the front page of this data sheet.

For convenience, a table of standard 1% resistor values is

shown in Table 1.

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 1.21

Ω,

12.1

Ω, 121Ω, 1.21k etc.

Bypass Capacitors

Input bypassing using a 1

µF 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 10

µF capacitor

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

capacitor to 20

µF 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 possi-

bility of oscillation due to unknown reactive load, a 1

µF

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 100

µF

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.

LT338A

D1

1N4002

R1

NOT

NEEDED

138A/338A F02

ADJ

R2

10

µF

100

µF

+

Figure 2

Load Regulation

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

possible to provide true remote load sensing. Load regu-

lation will be limited by the resistance of the wire connect-

ing 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 connec-

tion, with the bottom of the output divider returned to the