AAT3140

High Efficiency 1X/1.5X/2X Charge Pump

for White LED Applications

3140.2007.03.1.3

11

Applications Information

Constant Current Output Level Settings

The constant current source amplitudes for D1 to D4

are set via the serial interface according to a loga-

rithmic scale where each code is 1dB greater than

the previous code. In this manner, LED brightness

appears linear with each increasing code count.

Because the outputs D1 to D4 are true independent

constant current sources, the voltage observed on

any single given output will be determined by the

actual forward voltage (V

Since the output current of the AAT3140 is pro-

grammable, no PWM (pulse width modulation) or

additional control circuitry are needed to control

LED brightness. This feature greatly reduces the

burden on a microcontroller or system IC to manage

LED or display brightness, allowing the user to "set

it and forget it." With its high-speed serial interface

(1MHz data rate), the output current of the AAT3140

can be changed successively to brighten or dim

LEDs in smooth transitions (e.g., to fade out) or in

abrupt steps, giving the user complete programma-

bility and real-time control of LED brightness.

The individual current level settings are each

approximately 1dB apart for settings above Code 8

(see Figure 1). The current level settings below

Code 8 are more than 1dB apart and serve the

needs for transmissive displays and other low-cur-

rent applications.

Figure 1: Normalized Current Level Settings.

Table 1: Constant Current Source Output Nominal Programming Levels.

I

I

I

I

Code

(mA)

Code

(mA)

Code

(mA)

Code

(mA)

1

0.0

9

2.0

17

5.1

25

13

2

0.1

10

2.2

18

5.6

26

15

3

0.2

11

2.5

19

6

27

17

4

0.4

12

2.8

20

7

28

19

5

0.5

13

3.2

21

8

29

21

6

0.7

14

3.5

22

9

30

24

7

1.1

15

4.0

23

10

31

27

8

1.8

16

4.5

24

12

32

30

0.00

0.01

0.10

1.00

123456

789

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Code