AAT3194

High Efficiency 1.5X Fractional Charge Pump For White LED Applications

PRODUCT DATASHEET

3194.2008.05.1.1

9

www .analogictech.com

Applications Information

Current Level Settings

LED current level is set via the serial interface according

to a logarithmic scale where each code is 1dB greater

than the previous code. In this manner, the LED bright-

ness appears linear with each increasing code.

Code

20mA max

Code

20mA max

1

0.549

17

3.529

2

0.627

18

4.000

3

0.706

19

4.471

4

0.784

20

5.020

5

0.863

21

5.647

6

1.020

22

6.353

7

1.098

23

7.059

8

1.255

24

7.922

9

1.412

25

8.941

10

1.569

26

10.039

11

1.804

27

11.216

12

1.961

28

12.627

13

2.275

29

14.118

14

2.510

30

15.843

15

2.824

31

17.804

16

3.137

32

20.000

Table 1: Current Level Settings.

EN/SET Serial Interface

The current source output magnitude is controlled by the

EN/SET pin using AnalogicTech’s Simple Serial Control

the EN/SET pin and decodes them into 32 individual cur-

rent level settings each 1dB apart. Code 32 is full scale,

and Code 1 is full scale attenuated by 31dB. The modulo

32 interface wraps states back to state 1 after the 32nd

clock, so 1dB of attenuation is achieved by clocking the

-9dB to -10dB”). The counter can be clocked at speeds

up to 1MHz, so intermediate states are not visible. The

first rising edge of EN/SET enables the IC and initially

sets the output LED current to -31dB, the lowest setting

equal to 525µA. Once the final clock cycle is input for the

desired brightness level, the EN/SET pin is held high to

maintain the device output current at the programmed

level. The device is disabled 500µs after the EN/SET pin

transitions to a logic low state.

The EN/SET timing is designed to accommodate a wide

range of data rates. After the first rising edge of EN/SET,

the charge pump is enabled and reaches full capacity

multiple clock pulses may be entered on the EN/SET pin

to set the final output current level with a single burst of

clocks. Alternatively, the EN/SET clock pulses may be

entered one at a time to gradually increase the LED

brightness over any desired time period. A constant cur-

rent is sourced as long as EN/SET remains in a logic high

state. The current source outputs are switched off after

timeout period (see Figure 2).

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1

2

3

4

5

6

7

8

9 1011 121314 15161718 192021 222324 252627 282930 3132

Code

Figure 1: Normalized Current Level Settings.