AAT2803 datasheet(12/16 Pages) AAT

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AAT2803

Six-Channel White LED Backlight

Charge Pump with Flash

2803.2006.12.1.6

Constant Current Output Level Settings

The constant current sink levels for the white LED

backlight (D1 to D6) are set via the serial interface

according to a logarithmic scale where each code is

1dB lower than the previous code. In this manner,

LED brightness appears linear with each increasing

code count. Because the inputs D1 to D6 are true

independent constant current sinks, the voltage

observed on any single given input will be determined

by the actual forward voltage (VF) for the LED being

driven.

Since the input current sinks are programmable, no

PWM (pulse width modulation) or additional control

circuitry is needed to control LED brightness. This fea-

ture 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 input sink

current 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 programmabil-

ity and real-time control of LED brightness.

The 16 individual current level settings are each

approximately 1dB apart (see Current Level Settings

table). Code 1 is full scale, Code 15 is full scale atten-

uated by 14dB, and Code 16 is reserved as a "no cur-

rent" setting.

Table 1: Current Level Settings.

AS2Cwire Serial Interface

The Advanced Simple Serial Control (AS2Cwire)

single wire interface is used to set the possible

combinations of current levels and LED channel

states. AS2Cwire has addressing capability for

multiple data registers. With multiple data regis-

ters, the backlight charge pump main and sub-

channels can be programmed together or inde-

pendently from one another.

AS2Cwire relies on the number of rising edges of

the EN/SET pin to address and load the registers.

AS2Cwire latches data or address after the

EN/SET pin has been held high for time TLAT.

Address or data is differentiated by the number of

EN/SET rising edges. Since the data registers are

4 bits each, the differentiating number of pulses is

24 or 16, so that Address 1 is signified by 17 rising

edges, Address 2 by 18 rising edges, and so forth.

Data is set to any number of rising edges between

1 and including 16. A typical write protocol is a

burst of EN/SET rising edges, signifying a partic-

ular Address, followed by a pause with EN/SET

held high for the TLAT timeout period, a burst of ris-

ing edges signifying Data, and a TLAT timeout for

the data registers. Once an address is set, then

multiple writes to the corresponding data register

are allowed.

When EN/SET is held low for an amount of time

greater than TOFF, the backlight charge pump

enters into shutdown mode and draws less than

1μA from the supply. Address 1 is the default

address on the first rising edge after the backlight

charge pump has been disabled. Whenever shut-

down mode is entered, all registers are reset to 1.

30mA Max 20mA Max

15mA Max

Data

IOUT (mA)

30.0

20.0

15.0

26.7

17.8

13.3

23.8

15.9

11.9

21.4

14.3

10.7

19.0

12.7

9.5

16.7

11.1

8.3

15.2

10.2

7.6

13.3

8.9

6.7

11.9

7.9

6.0

10.5

7.0

5.2

9.5

6.3

4.8

8.6

5.7

4.3

7.6

5.1

3.8

6.7

4.4

3.3

6.2

4.1

3.1

0.0

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