AAT2842
2
High-Current Charge Pump with S Cwire™
Control and Dual LDO for Backlight and Flash
The backlight and flash LED currents are controlled
The AAT2842 features separate control interfaces
for the backlight and flash current control. The
backlight current features 16 current steps, each as
a percentage of the maximum backlight current set
by the BSET resistance. The flash has 16 current
level settings, again as a percentage of the maxi-
mum flash current set by the FSET resistance (see
Tables 1 and 2). Initiating a flash current also initi-
ates the flash timer which is programmed via an
external capacitor CT.
by a combination of an external programming
resistor from BSET (for backlight) or FSET (for
flash) to AGND and the backlight or flash serial
2
S Cwire interface BENS or FENS. The program-
ming resistor sets the maximum LED current for
2
each channel, and the serial S Cwire interface con-
trols the LED current relative to the maximum.
To drive backlight LEDs with optimal absolute
accuracy and channel-to-channel matching, the
maximum output current is set to 30mA with a
280kΩ resistor connected at the BSET pin of the
AAT2842.
Calculate the flash time T by the following equation:
T = 10 · CT
Using Backlight LED Outputs for
Low-Current LED Applications
where T is in seconds and CT is in µF.
For example, for a 0.1µF capacitor:
The AAT2842's backlight current outputs can be pro-
grammed to drive lower current LEDs, such as those
used for keypad applications. For best low-current
accuracy and matching, the preferred method is to
use a 280kΩ resistor for RBSET and then set the
desired current output using the product's S Cwire
interface, as shown in Table 1.
T = 10 · 0.1μF
= 1s
2
To disable the flash timer, connect CT to AGND.
If any one of the current sinks is not used, connect
the unused current sink to OUT. The current con-
troller monitors the current sink voltage and, if it is
connected to OUT, then it is assumed that the cur-
rent sink is not used or that the LED is shorted, and
the controller turns off that current sink.
Data
1
2
3
4
BL% of BSET
100
82
70
59
5
6
7
8
49.2
41.0
33.0
28.2
23.5
18.7
15.4
12.3
7.7
2
S Cwire Serial Interface
2
The S Cwire serial interface records rising edges of
the EN/SET pin and decodes them into 16 different
2
states. The S Cwire interface has flexible timing;
9
data can be clocked-in at speeds greater than
1MHz or much slower, such as 15kHz. After data is
submitted, EN/SET is held high to latch the data.
10
11
12
13
14
15
16
Once EN/SET has been held in the logic high state
for time TLAT, the programmed current becomes
active and the internal data register is reset to zero.
For subsequent current level programming, the
number of rising edges corresponding to the
desired code must be entered on the EN/SET pin.
4.1
2.4
0.2
Table 1: Backlight Current Register: BL1-BL4
(RBSET = 280kΩ).
2842.2007.09.1.2
11
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