AAT2842 Analogic Corporation, AAT2842 Datasheet - Page 11

• Manufacturer Part Number: AAT2842
• Manufacturer: Analogic Corporation

The backlight and flash LED currents are controlled
by a combination of an external programming
resistor from BSET (for backlight) or FSET (for
flash) to AGND and the backlight or flash serial
S
ming resistor sets the maximum LED current for
each channel, and the serial S
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. Depending upon the application and the
white LEDs chosen, the preferred method to
decrease the maximum BL output current is to use
a 280kΩ resistor for R
current output using the product's S
face, as shown in Table 1.
Using Backlight LED Outputs for
Low-Current LED Applications
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 R
desired current output using the product's S
interface, as shown in Table 1.
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.
S
The S
the EN/SET pin and decodes them into 16 different
states. The S
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.
Once EN/SET has been held in the logic high state
for time T
active and the internal data register is reset to zero.
For subsequent current level programming, the
2842.2006.10.1.0
2
2
Cwire interface BENS or FENS. The program-
Cwire Serial Interface
2
Cwire serial interface records rising edges of
LAT
, the programmed current becomes
2
Cwire interface has flexible timing;
BSET
and then set the desired
BSET
2
Cwire interface con-
and then set the
2
Cwire inter-
Control and Dual LDO for Backlight and Flash
2
Cwire
High-Current Charge Pump with S
number of rising edges corresponding to the
desired code must be entered on the EN/SET pin.
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 B
level settings, again as a percentage of the maxi-
mum flash current set by the F
Tables 1 and 2). Initiating a flash current also initi-
ates the flash timer which is programmed via an
external capacitor C
Calculate the flash time T by the following equation:
where T is in seconds and C
For example, for a 0.1µF capacitor:
To disable the flash timer, connect C
Table 1: Backlight Current Register: BL1-BL4
SET
Data
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
9
resistance. The flash has 16 current
(R
T = 10 × 0.1µF
BSET
T = 10 × C
= 1s
T
.
= 280kΩ Ω ).
T
T
is in µF.
SET
BL% of B
resistance (see
49.2
41.0
33.0
28.2
23.5
18.7
15.4
12.3
100
T
7.7
4.1
2.4
0.2
82
70
59
2
to AGND.
Cwire™
SET
11