AAT2805
Dual High Efficiency Charge Pump
for White LED and Flash Applications
The AAT2805 requires six external components:
three 1µF ceramic capacitors for the charge pump
flying capacitors (C1, C2, and C3), one 1µF ceramic
input capacitor (CIN), one 0.33µF to 1µF ceramic
capacitor for backlight charge pump output capacitor
(COUT), and one 1µF ceramic capacitor for flash
charge pump output capacitor (COUT).
Max IOUT (mA)
Data
1
20mA
20.0
17.8
15.9
14.3
12.7
11.1
10.2
8.9
30mA
30.0
26.7
23.8
21.4
19.0
16.7
15.2
13.3
11.9
10.5
9.5
15mA
15.0
13.3
11.9
10.7
9.5
2
3
4
5
6
8.3
Constant Current Output Level Settings
7
7.6
The constant current level for the LED channels is set
via the AS2Cwire serial interface according to a loga-
rithmic scale. The current level spacing is 1dB
between adjacent constant current settings. In this
manner, LED brightness appears to change linearly
when the settings are traversed. Because the inputs
D1 to D4 are true independent constant current
sinks, the voltage observed on any single given input
will be determined by the difference between VOUT
and the actual forward voltage (VF) of the LED being
driven.
8
6.7
9
7.9
6.0
10
11
12
13
14
15
16
7.0
5.2
6.3
4.8
5.7
8.6
4.3
5.1
7.6
3.8
4.4
6.7
3.3
4.1
6.2
3.1
0.0
0.0
0.0
Table 1: Constant Current Programming Levels.
Since the constant current levels are programmable,
no PWM (pulse width modulation) or additional con-
trol circuitry is needed to control LED brightness.
This feature greatly reduces the burden on a micro-
controller 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 LED current drive can be changed suc-
cessively to brighten or dim LEDs, in smooth transi-
tions (e.g., to fade-out) or in abrupt steps, giving the
user complete programmability and real-time control
of LED brightness.
Data
D1-D3 (mA)
D4 (mA)
1
2
0
0
0
0
3
0
0
4
0
0
5
0
0.05
0.5
1
6
0
7
0
8
0
2
9
0.05
0.5
1
0
For each Max Current scale (see Table 1), there are
16 current level settings separated from one another
by approximately 1dB. Code 1 is full-scale current
and Code 15 is full-scale current attenuated by
roughly 14dB. Code 16 is reserved as a "no current"
setting. When programming the charge pump, it will
default to the 20mA maximum scale.
10
11
12
13
14
15
16
0
0
2
0
0.05
0.5
1
0.05
0.5
1
2
2
The AAT2805 offers an additional Low Current mode
with reduced quiescent current (see Table 2). This
mode is especially useful for low-current applications
where a continuous, low-current state is maintained.
The reduction in quiescent current significantly
reduces the impact due to maintaining a continuous
backlighting state.
Table 2: Low Current Register Settings.
2805.2006.04.1.7
11
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
