AAT2805 Datasheet, PDF (11/17 Pages) Advanced Analogic Technologies – Dual High Efficiency Charge Pump for White LED and Flash Applications

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 (C_IN), one 0.33µF to 1µF ceramic
capacitor for backlight charge pump output capacitor
(C_OUT), and one 1µF ceramic capacitor for flash
charge pump output capacitor (C_OUT).
Constant Current Output Level Settings
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 V_OUT
and the actual forward voltage (V_F) of the LED being
driven.
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 microcon-
troller or system IC to manage LED or display bright-
ness, 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 successively to
brighten or dim LEDs, in smooth transitions (e.g., to
fade-out) or in abrupt steps, giving the user complete
programmability and real-time control of LED bright-
ness.
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.
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.
Data
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
20mA
20.0
17.8
15.9
14.3
12.7
11.1
10.2
8.9
7.9
7.0
6.3
5.7
5.1
4.4
4.1
0.0
Max I_OUT (mA)
30mA
30.0
26.7
23.8
21.4
19.0
16.7
15.2
13.3
11.9
10.5
9.5
8.6
7.6
6.7
6.2
0.0
15mA
15.0
13.3
11.9
10.7
9.5
8.3
7.6
6.7
6.0
5.2
4.8
4.3
3.8
3.3
3.1
0.0
Table 1: Constant Current Programming Levels.
Data
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
D1-D3 (mA)
0
0
0
0
0
0
0
0
0.05
0.5
1
2
0.05
0.5
1
2
D4 (mA)
0
0
0
0
0.05
0.5
1
2
0
0
0
0
0.05
0.5
1
2
Table 2: Low Current Register Settings.
2805.2005.05.1.2
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