AAT2801
Multi-Mode Charge Pump for
White LED Backlight and Flash Applications
on the voltage at the current source output. Since
Applications Information—
White LED Backlight Section
the LED to LED VF can vary as much as 1 volt, this
function significantly enhances overall efficiency of
the white LED backlight charge pump section when
the battery input voltage level is greater than the
voltage required at any current source output.
Constant Current Output Level Settings
The constant current source amplitudes for D1 to D4
are set via the serial interface according to a loga-
rithmic scale, where each code is 1dB greater than
the previous code. In this manner, LED brightness
appears linear with each increasing code count.
Because the outputs D1 to D4 are true independent
constant current sources, the voltage observed on
any single given output will be determined by the
actual forward voltage (VF) for the LED being driven.
Flash Driver
For flash applications, the voltage doubling
charge pump section is used to support the high
current demand required by the application.
Charge pump regulation is achieved by sensing
the output voltage through an internal resistor
divider network. The free running charge pump
switching frequency is approximately 1MHz. The
charge pump is designed to deliver up to 120mA
of continuous current and 250mA of pulsed cur-
rent for up to 500ms.
Since the output current of the AAT2801 is program-
mable, no PWM (pulse width modulation) or addi-
tional control circuitry is needed to control LED
brightness. This feature 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 output current of the AAT2801 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 brightness.
The AAT2801 requires only six external compo-
nents: three 1µF ceramic capacitors for the
charge pump flying capacitors (C1, C2 and C3),
one 1µF ceramic input capacitor (CIN), one 1µF
ceramic charge pump output capacitor (CCP), and
one 1µF for the flash output (CF). The four con-
stant current outputs (D1 to D4) drive four individ-
ual LEDs with a maximum current of 30mA each.
The EN/SET S2Cwire serial interface enables the
AAT2801 and sets the current source magni-
tudes. A separate flash enable pin (EN_FLSH) is
provided to independently control the flash charge
pump circuit.
The 32 individual current level settings are each
approximately 1dB apart (see Current Level
Settings below). Code 32 is full scale; Code 2 is full
scale attenuated by 31dB; and Code 1 is reserved
as a "no current" setting.
Normalized Current Level Settings
1.00
0.10
0.01
0.00
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Code
14
2801.2006.04.1.3
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
