AAT2806
Dual High Efficiency Charge Pump
for White LED and Flash Applications
The flash charge pump is a charge pump doubler
with regulated output voltage. It is designed to deliv-
er 120mA of continuous current and 250mA of pulsed
current.
IOUT (mA)
Data
1
2
3
4
5
6
7
8
D1-D3
20
14
10
7
Sub-Group D4
20
14
10
7
0
0
The AAT2806 requires six external components:
three 1µF ceramic capacitors for the charge pump
flying capacitors (C1, C2, and C3), one 1µF ceram-
ic input capacitor (CIN), one 0.33µF to 1µF ceramic
capacitor for the backlight charge pump output
capacitor (COUT), and one 1µF ceramic capacitor
for the flash charge pump output capacitor (COUT).
20
14
10
7
0
0
0
0
0.05
0.5
1
0
0
9
20
14
10
7
0.05
0.5
1
10
11
12
13
14
15
16
Constant Current Output Level Settings
The constant current level for the LED channels is set
2
via the S Cwire serial interface (see Table 1).
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.
2
2
Table 1: Constant Current Programming Levels.
Since the constant current levels are programma-
ble, no PWM (pulse width modulation) or addition-
al control circuitry is needed to control LED bright-
ness. This feature greatly reduces the burden on a
microcontroller or system IC to manage LED or dis-
play brightness, allowing the user to "set it and for-
get 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, giv-
ing the user complete programmability and real-
time control of LED brightness.
2
S Cwire Serial Interface
The current sink input magnitude on the backlight
charge pump is controlled by AnalogicTech's
2
S Cwire serial digital input. The interface relies on
the number of rising edges of the EN/SET pin to
2
address and load the registers. S Cwire latches
data or address after the EN/SET pin has been
held high for time TLAT. The interface records rising
edges of the EN/SET pin and decodes them into 16
different states, as indicated in Table 1. There are
four brightness levels for the main or sub-display
group with the possibility of individually turning ON
or OFF each group. To further optimize power effi-
ciency, the device also offers four low-current lev-
els for dim LED operation (Data 13 to 16). During
this low-current mode, the internal supply current
reduces to only 50µA typical.
The AAT2806 offers an additional Low Current mode
with reduced quiescent current (Data 13 to 16). 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.
The counter can be clocked at speeds up to 1MHz,
such that intermediate states are not visible. The
first rising edge of EN/SET enables the IC and ini-
tially sets the output LED current to 20mA. Once
the final clock cycle is input for the desired bright-
ness level, the EN/SET pin is held high to maintain
the device output current at the programmed level.
The device is disabled 500µs (TOFF) after the
EN/SET pin transitions to a logic low state.
2806.2007.09.1.9
11
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
