AAT2806 datasheet(12/16 Pages) AAT

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AAT2806

Dual High Efficiency Charge Pump

for White LED and Flash Applications

2806.2006.04.1.8

Disabled Current Sinks

The backlight charge pump is equipped with an

auto-disable feature to protect against an LED fail-

ure condition. Current sink inputs that are not used

should be disabled. To disable and properly termi-

nate unused current sink inputs, they must be tied

to VOUT. If left unconnected or terminated to

ground, the part will be forced to operate in 2X

charge pump mode.

Properly terminating unused current sink inputs is

important to prevent the charge pump modes from

activating prematurely. When properly terminated,

only a small sense current flows for each disabled

channel. The sense current for each disabled

channel is less than 10µA.

Applications Information

LED Selection

The AAT2806 is specifically intended for driving

white LEDs. However, the device design will allow

the AAT2806 to drive most types of LEDs with for-

ward voltage specifications ranging from 2.0V to

4.3V. LED applications may include main display

backlighting, camera photo-flash applications,

color (RGB) LEDs, infrared (IR) diodes for remotes,

and other loads benefiting from a controlled output

current generated from a varying input voltage.

Since the D1 to D4 input current sinks are matched

with negligible voltage dependence, the LED

brightness will be matched regardless of the spe-

cific LED forward voltage (VF) levels.

In some instances (e.g., in high-luminous-output

applications such as photo-flash), it may be neces-

sary to drive high-VF type LEDs. The low-dropout

current-sinks in the AAT2806 make it capable of

driving LEDs with forward voltages as high as 4.3V

at full current from an input supply as low as 3.0V.

Outputs can be paralleled to drive high-current

LEDs without complication.

Device Switching Noise Performance

The AAT2806 operates at a fixed frequency of

approximately 1MHz to control noise and limit har-

monics that can interfere with the RF operation of cel-

lular telephone handsets or other communication

devices. Back-injected noise appearing on the input

pin of the charge pump is 20mV peak-to-peak, typi-

cally ten times less than inductor-based DC/DC

boost converter white LED backlight solutions. The

AAT2806 soft-start feature prevents noise transient

effects associated with inrush currents during start-

up of the charge pump circuit.

Capacitor Selection

Careful selection of the six external capacitors CIN,

C1, C2, C3, and COUT (for backlight and flash) is

important because they will affect turn-on time, out-

put ripple, and transient performance. Optimum per-

formance will be obtained when low equivalent series

resistance (ESR) ceramic capacitors are used. In

general, low ESR may be defined as less than

100m

Ω. A value of 1µF for all six capacitors is a good

starting point when choosing capacitors.

S2Cwire Serial Interface Timing Diagram

EN/SET

n-1

n ≤ 16

Data Reg

LAT

TOFF

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