AAT2803 datasheet(14/16 Pages) AAT

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AAT2803

Six-Channel White LED Backlight

Charge Pump with Flash

2803.2006.12.1.6

Max Current Register Settings—Address 4

Low Current Register Settings—Address 5

Disabled Current Sinks

The backlight charge pump is equipped with an auto-

disable feature to protect against an LED failure con-

dition. Current sink inputs that are not used should

be disabled.

To disable and properly terminate

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 chan-

nel is less than 120μA.

Applications Information

LED Selection

Although the AAT2803 is specifically intended for

driving white LEDs, the device can also be used to

drive most types of LEDs with forward voltage

specifications ranging from 2.0V to 4.7V.

LED

applications may include main and sub-LCD dis-

play 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.

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 AAT2803 make it capable of

driving LEDs with forward voltages as high as 4.7V

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 AAT2803 operates at a fixed frequency of

approximately 1MHz to control noise and limit har-

monics that can interfere with the RF operation of

cellular telephone handsets or other communica-

tion devices. Back-injected noise appearing on the

input pin of the charge pump is 20mV peak-to-

peak, typically ten times less than inductor-based

DC/DC boost converter white LED backlight solu-

tions. The AAT2803 soft-start feature prevents

noise transient effects associated with inrush cur-

rents 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,

output ripple, and transient performance. Optimum

performance 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 four capacitors

is a good starting point when choosing capacitors.

If the LED current sources are only programmed

for light current levels, then the capacitor size may

be decreased.

Data

D1-D4 (mA)

D5-D6 (mA)

0.05

0.5

0.05

0.5

0.05

0.5

Data

Max Current

20mA Max Scale

30mA Max Scale

15mA Max Scale

Low Current Mode

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