AAT2803 Datasheet, PDF (14/16 Pages) Advanced Analogic Technologies – Six-Channel White LED Backlight Charge Pump with Flash

Max Current Register Settings—Address 4
Data
1
2
3
4
Max Current
20mA Max Scale
30mA Max Scale
15mA Max Scale
Low Current Mode
Low Current Register Settings—Address 5
Data
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
D1-D4 (mA)
0
0
0
0
0
0
0
0
0.05
0.5
1
2
0.05
0.5
1
2
D5-D6 (mA)
0
0
0
0
0.05
0.5
1
2
0
0
0
0
0.05
0.5
1
2
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 V_OUT.
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.
AAT2803
Six-Channel White LED Backlight
Charge Pump with Flash
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-V_F 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 C_IN,
C₁, C₂, C₃, and C_OUT (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 100mΩ. 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.
14
2803.2006.12.1.6