AAT2806
Dual High Efficiency Charge Pump
for White LED and Flash Applications
2
S Cwire Serial Interface Timing Diagram
THI
TOFF
TLO
TLAT
EN/SET
1
2
n-1
n ≤ 16
0
n
0
Data Reg
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.
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.
Device Switching Noise Performance
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.
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.
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.
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
100mΩ. Avalue of 1µF for all six capacitors is a good
starting point when choosing capacitors.
12
2806.2007.09.1.9
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
