AAT3123/23A/24
High Efficiency 1X/1.5X Fractional Charge
Pump for White LED Applications
EN/SET Serial Interface
The current source output magnitude is controlled
by the EN/SET pin, using AnalogicTech’s Simple
Serial Control (S
2
Cwire) interface. The interface
records rising edges of the EN/SET pin, and
decodes them into 32 individual current level set-
tings each 1dB apart. Code 32 is full scale, and
Code 1 is full scale attenuated by 31dB. The modu-
lo 32 interface wraps states back to State 1 after the
32nd clock, so 1dB of attenuation is achieved by
clocking the EN/SET pin 31 times. The counter can
be clocked at speeds up to 1MHz, so intermediate
states are not visible. The first rising edge of
EN/SET enables the IC and initially sets the output
LED current to -31dB, the lowest setting equal to
525µA. Once the final clock cycle is input for the
desired brightness level, the EN/SET pin is held
high to maintain the device output current at the
programmed level. The device is disabled 500µs
after the EN/SET pin transitions to a logic low state.
The EN/SET timing is designed to accommodate a
wide range of data rates. After the first rising edge
of EN/SET, the charge pump is enabled and reach-
es full capacity after the soft-start time (T
SS
). During
the soft-start time, multiple clock pulses may be
entered on the EN/SET pin to set the final output
current level with a single burst of clocks.
Alternatively, the EN/SET clock pulses may be
entered one at a time to gradually increase LED
brightness over any desired time period. A constant
current is sourced as long as EN/SET remains in a
logic high state. The current source outputs are
switched off after EN/SET has remained in a low
state for at least the t
OFF
timeout period.
EN/SET Timing
t
HI
t
LO
t
OFF
EN/SET
Code
OFF
1
2
3
OFF
LED Selection
The AAT3123/23A/24 is specifically intended for driv-
ing white LEDs. However, the device design will
allow the AAT3123/23A/24 to drive most types of
LEDs with forward voltage specifications ranging
from 2.0V to 4.3V. LED applications may include
main and sub-LCD 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 D6 output cur-
rent sources are matched with negligible voltage
dependence, LED brightness will be matched
regardless of the specific LED forward voltage (V
F
)
levels.
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 sources in the AAT3123/23A/24 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 AAT3123/23A/24 operates at a fixed frequency
of approximately 1MHz to control noise and limit
harmonics 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 solutions. The
AAT3123/23A/24 soft-start feature prevents noise
transient effects associated with inrush currents
during start-up of the charge pump circuit.
3123.2005.12.1.4
9
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
