PRODUCT DATASHEET
AAT3123/23A/24
ChargePump
TM
High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications
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 gener-
ated from a varying input voltage. Since the D1 to D6
output current sources are matched with negligible volt-
age dependence, LED brightness will be matched regard-
less of the specific LED forward voltage (V
F
) levels.
In some instances (e.g., in high luminous output applica-
tions such as photo flash), it may be necessary 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.
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 cur-
rent level settings each 1dB apart. Code 32 is full scale,
and Code 1 is full scale attenuated by 31dB. The modulo
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 ini-
tially 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 pro-
grammed 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 reaches 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.
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 tele-
phone handsets or other communication 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 back-
light solutions. The AAT3123/23A/24 soft-start feature
prevents noise transient effects associated with inrush
currents during start-up of the charge pump circuit.
Power Efficiency and Device Evaluation
The charge pump efficiency discussion in the following
sections only accounts for efficiency of the charge pump
section itself. Due to the unique circuit architecture and
design of the AAT3123/23A/24, it is very difficult to
measure efficiency in terms of a percent value compar-
ing input power over output power.
LED Selection
The AAT3123/23A/24 is specifically intended for driving
white LEDs. However, the device design will allow the
AAT3123/23A/24 to drive most types of LEDs with for-
ward voltage specifications ranging from 2.0V to 4.3V.
EN/SET Timing
t
LO
t
HI
t
OFF
EN/SET
Code
OFF
1
2
3
OFF
3123.2008.03.1.5
www.analogictech.com
9
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
