AAT3123 ETC, AAT3123 Datasheet - Page 9
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AAT3123
Manufacturer Part Number
AAT3123
Description
High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications
Manufacturer
ETC
Datasheet
1.AAT3123.pdf
(18 pages)
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EN/SET Serial Interface
The current source output magnitude is controlled
by the EN/SET pin, using AnalogicTech’s Simple
Serial Control (S
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
EN/SET Timing
LED Selection
The AAT3123/24 is specifically intended for driving
white LED's. However, the device design will allow
the AAT3123/24 to drive most types of LED's with
forward voltage specifications ranging from 2.0V to
4.3V. LED applications may include main and sub-
LCD display backlighting, camera photo-flash appli-
cations, color (RGB) LED's, infrared (IR) diodes for
remotes, and other loads benefiting from a controlled
output-current generated from a varying input-volt-
age. Since the D1 to D6 output current sources are
matched with negligible voltage dependence, the
LED brightness will be matched regardless of the
specific LED forward voltage (V
In some instances (e.g. in high-luminous-output
applications such as photo-flash) it may be neces-
sary to drive high-V
3123.2003.09.1.1
2
EN/SET
Cwire) interface. The interface
Code
F
type LED's. The low-dropout
OFF
F
) levels.
1
High Efficiency 1X/1.5X Fractional Charge
t
LO
t
HI
Pump for White LED Applications
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 (tSS). 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 the 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
current-sources in the AAT3123/24 makes it capa-
ble of driving LED's 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 LED's without complication.
Device Switching Noise Performance
The AAT3123/24 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 AAT3123/24 soft-start feature prevents
noise transient effects associated with in-rush cur-
rents during the start up of the charge pump circuit.
2
3
t
OFF
OFF
timeout period.
OFF
9