PRODUCT DATASHEET
AAT3123/23A/24
ChargePump
TM
High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications
Functional Block Diagram
VIN
Soft-Start
Control
1MHz
Oscillator
Voltage
Reference
1x/1.5x
Charge
Pump
C1+
C1-
C2+
C2-
OUT
Current
Current
Reference
D1
Quad
Output
DAC
D2
D3
D4
EN/SET
S
2
Cwire
Interface
32 x 8 bit
ROM
Dual
Output
DAC
D5*
D6*
*AAT3124 only
GND
Functional Description
The AAT3123/23A/24 is a dual mode load switch (1X)
and high efficiency (1.5X) fractional charge pump device
intended for white LED backlight applications. The frac-
tional charge pump consists of a low dropout linear volt-
age regulator followed by a 1.5X charge pump with
multiple current-source outputs. To maximize power
conversion efficiency, an internal feedback control sens-
ing circuit monitors the voltage required on the constant
current source outputs. This control circuit then sets the
load switch and charge pump functions based upon the
input voltage level versus the output voltage level need-
ed. This function significantly enhances overall device
efficiency when the input voltage level is greater than
the voltage required at the constant current source out-
puts. For the AAT3123/23A, the 1X load switch/1.5X
charge pump mode is decided on the voltage sensed on
the output D1. The AAT3124 bases the 1X load switch/1.5X
charge pump mode decision on the voltage levels sensed
on either the D1-D4 output group or the D5-D6 output
group, whichever is greater. Switchover between the
1.5X (charge-pump) operating mode and the 1X (load
switch) mode occurs automatically (as a function of
input and output voltages) and does not require user
intervention to maintain maximum efficiency.
The AAT3123/23A/24 requires only four external compo-
nents: two 1μF ceramic capacitors for the charge pump
flying capacitors (C
1
and C
2
), one 1μF ceramic input
capacitor (C
IN
), and one 0.33μF to 1μF ceramic output
capacitor (C
OUT
). The LDO/1.5X charge pump output is
converted into four (D1 to D4) or six (D1 to D4 and D5 to
D6) constant current outputs to drive four or six individ-
ual LEDs with a maximum current of 20mA each. The
current source output magnitude is controlled by the EN/
SET serial data S
2
Cwire interface. The interface records
rising edges of the EN/SET pin and decodes them into 32
individual current level settings each 1dB apart (see Table
1, Current Level Settings). Code 32 is full scale, and Code
1 is full scale attenuated by 31dB. The modulo 32 inter-
face wraps states back to State 1 after the 32nd clock.
With each EN/SET pulse, the output current increases by
1dB. To decrease the output current by 1dB, 31 EN/SET
clock pulses are required. The counter can be clocked at
speeds up to 1MHz, so intermediate states are not visi-
ble. 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 pro-
grammed level. The device is disabled 500μs after the
EN/SET pin transitions to a logic low state.
3123.2008.03.1.5
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