AAT3123/23A/24
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 fractional charge pump consists
of a low dropout linear voltage regulator followed
by a 1.5X charge pump with multiple current-
source outputs. To maximize power conversion
efficiency, an internal feedback control sensing cir-
cuit 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 out-
put voltage level needed. This function significant-
ly enhances overall device efficiency when the
input voltage level is greater than the voltage
required at the constant current source outputs.
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, whichev-
er 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.
3123.2005.12.1.4
The AAT3123/23A/24 requires only four external
components: 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) con-
stant current outputs to drive four or six individual
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 inter-
face records rising edges of the EN/SET pin and
decodes them into 32 individual current level set-
tings 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 interface
wraps states back to State 1 after the 32nd clock.
With each EN/SET pulse, the output current increas-
es 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 intermedi-
ate 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 pro-
grammed level. The device is disabled 500µs after
the EN/SET pin transitions to a logic low state.
7
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
