AAT3158
High Efficiency 1X/1.5X/2X Charge Pump
for White LED Applications
Functional Block Diagram
C1+
C1- C2+
C2-
V
IN
1X, 1.5X, 2X
Charge Pump
V
OUT
Soft-Start
Control
1MHz
Oscillator
Voltage
Reference
D/A
6 x 16 bit
ROM
D/A
D/A
D/A
D1
D2
D3
D4
GND
EN/SET
S
2
Cwire
Interface
Functional Description
The AAT3158 is a tri-mode load switch (1X) and
high efficiency (1.5X or 2X) charge pump device
intended for white LED backlight applications. To
maximize power conversion efficiency, an internal
sensing circuit monitors the voltage required on
each constant current sink input and sets the load
switch and charge pump modes based on the input
battery voltage and the current sink input voltage.
As the battery discharges over time, the AAT3158
charge pump is enabled when any of the four cur-
rent sink inputs nears dropout. The charge pump
initially starts in 1.5X mode. If the charge pump
output droops enough for any current source out-
put to become close to dropout, the charge pump
will automatically transition to 2X mode. The
AAT3158 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 charge pump output capacitor (C
OUT
).
The four constant current sink inputs (D1 to D4)
can drive four individual LEDs with a maximum cur-
rent of 20mA each. The unused sink inputs must be
8
connected to V
OUT
; otherwise, the part will operate
only in 2X charge pump mode. The S
2
Cwire serial
interface enables the AAT3158 and sets the cur-
rent sink magnitudes.
Constant Current Output Level Settings
The constant current sink levels for D1 to D4 are
set via the serial interface according to a logarith-
mic scale for the first 12 codes, and a separate low
current scale for the last 4 codes. For the first 12
codes, each code is approximately 1.5dB lower
than the previous code. In this manner, LED
brightness appears linear with each increasing
code count. Because the inputs D1 to D4 are true
independent constant current sinks, the voltage
observed on any single given input will be deter-
mined by the actual forward voltage (V
F
) for the
LED being driven.
Since the input current sinks of the AAT3158 are
programmable, no PWM (pulse width modulation) or
additional control circuitry is needed to control LED
brightness. This feature greatly reduces the burden
on a microcontroller or system IC to manage LED or
display brightness, allowing the user to "set it and
3158.2006.02.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
