AAT3175
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Flash Applications
Functional Block Diagram
C1+
C1- C2+
C2-
VIN
1X, 1.5X, 2X
Charge Pump
VOUT
Soft Start
Control
1MHz
Oscillator
Voltage
Reference
D/A
D/A
6 x 16 bit
ROM
D/A
D/A
D/A
D1
D2
D3
D4
EN/SET
S
2
Cwire
Interface
D/A
GND
Functional Description
The AAT3175 is a tri-mode load switch (1X) and
high efficiency (1.5X or 2X) charge pump device
intended for white LED flash applications. To max-
imize 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 AAT3175
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 AAT3175 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 charge pump output capacitor (C
OUT
).
The four constant current sink inputs (D1 to D4) can
drive four individual LEDs with a maximum current
of 50mA each for D2 and D3. D1 and D4 are driven
by two parallel current sources, resulting in a maxi-
mum of 100mA each. Unused sink inputs are con-
nected to V
OUT
; otherwise, the part will operate only
in 2X charge pump mode. The S
2
Cwire serial inter-
face enables the AAT3175 and sets the current sink
magnitudes. The D1~D4 current configuration is
optimized to drive either 2-in-1 LEDs by combining
D1/D2, D3/D4, or 3-in-1 LEDs by combining D2/D3.
Constant Current Output Level Settings
The constant current sink levels for D1 to D4 are set
via the serial interface according to a logarithmic
scale where each code is 1dB lower than the previ-
ous 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 determined by the actual forward
voltage (V
F
) for the LED being driven.
Since the input current sinks of the AAT3175 are
programmable, no PWM (pulse width modulation) or
additional control circuitry is needed to control LED
brightness. This feature greatly reduces the burden
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3175.2006.01.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
