AAT3157 Datasheet, PDF (8/11 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump For White LED Applications

Functional Block Diagram
AAT3157
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
C1+ C1- C2+ C2-
IN
EN/SET
1X, 1.5X, 2X
Charge Pump
Soft Start
Control
1MHz
Oscillator
Voltage
Reference
S²Cwire
Interface
D/A
6 x 16 bit
ROM
D/A
D/A
OUTCP
D1
D2
D3
GND
Functional Description
The AAT3157 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 AAT3157
charge pump is enabled when any of the three 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
AAT3157 requires only four external components:
two 1µF ceramic capacitors for the charge pump
flying capacitors (C₁ and C₂), one 1µF ceramic
input capacitor (C_IN), and one 0.33µF to 1µF
ceramic charge pump output capacitor (C_OUT).
The three constant current sink inputs (D1 to D3)
can drive three individual LEDs with a maximum
current of 20mA each. The unused sink inputs
must be connected to OUTCP, otherwise the part
will operate only in 2X charge pump mode. The
S²Cwire serial interface enables the AAT3157 and
sets the current sink magnitudes.
Constant Current Output Level Settings
The constant current sink levels for D1 to D3 are set
via the serial interface according to a logarithmic
scale for the first 12 codes and a separate low-cur-
rent scale for the last four 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 D3 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 AAT3157 are
programmable, no PWM (pulse width modulation) or
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