AAT3151B_08 Datasheet, PDF (8/13 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X Charge Pump For White LED Applications

ChargePump^TM
PRODUCT DATASHEET
AAT3151B
High Efficiency 1X/1.5X Charge Pump For White LED Applications
Functional Block Diagram
C1+ C1- C2+ C2-
VIN
EN/SET
Soft-Start
Control
1MHz
Oscillator
Voltage
Reference
AS²Cwire
Interface
1X and 1.5X
Charge Pump
D/A
D/A
6 x 16 bit
ROM
D/A
D/A
VOUT
D1
D2
D3
D4
GND
Functional Description
The AAT3151B is a dual mode load switch (1X) and high
efficiency (1.5X) charge pump device intended for white
LED backlight applications. To maximize power conver-
sion 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 volt-
age. As the battery discharges over time, the AAT3151B
charge pump is enabled when any of the four current
sinks near dropout.
The AAT3151B 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 1μF ceramic charge pump output capaci-
tor (C_OUT).
The AAT3151B constant current sinks can drive four indi-
vidual LEDs with a maximum current of 30mA per chan-
nel. The AS²Cwire serial interface enables the AAT3151B
and sets the constant current sink magnitudes.
Constant Current Output Level Settings
The constant current level for the LED channels is set via
the AS²Cwire serial interface according to a logarithmic
scale. In this manner, LED brightness appears to change
linearly when the settings in the scale are traversed.
Because the inputs D1 to D4 are true independent con-
stant current sinks, the voltage observed on any single
given input will be determined by the difference between
V_OUT and the actual forward voltage (V_F) of the LED being
driven.
Since the constant current levels for the AAT3151B are
programmable, no PWM (pulse width modulation) or
additional control circuitry are 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 forget it.”
With its high-speed serial interface (>1MHz data rate),
the LED current drive can be changed successively to
brighten or dim LEDs in smooth transitions (e.g., to fade
out) or in abrupt steps, giving the user complete pro-
grammability and real-time control of LED brightness.
For each Max Current scale, there are 16 current level
settings separated from one another by approximately
1dB. Code 1 is full-scale current and Code 15 is full-scale
current attenuated by roughly 14dB. Code 16 is reserved
as a “no current” setting (see Table 1).
The AAT3151B has an additional Low Current mode with
reduced quiescent current. This mode is especially useful
for low-current applications where a continuous, low-
current state is maintained. The reduction in quiescent
current significantly reduces the impact due to maintain-
ing a continuous backlighting state.
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3151B.2008.02.1.1