AAT3151B_08 Datasheet, PDF (11/13 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
Applications Information
LED Selection
The AAT3151B is specifically intended for driving white
LEDs. However, the device design will allow the AAT3151B
to drive most types of LEDs with forward voltage speci-
fications ranging from 2.0V to 4.3V. LED applications
may include main and sub-LCD display backlighting,
camera photo-flash applications, color (RGB) LEDs,
infrared (IR) diodes for remotes, and other loads bene-
fiting from a regulated output current generated from a
varying input voltage. Since the D1 to D4 constant cur-
rent sinks are matched with negligible voltage depen-
dence, the LED brightness will be matched regardless of
the specific LED forward voltage (V_F) levels.
In some instances (e.g., in high luminous output appli-
cations such as photo flash), it may be necessary to
drive high-V_F type LEDs. The low dropout current sinks
in the AAT3151B make it capable of driving LEDs with
forward voltages as high as 4.3V at full current from an
input supply as low as 3.0V. Current sinks can be paral-
leled to drive high-current LEDs without complication.
Device Switching Noise Performance
The AAT3151B operates at a fixed frequency of approxi-
mately 1MHz to control noise and limit harmonics that
can interfere with the RF operation of cellular telephone
handsets or other communication devices. Back-injected
noise appearing on the input pin of the charge pump is
20mV peak-to-peak, typically ten times less than induc-
tor-based DC/DC boost converter white LED backlight
solutions. The AAT3151B soft-start feature prevents
noise transient effects associated with inrush currents
during start-up of the charge pump circuit.
Power Efficiency and Device Evaluation
The charge pump efficiency discussion in the following
sections only accounts for the efficiency of the charge
pump section itself. Due to the unique circuit architec-
ture and design of the AAT3151B, it is very difficult to
measure efficiency in terms of a percent value compar-
ing input power over output power.
Since the AAT3151B inputs are pure constant current
sinks and typically drive individual loads, it is difficult to
measure the output voltage for a given input (D1 to D4)
to derive an overall output power measurement. For any
given application, white LED forward voltage levels can
differ, yet the load drive current will be maintained as a
constant.
This makes quantifying output power a difficult task
when taken in the context of comparing to other white
LED driver circuit topologies. A better way to quantify
total device efficiency is to observe the total input power
to the device for a given LED current drive level. The
best white LED driver for a given application should be
based on trade-offs of size, external component count,
reliability, operating range, and total energy usage...not
just % efficiency.
The AAT3151B efficiency may be quantified under very
specific conditions and is dependent upon the input volt-
age versus the output voltage seen across the loads
applied to inputs D1 through D4 for a given constant
current setting. Depending on the combination of V_IN
and voltages sensed at the current sinks, the device will
operate in load switch mode. When any one of the volt-
ages sensed at the current sinks nears dropout, the
device will operate in 1.5X charge pump mode. Each of
these modes will yield different efficiency values. Refer
to the following two sections for explanations for each
operational mode.
Load Switch Mode Efficiency
The AAT3151B load switch mode is operational at all
times and functions alone to enhance device power con-
version efficiency when V_IN is greater than the voltage
across the load. When in load switch mode, the voltage
conversion efficiency is defined as output power divided
by input power:
η
=
P_OUT
P_IN
The expression to define the ideal efficiency (η) can be
rewritten as:
-or-
η = ^POUT = ^{VOUT · IOUT} = ^VOUT
P_IN
V_IN · I_OUT
V_IN
η(%)
=
100
⎛
⎝
V_{OUT ⎞}
V_IN ⎠
3151B.2008.02.1.1
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