AAT3120 Datasheet, PDF (9/14 Pages) Advanced Analogic Technologies – 3-Channel Charge Pump For White LED Applications

Applications Information
LED Selection
The AAT3120 is specifically intended for driving
white LEDs. However, the device design will allow
the AAT3120 to drive most types of LEDs with for-
ward voltage specifications ranging from 2.0V to
4.3V. LED applications may include main and sub-
LCD display backlighting, camera photo-flash appli-
cations, color (RGB) LEDs, infrared (IR) diodes for
remotes, and other loads benefiting from a controlled
output current generated from a varying input volt-
age. Since the D1 to D3 output current sources are
matched with negligible voltage dependence, 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
applications such as photo-flash) it may be neces-
sary to drive high-V_F type LEDs. The low-dropout
current-sources in the AAT3120 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.
Outputs can be paralleled to drive high current
LEDs without complication.
Device Switching Noise Performance
The AAT3120 operates at a fixed frequency of
approximately 1MHz to control noise and limit har-
monics that can interfere with the RF operation of
cellular telephone handsets or other communica-
tion devices. Back-injected noise appearing on the
input pin of the charge pump is 20mV peak-to-
peak, typically ten times less than inductor-based
DC/DC boost converter white LED backlight solu-
tions. The AAT3120 soft-start feature prevents
noise transient effects associated with in-rush cur-
rents during start up of the charge pump circuit.
Power Efficiency and Device
Evaluation
The charge pump efficiency discussion in the fol-
lowing sections only accounts for efficiency of the
charge pump section itself. Due to the unique circuit
architecture and design of the AAT3120, it is very
difficult to measure efficiency in terms of a percent
value comparing input power over output power.
AAT3120
3-Channel Charge Pump
For White LED Applications
Since the AAT3120 outputs are pure constant cur-
rent sources and typically drive individual loads, it is
difficult to measure the output voltage for a given
output (D1 to D3) to derive an overall output power
measurement. For any given application, white LED
forward voltage levels can differ, yet the output 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 driv-
er 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 components
count, reliability, operating range, and total energy
usage...not just % efficiency.
The AAT3120 efficiency may be quantified under
very specific conditions and is dependent upon the
input voltage versus the output voltage seen
across the loads applied to outputs D1 through D3
for a given constant current setting. Depending
upon the case of V_IN being greater than the specif-
ic voltage seen across the load on D1, the device
will operate in "load switch" mode. If the voltage
seen on the constant current source output is less
than V_IN, then the device will operate in 1.5X
charge pump mode. Each of these two modes will
yield different efficiency values. One should refer
to the following two sections for explanations of
each operational mode.
Load Switch Mode Efficiency
The AAT3120 load switch mode is operational at all
times. It functions alone to enhance device power
conversion efficiency when the condition exists
where V_IN is greater than the voltages at the con-
stant current source outputs. When in "load switch"
mode, the voltage conversion efficiency is defined
as output power divided by input power:
η
=
P_OUT
P_IN
3120.2005.11.1.1
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