AAT3141_07 Datasheet, PDF (13/16 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications

AAT3141
High Efficiency 1X/1.5X/2X Charge Pump
for White LED Applications
LED Selection
The AAT3141 is specifically intended for driving
white LEDs. However, the device design will allow
the AAT3141 to drive most types of LEDs with for-
ward voltage specifications ranging from 1.0V 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 D4 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 AAT3141 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.
Termination of Unused Current Source
Outputs
If any outputs (D1 to D4) are not used, they should
be terminated by connecting the respective unused
output directly to ground. This is required to assure
correct charge pump mode operation. If a given
unused output is not terminated to ground, it will
appear to the respective output’s feedback control
as an infinite impedance load and the unused con-
stant current source will be set for a maximum volt-
age. Although no current flows from the unused
current source, the voltage sensed at that node
could force the charge pump control to activate the
charge pump when it is not necessary. To prevent
this effect, simply connect the unused constant cur-
rent source outputs to ground.
Device Switching Noise Performance
The AAT3141 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
3141.2007.03.1.3
DC/DC boost converter white LED backlight solu-
tions. The AAT3141 soft-start feature prevents
noise transient effects associated with inrush cur-
rents during start-up of the charge pump circuit.
Power Efficiency and Device Evaluation
The charge pump efficiency discussion in the follow-
ing sections only accounts for efficiency of the
charge pump section itself. Due to the unique circuit
architecture and design of the AAT3141, it is very dif-
ficult to measure efficiency in terms of a percent
value comparing input power over output power.
Since the AAT3141 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 D4) to derive an overall output power
measurement. For any given application, white
LED forward voltage levels can differ, yet the out-
put 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 applica-
tion should be based on trade-offs of size, external
component count, reliability, operating range, and
total energy usage...not just % efficiency.
The AAT3141 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 D4 for a
given constant current setting. Depending upon the
case of V_IN being greater than the specific voltage
seen across the load on D1 through D4, 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 or 2X charge
pump mode. Each of these modes will yield differ-
ent efficiency values. Refer to the following two sec-
tions for explanations of each operational mode.
Load Switch Mode Efficiency
The AAT3141 load switch mode is operational at all
times and functions alone to enhance device power
conversion efficiency when V_IN is greater than volt-
age across the load connected to the constant cur-
rent source outputs. When in load switch mode,
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