AAT3134
High Efficiency 1X/1.5X Fractional
Charge Pump for White LED Applications
EN/SET Timing Diagram
tHI
tOFF
tLO
EN/SET
OFF
OFF
Code
1
2
3
ically ten times less than inductor-based DC/DC
boost converter white LED backlight solutions. The
AAT3134 soft-start feature prevents noise transient
effects associated with inrush currents during start-
up of the charge pump circuit.
LED Selection
The AAT3134 is specifically intended for driving
white LEDs. However, the device design will allow
the AAT3134 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 D6 output current sources are
matched with negligible voltage dependence, the
LED brightness will be matched regardless of the
specific LED forward voltage (VF) levels.
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 AAT3134, it is very dif-
ficult to measure efficiency in terms of a percent
value comparing input power over output power.
Since the AAT3134 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 D6) 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.
In some instances (e.g., in high luminous output
applications such as photo flash), it may be neces-
sary to drive high-VF type LEDs. The low dropout
current sources in the AAT3134 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.
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
components count, reliability, operating range, and
total energy usage...not just % efficiency.
Device Switching Noise Performance
The AAT3134 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 communication
devices. Back-injected noise appearing on the input
pin of the charge pump is 20mV peak-to-peak, typ-
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3134.2005.12.1.1
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