AAT3169_08 Datasheet, PDF (11/13 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications

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PRODUCT DATASHEET
AAT3169
High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications
Applications Information
LED Selection
Although the AAT3169 is designed for driving white
LEDs, the device also can be used to drive most types of
LEDs with forward voltages from 2.0V to 4.7V. LED appli-
cations may include main and sub-LCD display back-
lighting, camera photo-flash applications, color (RGB)
LEDs, infrared (IR) diodes for remotes, and other loads
benefiting from a controlled output current generated
from a varying input voltage. Since the D1 to D6 input
current 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 AAT3169 make it capable of driving LEDs with
forward voltages as high as 4.7V 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 AAT3169 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 AAT3169 soft-start feature prevents noise
transient effects associated with inrush currents during
start-up of the charge pump circuit.
Capacitor Selection
Careful selection of the four external capacitors (C_IN, C₁,
C₂, and C_OUT) is important because they will affect turn-
on time, output ripple, and transient performance.
Optimum performance will be obtained when low ESR
ceramic capacitors are used; in general, low ESR may be
defined as less than 100mΩ. A value of 1µF for all four
capacitors is a good starting point when choosing capac-
itors. If the LED current sources are programmed only
for light current levels, then the capacitor size may be
decreased.
Capacitor Characteristics
Ceramic composition capacitors are highly recommend-
ed over all other types of capacitors for use with the
AAT3169. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor typically has very low equiva-
lent series resistance (ESR), is lowest cost, has a small-
er PCB footprint, and is non-polarized. Low ESR ceramic
capacitors help maximize charge pump transient
response. Since ceramic capacitors are non-polarized,
they are not prone to incorrect connection damage.
Equivalent Series Resistance
ESR is an important characteristic to consider when
selecting a capacitor. ESR is resistance internal to a
capacitor that is caused by the leads, internal connec-
tions, size or area, material composition, and ambient
temperature. Capacitor ESR is typically measured in mil-
liohms for ceramic capacitors and can range to more
than several ohms for tantalum or aluminum electrolytic
capacitors.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1µF are typically made
from NPO or C0G materials. NPO and C0G materials
generally have tight tolerance and are very stable over
temperature. Larger capacitor values are usually com-
posed of X7R, X5R, Z5U, or Y5V dielectric materials.
Large ceramic capacitors (i.e., greater than 2.2µF) are
often available in low-cost Y5V and Z5U dielectrics, but
capacitors greater than 1µF are not typically required for
AAT3169 applications.
Capacitor area is another contributor to ESR. Capacitors
that are physically large will have a lower ESR when
compared to an equivalent material smaller capacitor.
These larger devices can improve circuit transient
response when compared to an equal value capacitor in
a smaller package size.
3169.2008.03.1.3
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