AAT3159 Datasheet, PDF (12/15 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications

ChargePump^TM
PRODUCT DATASHEET
AAT3159
High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications
Device Switching Noise Performance
The AAT3159 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 AAT3159 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, C_OUT) is important because they will affect turn-on
time, output ripple, and transient performance. Optimum
performance will be obtained when low equivalent series
resistance (ESR) ceramic capacitors are used; in gener-
al, low ESR may be defined as less than 100mΩ. A value
of 1μF for all four capacitors is a good starting point
when choosing capacitors. If the LED current sources
are only programmed 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
AAT3159. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor typically has very low ESR, is
lowest cost, has a smaller PCB footprint, and is non-
polarized. Low ESR ceramic capacitors help maximize
charge pump transient response. Since ceramic capaci-
tors 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 a 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
milliohms 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
typically have tight tolerance and are stable over tem-
perature. Larger capacitor values are typically com-
posed of X7R, X5R, Z5U, or Y5V dielectric materials.
Large ceramic capacitors, typically greater than 2.2μF,
are often available in low-cost Y5V and Z5U dielectrics,
but capacitors greater than 1μF are typically not required
for AAT3159 applications. Capacitor area is another con-
tributor 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.
PCB Layout
To achieve adequate electrical and thermal performance,
careful attention must be given to the printed circuit
board (PCB) layout of the AAT3159. Figures 5 and 6
illustrate an example PCB layout for the AAT3159 (eval-
uation board). The flying capacitors (C1 and C2), input
capacitor (C4), and output capacitor (C3) should be con-
nected as close as possible to the IC. In addition to the
external passive components being placed as close as
possible to the IC, all traces connecting the AAT3159
should be as short and wide as possible to minimize path
resistance and potential coupling.
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