AAT3128
RGB LED Fashion Lighting Controller
Capacitor Selection
Ceramic Capacitor Materials
Careful selection of the four external capacitors CIN,
C1, C2, and COUT is important because they will
affect turn-on time, output ripple, and transient per-
formance. Optimum performance will be obtained
when low equivalent series resistance (ESR)
ceramic capacitors are used. In general, low ESR
may be defined as less than 50mΩ. For best over-
all performance, 1μF capacitors are recommended.
Capacitors used in AAT3128 applications range in
value from 0.47µF to 2.2µF. Nominally a 1µF
capacitor with a minimum voltage specification
exceeding 6V is recommended. Ceramic capaci-
tors in this range are typically composed of X7R,
X5R, Z5U, or Y5V dielectric materials. The use of
X5R and X7R dielectric capacitors is strongly rec-
ommended due to their inherent low ESR charac-
teristics and improved tolerance specifications over
the device operating temperature range.
Depending on ripple requirements, the input and
output capacitor values can be increased or
decreased accordingly. If lower ripple is desired,
increase the capacitor value. The peak-to-peak rip-
ple seen at the input or output increases or
decreases with capacitor size.
Capacitors that are physically larger will have a
lower ESR when compared to a smaller case size
capacitor made from an equivalent material. These
larger devices can improve circuit transient
response when compared to an equal value capac-
itor in a smaller package size.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3128. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
trolytic counterparts. 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 capacitors are
non-polarized, they are not prone to incorrect con-
nection damage.
Layout Considerations
The AAT3128 is a high-performance device that
operates with a high switching frequency. To
ensure optimal device performance and accommo-
date the fast switched capacitor transients generat-
ed by the AAT3128, special attention should be
given to the printed circuit board (PCB) layout. The
PCB traces connecting C1 and C2 experience fast
current transients. As a result, the trace lengths for
C1 and C2 should be minimized by placing the
components as closely as possible to the IC. This
will minimize path resistance and reduce ringing
due to the inductance associated with trace length.
For best IC performance, CIN and COUT are placed
as closely as possible to the IN and CP pins and
are connected to a solid ground.
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 connections, size or area, material compo-
sition, and ambient temperature. Capacitor ESR is
typically measured in milliohms for ceramic capac-
itors and can range to more than several ohms for
tantalum or aluminum electrolytic capacitors.
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3128.2007.09.1.0
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