AAT3156 Datasheet, PDF (13/14 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump For White LED Applications

AAT3156
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
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 Equivalent Series Resistance
(ESR) ceramic capacitors are used. In general, low
ESR may be defined as less than 100mΩ. A capac-
itor value of 1µF for all four capacitors is a good
starting point when choosing capacitors. If the con-
stant current sinks are only programmed for light
current levels, then the capacitor size may be
decreased.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3156. 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 to maximize charge pump
transient response. Since ceramic capacitors are
non-polarized, they are not prone to incorrect con-
nection damage.
Equivalent Series Resistance
ESR is an important characteristic to consider
when selecting a capacitor. ESR is a resistance
internal to a capacitor which 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.
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 sta-
ble over temperature. Larger capacitor values are
typically composed of X7R, X5R, Z5U, or Y5V
dielectric materials. Large ceramic capacitors, typ-
ically greater than 2.2µF, are often available in low-
cost Y5V and Z5U dielectrics, but capacitors
greater than 1µF are typically not required for
AAT3156 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.
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