AAT3122 Datasheet, PDF (10/12 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications

AAT3122
High Efficiency 1X/1.5X Fractional 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 per-
formance. Optimum performance will be obtained
when low ESR ceramic capacitors are used. In
general, low ESR may be defined as less than
100mΩ. When choosing the four capacitors, a
capacitor value of 1µF is a good starting point. 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 recom-
mended over all other types of capacitors for use
with the AAT3122 products. Ceramic capacitors
offer many advantages over their tantalum and alu-
minum electrolytic counterparts. A ceramic capaci-
tor 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
connection damage.
Equivalent Series Resistance: ESR is an impor-
tant characteristic to consider when selecting a
capacitor. ESR is a resistance internal to a capac-
itor that is caused by the leads, internal connec-
tions, size or area, material composition, and ambi-
ent temperature. Capacitor ESR is typically meas-
ured 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 tempera-
ture. Large capacitor values are typically com-
posed of X7R, X5R, Z5U, or Y5V dielectric materi-
als. Large ceramic capacitors, typically greater
than 2.2µF, are often available in low-cost Y5V and
Z5U dielectrics, but large capacitors are not
required in the AAT3122 application.
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.
Thermal Protection
The AAT3122 has a thermal protection circuit that
will shut down the charge pump and current out-
puts if the die temperature rises above the thermal
limit due to short-circuit conditions.
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