AAT3122 View Datasheet(PDF) - Advanced Analogic Technologies
Part Name
Description
MFG CO.
AAT3122
Advanced Analogic Technologies
'AAT3122' PDF : 12 Pages View PDF
AAT3122
High Efficiency 1X/1.5X Fractional Charge
Pump for White LED Applications
Capacitor Selection
Careful selection of the four external capacitors CIN,
C1, C2, COUT is important because they will affect
turn on time, output ripple and transient performance.
Optimum performance will be obtained when low
ESR (<100mΩ) ceramic capacitors are used. In gen-
eral, 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): 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 composition
and ambient temperature. Capacitor ESR is typi-
cally measured in milliohms for ceramic capacitors
and can range to more than several ohms for tan-
talum or aluminum electrolytic capacitors.
Ceramic Capacitor Materials: Ceramic capacitors
less than 0.1µF are typically made from NPO or COG
materials. NPO and COG materials typically have
tight tolerance and are stable over temperature. Large
capacitor values are typically composed of X7R, X5R,
Z5U or Y5V dielectric materials. Large ceramic capac-
itors, 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.
10
3122.2004.08.1.2
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