AAT3152
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
formance. Optimum performance will be obtained
ly measured in milliohms for ceramic capacitors and
can range to more than several ohms for tantalum
or aluminum electrolytic capacitors.
when low Equivalent Series Resistance (ESR)
ceramic capacitors are used. In general, 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 lev-
els, then the capacitor size may be decreased.
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
AAT3152 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.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3152. 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
Thermal Protection
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 composition,
and ambient temperature. Capacitor ESR is typical-
The AAT3152 has a built-in thermal protection cir-
cuit that will shut down the charge pump if the die
temperature rises above the thermal limit, as is the
case during a short circuit of the VOUT pin.
3152.2005.08.1.2
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AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
