AAT3151
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
For a charge pump with an output of 5 volts and a
ceramic capacitors help maximize charge pump
transient response. Since ceramic capacitors are
non-polarized, they are not prone to incorrect con-
nection damage.
nominal input of 3.5 volts, the theoretical efficiency
is 95%. Due to internal switching losses and IC
quiescent current consumption, the actual efficien-
cy can be measured at 93%. These figures are in
close agreement for output load conditions from
1mA to 100mA. Efficiency will decrease as load
current drops below 0.05mA or when level of VIN
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, inter-
nal 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 tanta-
lum or aluminum electrolytic capacitors.
approaches VOUT
.
Capacitor Selection
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
performance. Optimum performance will be
obtained when low equivalent series resistance
(ESR) (<100mΩ) ceramic capacitors are used. In
general, low ESR may be defined as less than
100mΩ. A capacitor value of 1µF for all four capac-
itors is a good starting point when choosing capac-
itors. If the constant current sinks are only pro-
grammed for light current levels, then the capacitor
size may be decreased.
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 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
AAT3151 applications.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3151. 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
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.
3151.2004.10.0.6
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