AAT3194
High Efficiency 1.5X Fractional Charge
Pump For White LED Applications
current drops below 0.05mA or when the level of
the power supply of IN approaches OUT. Refer to
the Typical Characteristics section of this datasheet
for measured plots of efficiency versus input volt-
age and output load current for the given charge
pump output voltage options.
formance. 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 value of
1µF for all four capacitors is a good starting point
when choosing capacitors. If the LED current
sources are only programmed for minimal current
levels, then the capacitor size may be decreased.
Power Efficiency and Device Evaluation
The charge pump efficiency discussion in the previ-
ous section only accounts for efficiency of the
charge pump section itself. Due to the unique cir-
cuit architecture and design of the AAT3194, it is
very difficult to measure efficiency in terms of a per-
cent value comparing input power over output
power. Since the device outputs are pure constant
current sources, it is difficult to measure the output
voltage for a given output (D1 to D4) to derive an
output power measurement. For any given appli-
cation, white LED forward voltage levels can differ,
yet the output drive current will be maintained as a
constant. This makes quantifying output power a
difficult task when taken in the context of comparing
to other white LED driver circuit topologies. A bet-
ter way to quantify total device efficiency is to
observe the total input power to the device for a
given LED current drive level. The best white LED
driver for a given application should be based on
trade-offs of size, external components count, relia-
bility, operating range, and total energy usage...not
just "% efficiency."
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3194. 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 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 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 generally
have tight tolerance and are very stable over tem-
perature. Larger capacitor values are usually com-
posed of X7R, X5R, Z5U, or Y5V dielectric materi-
als. Large ceramic capacitors (i.e., greater than
2.2µF) are often available in low-cost Y5V and Z5U
dielectrics, but capacitors greater than 1µF are not
typically required for AAT3194 applications.
AAT3194 Input Power vs. LED Current
700
VIN = 3.6V
600
500
400
300
200
100
0
0
20
40
60
80
100
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.
Output (LED) Current (mA)
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 per-
3194.2006.09.1.0
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