AAT3113, AAT3114/14A
High Efficiency 1.5X Fractional Charge
Pumps For White LED Applications
current drops below 0.05mA or when the level of
VIN approaches VOUT Refer to the Typical
Characteristics section of this datasheet for meas-
ured plots of efficiency versus input voltage and
output load current for the given charge pump out-
put 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 AAT3113 and
AAT3114/14A, it is very difficult to measure efficien-
cy in terms of a percent 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 D6) to derive an output power measurement. For
any given application, 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 con-
text of comparing to other white LED driver circuit
topologies. A better way to quantify total device effi-
ciency 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, reliability, 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 AAT3113 and AAT3114/14A products.
Ceramic capacitors offer many advantages over
their tantalum and aluminum electrolytic counter-
parts. 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 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 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 AAT3113 or AAT3114/14A
applications.
AAT3113/AAT3114 Input Power
vs. LED Current
700
VIN = 3.6V
600
500
400
300
200
100
0
0
20
40
60
80
100
120
140
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-
3113.2006.07.1.9
11
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
