AAT3113/4
High Efficiency 1.5X Fractional Charge
Pumps For White LED Applications
Power Efficiency and Device Evaluation:
Capacitor Characteristics
The charge pump efficiency discussion of the previ-
ous section only accounts for the efficiency of the
charge pump section itself. Due to the unique circuit
architecture and design of the AAT3113/14, it is very
difficult to measure efficiency in terms of a percent
value comparing input power over output power.
Since the AAT3113/14 outputs are pure constant cur-
rent sources, it is difficult to measure the output volt-
age 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 context of comparing to other
white LED driver circuit topologies. A better 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 component count, reliability, operating range
and total energy usage...Not just "% efficiency".
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3113/4 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 tempera-
ture. Large capacitor values are typically com-
posed of X7R, X5R, Z5U or Y5V dielectric materi-
als. Large ceramic capacitors, typically greater
than 2.2µF are often available in low cost Y5V and
Z5U dielectrics, but large capacitors are not
required in the AAT3113/4 application.
AAT3114 Input Power vs. LED Current
700
VIN = 3.6V
600
500
400
300
200
100
0
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.
0
20
40
60
80
100
120
140
Output (LED) Current (mA)
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 per-
formance. Optimum performance will be obtained
when low 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
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.
Thermal Protection
The AAT3113/4 have a thermal protection circuit
that will shut down the charge pump and current
outputs if the die temperature rises above the ther-
mal limit. However, thermal resistance of the QFN
package is so low, that if, in the case of the
AAT3114, all six outputs are shorted to ground at
maximum 20mA output level, the die temperature
will not rise sufficiently to trip the thermal protection.
The thermal protection will only trip if COUT is short-
ed to ground and the ambient temperature is high.
10
3113.2004.07.1.4
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
