AAT2806
Dual High Efficiency Charge Pump
for White LED and Flash Applications
Capacitor Characteristics
Charge Pump Power Efficiency
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use with
the AAT2806. Ceramic capacitors offer many advan-
tages over their tantalum and aluminum electrolytic
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 connection damage.
Backlight Charge Pump: The charge pump effi-
ciency discussion in the following sections only
accounts for the efficiency of the charge pump sec-
tion itself. Due to the unique circuit architecture, it
is very difficult to measure efficiency in terms of a
percent value comparing input power over output
power. Since the outputs are pure constant current
sinks and typically drive individual loads, it is diffi-
cult to measure the output voltage for a given out-
put (D1 to D4) to derive an overall output power
measurement. For any given application, white
LED forward voltage levels can differ, yet the out-
put drive current will be maintained as a constant.
Equivalent Series Resistance
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 con-
nections, size or area, material composition, and
ambient temperature. Capacitor ESR is typically
measured in milliohms for ceramic capacitors and
can range to more than several ohms for tantalum or
aluminum electrolytic capacitors.
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 applica-
tion should be based on trade-offs of size, external
components count, reliability, operating range, and
total energy usage...not just "% efficiency."
Ceramic Capacitor Materials
Efficiency of the AAT2806 may be quantified under
very specific conditions and is dependent upon the
input voltage versus the output voltage seen across
the loads applied to outputs D1 through D4 for a given
constant current setting. Depending on the combina-
tion of VIN and voltages sensed at the current sinks,
the device will operate in load switch mode. When
any one of the voltages sensed at the current sinks
nears dropout, the device will operate in 1.5X or 2X
charge pump mode. Each of these modes will yield
different efficiency values. Refer to the following two
sections for explanations for each operational mode.
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 temperature. Larger capacitor val-
ues are usually composed of X7R, X5R, Z5U, or
Y5V dielectric materials. 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
AAT2806 applications.
Capacitor area is another contributor to ESR.
Capacitors that are physically large will have a lower
ESR when compared to an equivalent material small-
er capacitor. These larger devices can improve cir-
cuit transient response when compared to an equal
value capacitor in a smaller package size.
Load Switch Mode Efficiency: The load switch
mode is operational at all times and functions alone
to enhance device power conversion efficiency when
VIN is greater than the voltage across the load. When
in load switch mode, the voltage conversion efficien-
cy is defined as output power divided by input power:
Thermal Protection
The AAT2806 has a thermal protection circuit that
will shut down the two charge pumps if the die tem-
perature rises above the thermal limit.
POUT
η =
PIN
2806.2006.04.1.8
13
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
