AAT2805 Datasheet, PDF (14/17 Pages) Advanced Analogic Technologies – Dual High Efficiency Charge Pump for White LED and Flash Applications

AAT2805
Dual High Efficiency Charge Pump
for White LED and Flash Applications
Device Switching Noise Performance
The AAT2805 operates at a fixed frequency of
approximately 1MHz to control noise and limit har-
monics that can interfere with the RF operation of cel-
lular telephone handsets or other communication
devices. Back-injected noise appearing on the input
pin of the charge pump is 20mV peak-to-peak, typi-
cally ten times less than inductor-based DC/DC
boost converter white LED backlight solutions. The
AAT2805 soft-start feature prevents noise transient
effects associated with in-rush currents during start
up of the charge pump circuit.
Capacitor Selection
Careful selection of the six external capacitors C_IN,
C1, C2, C3 and C_OUT (for backlight and flash) is
important because they will affect turn-on time, out-
put ripple, and transient performance. Optimum per-
formance 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 value of 1µF for all six capacitors is
a good starting point when choosing capacitors.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use with
the AAT2805. 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.
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, internal
connections, 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.
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 temperature. Larger capacitor values are typi-
cally composed of X7R, X5R, Z5U, or Y5V dielectric
materials. Large ceramic capacitors, typically greater
than 2.2µF, are often available in low-cost Y5V and
Z5U dielectrics, but capacitors greater than 1µF are
typically not required for AAT2805 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.
Thermal Protection
The AAT2805 has a thermal protection circuit that
will shut down the two charge pumps if the die tem-
perature rises above the thermal limit.
Charge Pump Power Efficiency
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.
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
component count, reliability, operating range, and
total energy usage...not just "% efficiency."
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