AAT2807A
Dual High Efficiency Charge Pump
for White LED and Flash Applications
Capacitor Characteristics
Applications Information
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT2807A. 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 to maximize charge pump
transient response. Since ceramic capacitors are
non-polarized, they are not prone to incorrect con-
nection damage.
LED Selection
The AAT2807A is specifically intended for driving
white LEDs. However, the device design will allow
the AAT2807A to drive most types of LEDs with for-
ward voltage specifications ranging from 2.0V to
4.3V. LED applications may include main display
backlighting, camera photo-flash applications,
color (RGB) LEDs, infrared (IR) diodes for remotes,
and other loads benefiting from a controlled output
current generated from a varying input voltage.
Since the D1 to D3 input current sinks are matched
with negligible voltage dependence, LED bright-
ness will be matched regardless of the specific
LED forward voltage (VF) levels.
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.
In some instances (e.g., in high-luminous-output
applications such as photo flash), it may be neces-
sary to drive high-VF type LEDs. The low-dropout
current sinks in the AAT2807A make it capable of
driving LEDs with forward voltages as high as 4.3V
at full current from an input supply as low as 3.0V.
Outputs can be paralleled to drive high-current
LEDs without complication.
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 sta-
ble over temperature. Larger capacitor values are
typically composed of X7R, X5R, Z5U, or Y5V
dielectric materials. Large ceramic capacitors, typ-
ically greater than 2.2µF, are often available in low-
cost Y5V and Z5U dielectrics, but capacitors
greater than 1µF are typically not required for
AAT2807A applications.
Device Switching Noise Performance
The AAT2807A has two independent fixed frequen-
cies of approximately 1MHz to control noise and
limit harmonics that can interfere with the RF oper-
ation of cellular telephone handsets or other com-
munication devices. Back-injected noise appearing
on the input pin of the charge pump is 20mV peak-
to-peak, typically ten times less than inductor-based
DC/DC boost converter white LED backlight solu-
tions. The AAT2807A soft-start feature prevents
noise transient effects associated with in-rush cur-
rents during start up of the charge pump circuit.
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.
Capacitor Selection
Careful selection of the six external capacitors CIN,
C1, C2, C3 and COUT (for backlight and flash) is
important because they will affect turn-on time, out-
put ripple, and transient performance. 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 six capacitors is
a good starting point when choosing capacitors.
Thermal Protection
The AAT2807A has a thermal protection circuit that
will shut down both charge pumps if the die tem-
perature rises above the thermal limit, as is the case
during a short circuit of the charge pump outputs.
10
2807A.2004.12.1.0
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