AAT2842 Analogic Corporation, AAT2842 Datasheet - Page 15

• Manufacturer Part Number: AAT2842
• Manufacturer: Analogic Corporation

boosted to twice the input voltage. The doubling
conversion ratio introduces a corresponding dou-
bling of the input current. For ideal conversion, the
2X mode efficiency is given by:
LED Selection
The AAT2842 is designed to drive high-intensity
white LEDs. It is particularly suitable for LEDs with
an operating forward voltage in the range of 4.2V to
1.5V.
The charge pump device can also drive other loads
that have similar characteristics to white LEDs. For
various load types, the AAT2842 provides a high-
current, programmable, ideal constant current
source.
Capacitor Selection
Careful selection of the four external capacitors
C
affect turn-on time, output ripple, and transient per-
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.
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT2842. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
2842.2006.10.1.0
IN
, C
1
, C
2
, and C
η =
V
V
OUT
IN
F
· I
· 2I
is important because they will
LED
IN
≈
2 · V
V
F
IN
Control and Dual LDO for Backlight and Flash
High-Current Charge Pump with S
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 maximize charge pump
transient response. Since ceramic capacitors are
non-polarized, they are not prone to incorrect con-
nection damage.
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 connections, size or area, material compo-
sition, and ambient temperature. Capacitor ESR is
typically measured in milliohms for ceramic capac-
itors 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 temperature. Larger capacitor val-
ues are usually composed of X7R, X5R, Z5U, or
Y5V dielectric materials. Large ceramic capacitors
are often available in lower-cost dielectrics, but
capacitors greater than 10µF are not typically
required for AAT2842 applications.
Capacitor area is another contributor to ESR.
Capacitors that are physically larger will have a
lower ESR when compared to an equivalent mate-
rial smaller capacitor. These larger devices can
improve circuit performance when compared to an
equal value capacitor in a smaller package size.
2
Cwire™
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