AAT2845
Four-Channel Backlight Driver with Dual LDOs
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. A value of
1µF for all four capacitors is a good starting point
when choosing capacitors. If the constant current
sinks are only programmed for light current levels,
then the capacitor size may be decreased.
1.5X and 2X Charge Pump Mode
Efficiency
The AAT2845 contains a fractional charge pump
which will boost the input supply voltage in the
event where VIN is less than the voltage required to
supply the output. The efficiency (η) can be simply
defined as a linear voltage regulator with an effec-
tive output voltage that is equal to one and one half
or two times the input voltage. Efficiency (η) for an
ideal 1.5X charge pump can typically be expressed
as the output power divided by the input power.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use with
the AAT2845. 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.
POUT
PIN
η =
In addition, with an ideal 1.5X charge pump, the
output current may be expressed as 2/3 of the
input current. The expression to define the ideal
efficiency (η) can be rewritten as:
Equivalent Series Resistance
POUT
PIN
VOUT · IOUT
VOUT
η =
=
=
VIN · 1.5IOUT 1.5VIN
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 composition,
and ambient temperature. Capacitor ESR is typi-
cally measured in milliohms for ceramic capacitors
and can range to more than several ohms for tanta-
lum or aluminum electrolytic capacitors.
-or-
VOUT
⎛
⎝
⎞
⎠
η(%) = 100
1.5V
IN
For a charge pump with an output of 5V and a nom-
inal input of 3.5V, the theoretical efficiency is 95%.
Due to internal switching losses and IC quiescent
current consumption, the actual efficiency can be
measured at 93%. These figures are in close
agreement for output load conditions from 1mA to
100mA. Efficiency will decrease substantially as
load current drops below 1mA or when the level of
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
(i.e., larger than 2.2µF) are often available in low-
cost Y5V and Z5U dielectrics, but capacitors larger
than 1µF are not typically required for AAT2845
applications.
VIN approaches VOUT
.
The same calculations apply for 2X mode, where
the output current then becomes 1/2 of the input
current.
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 four external capacitors
CIN, C1, C2, and COUT is important because they will
affect turn-on time, output ripple, and transient per-
14
2845.2006.12.1.0
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