AAT3142
High Efficiency 1X/1.5X/2X Charge Pump
for White LED Applications
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:
V
OUT
×
I
OUT
V
OUT
P
OUT
=
=
P
IN
V
IN
×
1.5I
OUT
1.5V
IN
maximize charge pump transient response. Since
ceramic capacitors are non-polarized, they are not
prone to incorrect connection damage.
Equivalent Series Resistance
η
=
-or-
⎛
V
OUT
⎞
⎝
1.5V
IN
⎠
η(%)
= 100
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.
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 agree-
ment for output load conditions from 1mA to 100mA.
Efficiency will decrease as load current drops below
0.05mA or when the level of V
IN
approaches V
OUT
.
Refer to the Typical Characteristics section of this
datasheet for measured plots of efficiency versus
input voltage and output load current for the given
charge pump output voltage options.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1µF are typically
made from NPO or C0G materials. NPO and C0G
materials have tight tolerance and are stable over
temperature. Large capacitor values are typically
composed of X7R, X5R, Z5U, or Y5V dielectric mate-
rials. Large ceramic capacitors, greater than 2.2µF,
are often available in low-cost Y5V and Z5U
dielectrics, but capacitors greater than 1µF are usu-
ally not required for AAT3142 applications.
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 C
IN
,
C
1
, C
2
, and C
OUT
is important because they will
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. A value of 1µF
for all four capacitors is a good starting point when
choosing capacitors. If the LED current sources are
only programmed for light current levels, then the
capacitor size may be decreased.
Thermal Protection
The AAT3142 has a thermal protection circuit that will
shut down the charge pump if the die temperature
rises above the thermal limit, as is the case during a
short-circuit of the CP pin.
Charge Pump Compatibility
The three-output AAT3142 is pin-compatible with
the AAT3140, AAT3123, and AAT3113 in TSOPJW-
12 packages. The AAT3142 offers an improved
overall efficiency, wider operating range, and the
ability to drive high-V
F
type LEDs at full current.
The AAT3142 is well suited for battery-powered
applications using single-cell lithium-ion/polymer
batteries and 3-series connected dry cells (3.6V).
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use with
the AAT3142. Ceramic capacitors offer many advan-
tages over their tantalum and aluminum electrolytic
counterparts. A ceramic capacitor has very low ESR,
is lowest cost, has a smaller PCB footprint, and is
non-polarized. Low ESR ceramic capacitors help
14
3142.2007.03.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
