AAT3140 Datasheet, PDF (14/16 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications

AAT3140
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:
η = ^POUT =
P_IN
V_OUT × I_OUT
V_IN × 1.5I_OUT
=
V_OUT
1.5V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT ⎞
1.5V_IN⎠
For a charge pump with an output of 5 volts and a
nominal input of 3.5 volts, the theoretical efficiency
is 95%. Due to internal switching losses and IC
quiescent current consumption, the actual efficien-
cy can be measured at 93%. These figures are in
close agreement 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 for measured plots of effi-
ciency versus input voltage and output load current
for the given charge pump output voltage options.
Capacitor Selection
Careful selection of the four external capacitors C_IN,
C1, C2, 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.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use with
the AAT3140. 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
14
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 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.
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. Large 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 AAT3140 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.
Thermal Protection
The AAT3140 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 four-output AAT3140 is pin-compatible with the
AAT3123, AAT3132, and AAT3113 in TSOPJW-12
packages. The AAT3140 offers an improved overall
efficiency, wider operating range, and the ability to
drive high-V_F type LEDs at full current. The
AAT3140 is well suited for battery-powered appli-
cations using single-cell lithium-ion/polymer batter-
ies and 3-series connected dry cells (3.6V).
3140.2005.03.1.1