AAT3134
High Efficiency 1X/1.5X Fractional
Charge Pump for White LED Applications
Capacitor Characteristics
Ceramic Capacitor Materials
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3134. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
trolytic counterparts. A ceramic capacitor has very
low ESR, is lowest cost, has a smaller PCB foot-
print, and is non-polarized. Low ESR ceramic
capacitors help to maximize charge pump transient
response. Since ceramic capacitors are non-polar-
ized, they are not prone to incorrect connection
damage.
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 AAT3134 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.
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 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.
Thermal Protection
The AAT3134 has a thermal protection circuit that
will shut down the internal LDO and charge pump if
the die temperature rises above the thermal limit,
as is the case during a short-circuit of the OUT pin.
VIN
C1+
C1
1µF
D1
RB*
D2
RB*
D3
RB*
D4
RB*
C1-
C2+
VOUT
CIN
COUT
C2
VBATTERY
AAT3134
1µF
1µF
1µF
C2-
Resistor R is optional
D1
D2
D3
D4
D5
D6
EN/SET
EN/SET
GND
D5
D6
R
R
Resistor R is optional
*In some applications, white LED forward voltages (VF) can vary significantly. Ballast resistors between the LED cathodes and ground are recommended
for balancing the forward voltage differences. The ballast resistor value may be approximated by the following equation:
VSOURCE - VF
=
RB
IF
Figure 2: AAT3134 Driving Two Groups of Paralleled White LEDs
(e.g., main and sub-LCD backlights).
12
3134.2005.12.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
