AAT3141
High Efficiency 1X/1.5X/2X Charge Pump
for White LED Applications
The expression to define the ideal efficiency (η)
can be rewritten as:
es VOUT. Refer to the Typical Characteristics section
for measured plots of efficiency versus input voltage
and output load current for the given charge pump
output voltage options.
POUT VOUT × IOUT VOUT
η =
=
=
PIN
VIN × IOUT
VIN
Capacitor Selection
Careful selection of the four external capacitors
CIN, C1, C2, COUT is important because they will
affect turn on time, output ripple and transient per-
formance. Optimum performance will be obtained
when low ESR (<100mΩ) ceramic capacitors are
used. In general, low ESR may be defined as less
than 100mΩ. A capacitor 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.
-or-
V
OUT
η(%) = 100
VIN
Charge Pump Section Efficiency
The AAT3141 contains a fractional charge pump
which will boost the input supply voltage in the
event where VIN is less then the voltage required
on the constant current source outputs. The effi-
ciency (η) can be simply defined as a linear voltage
regulator with an effective output voltage that is
equal to one and a half or two times the input volt-
age. 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 AAT3141. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
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.
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 (ESR)
ESR is an important characteristic to consider when
selecting a capacitor. ESR is a resistance internal
to a capacitor, which is caused by the leads, inter-
nal 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.
POUT
PIN
VOUT × IOUT
=
VOUT
η =
=
VIN × 1.5IOUT 1.5VIN
VOUT
η(%) = 100
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 quies-
cent 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 as load current
drops below 0.05mA or when level of VIN approach-
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 sta-
ble over temperature. Large capacitor values are
typically composed of X7R, X5R, Z5U or Y5V
dielectric materials. Large ceramic capacitors, typ-
ically greater than 2.2µF are often available in low
14
3141.2004.07.0.91
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
