AAT3141ITP-T1 View Datasheet(PDF) - Advanced Analogic Technologies
Part Name
Description
MFG CO.
AAT3141ITP-T1
Advanced Analogic Technologies
'AAT3141ITP-T1' PDF : 16 Pages View PDF
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:
-or-
η
=
POUT
PIN
=
VOUT × IOUT
VIN × IOUT
=
VOUT
VIN
η(%)
=
100
VOUT
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.
η
=
POUT
PIN
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 =
PIN
VOUT × IOUT
VIN × 1.5IOUT
=
VOUT
1.5VIN
η(%)
=
100
VOUT
1.5VIN
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-
14
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.
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.
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.
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.
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
3141.2004.07.0.91
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