AAT3151B_08 Datasheet, PDF (12/13 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X Charge Pump For White LED Applications

ChargePump^TM
PRODUCT DATASHEET
AAT3151B
High Efficiency 1X/1.5X Charge Pump For White LED Applications
Charge Pump Section Efficiency
The AAT3151B contains a fractional charge pump that
will boost the input supply voltage when V_IN is less than
the voltage required on the constant current sink inputs.
The efficiency (η) can be simply defined as a linear volt-
age regulator with an effective output voltage that is
equal to one and one half or two times the input voltage.
Efficiency (η) for an ideal 1.5X charge pump can typi-
cally be expressed as the output power divided by the
input power.
η
=
P_OUT
P_IN
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:
-or-
η = ^POUT = ^{VOUT · IOUT} = ^VOUT
P_IN V_IN · 1.5I_OUT 1.5V_IN
η(%)
=
100
⎛
⎝
V_OUT ⎞
1.5V_IN⎠
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 performance. Optimum
performance will be obtained when low equivalent series
resistance (ESR) ceramic capacitors are used. In gener-
al, 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 constant current sinks
are only programmed for light current levels, then the
capacitor size may be decreased.
Capacitor Characteristics
Ceramic composition capacitors are highly recommended
over all other types of capacitors for use with the
AAT3151B. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor 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 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 connec-
tions, size or area, material composition, and ambient
temperature. Capacitor ESR is typically measured in mil-
liohms 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 C0G materials. NPO and C0G materials gen-
erally have tight tolerance and are very stable over tem-
perature. Larger capacitor values are usually composed
of X7R, X5R, Z5U, or Y5V dielectric materials. Large
ceramic capacitors (i.e., greater than 2.2μF) are often
available in low-cost Y5V and Z5U dielectrics, but capac-
itors greater than 1μF are not typically required for
AAT3151B 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.
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3151B.2008.02.1.1