AAT3120 Datasheet, PDF (10/14 Pages) Advanced Analogic Technologies – 3-Channel Charge Pump For White LED Applications

The expression to define the ideal efficiency (η)
can be rewritten as:
η
=
P_OUT
P_IN
=
V_OUT × I_OUT
V_IN × I_OUT
= ^VOUT
V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT ⎞
V_IN ⎠
Charge Pump Section Efficiency
The AAT3120 contains a fractional charge pump
which will boost the input supply voltage in the
event where V_IN is less than 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 one half times the input voltage.
Efficiency (η) for an ideal 1.5X charge pump can
typically 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:
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
10
AAT3120
3-Channel Charge Pump
For White LED Applications
close agreement for output load conditions from
1mA to 100mA. Efficiency will decrease as load
current drops below 0.05mA or when level of V_IN
approaches V_OUT. Refer to the Typical Character-
istics section of this document for measured plots
of efficiency versus input voltage and output load
current for the given charge pump output voltage
options.
Ballast Resistors for Current Matching
In some applications, white LED forward voltages
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:
R_B =
V_SOURCE - V_F
I_F
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 lev-
els, 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 AAT3120. 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
help maximize charge pump transient response.
Since ceramic capacitors are non-polarized, they are
not prone to incorrect connection damage.
Equivalent Series Resistance
3120.2005.11.1.1