AAT3131
High Efficiency 1X/1.5X Fractional
Charge Pump for White LED Applications
different efficiency values. Refer to the following
two sections for explanations for each operational
mode.
input current. The expression to define the ideal
efficiency (η) can be rewritten as:
P
OUT
V
OUT
×
I
OUT
V
OUT
=
=
P
IN
V
IN
×
1.5I
OUT
1.5V
IN
Load Switch Mode Efficiency
The AAT3131 load switch mode is operational at all
times and functions alone to enhance device power
conversion efficiency when V
IN
is greater than volt-
age across the load connected to the constant cur-
rent source outputs. When in load switch mode,
the voltage conversion efficiency is defined as out-
put power divided by input power:
P
OUT
P
IN
-or-
η
=
η(%)
= 100
⎛
V
OUT
⎞
⎝
1.5V
IN
⎠
η
=
The expression to define the ideal efficiency (η)
can be rewritten as:
P
OUT
V
OUT
×
I
OUT
V
OUT
=
=
P
IN
V
IN
×
I
OUT
V
IN
η
=
-or-
For a charge pump with an output of 5V and a nom-
inal input of 3.5V, the theoretical efficiency is 95%.
Due to internal switching losses and IC quiescent
current consumption, the actual efficiency can be
measured at 93%. These figures are in close agree-
ment 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 Characteristics section of this
datasheet for measured plots of efficiency versus
input voltage and output load current for the given
charge pump output voltage options.
Capacitor Selection
⎛
V
OUT
⎞
η(%)
= 100
⎝
V
IN
⎠
Careful selection of the four external capacitors
C
IN
, C
1
, C
2
, 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.
Charge Pump Section Efficiency
The AAT3131 contains a fractional charge pump
which will boost the input supply voltage in the
event that V
IN
is less than the voltage required on
the constant current source outputs. The efficien-
cy (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.
P
OUT
P
IN
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use with
the AAT3131. 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.
11
η
=
In addition, with an ideal 1.5X charge pump, the
output current may be expressed as 2/3 of the
3131.2005.12.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
