AAT3123 ETC, AAT3123 Datasheet - Page 10

• Manufacturer Part Number: AAT3123
• Description: High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications
• Manufacturer: ETC

Power Efficiency and Device
Evaluation
The charge pump efficiency discussion in the fol-
lowing sections only account for the efficiency of
the charge pump section itself. Due to the unique
circuit architecture and design of the AAT3123/24,
it is very difficult to measure efficiency in terms of a
percent value comparing input power over output
power.
Since the AAT3123/24 outputs are pure constant
current sources and typically drive individual loads, it
is difficult to measure the output voltage for a given
output (D1 to D6) to derive an overall output power
measurement. For any given application, white LED
forward voltage levels can differ, yet the output drive
current will be maintained as a constant.
This makes quantifying output power a difficult task
when taken in the context of comparing to other white
LED driver circuit topologies. A better way to quantify
total device efficiency is to observe the total input
power to the device for a given LED current drive
level. The best White LED driver for a given applica-
tion should be based on trade-offs of size, external
component count, reliability, operating range and
total energy usage...Not just "% efficiency".
The AAT3123/24 efficiency may be quantified
under very specific conditions and is dependant
upon the input voltage versus the output voltage
seen across the loads applied to outputs D1
through D4 or D6 for a given constant current set-
ting. Depending upon the case of V
than the specific voltage seen across the load on
D1 (or D5 when the AAT3124 is used) the device
will operate in "Load Switch" mode. If the voltage
seen on the constant current source output is less
than V
pump mode. Each of these two modes will yield
different efficiency values. One should refer to the
following two sections for explanations for each
operational mode.
Load Switch Mode Efficiency
The AAT3123/24 load switch mode is operational
at all times and functions alone to enhance device
power conversion efficiency when the condition
exists where V
10
IN
then the device will operate in 1.5X charge
IN
is greater then voltage across the
IN
being greater
High Efficiency 1X/1.5X Fractional Charge
Pump for White LED Applications
load connected to the constant current source out-
puts. When in "Load Switch" mode, the voltage
conversion efficiency is defined as output power
divided by input power:
The expression to define the ideal efficiency ( ) can
be rewritten as:
η = P
or
Charge Pump Section Efficiency
The AAT3123/24 contains a fractional charge pump
which will boost the input supply voltage in the
event where V
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 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.
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 = P
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
close agreement for output load conditions from
1mA to 100mA.
current drops below 0.05mA or when level of V
approaches V
istics section for measured plots of efficiency ver-
sus input voltage and output load current for the
given charge pump output voltage options.
OUT
OUT
/P
/P
IN
IN
= (V
η(%) = 100(V
= (V
OUT
η(%) = 100(V
IN
OUT
. Refer to the Typical Character-
OUT
is less then the voltage required
Efficiency will decrease as load
η = P
η = P
x I
OUT
x I
OUT
)/(V
OUT
OUT
OUT
)/(V
IN
/P
/P
OUT
x 1.5I
IN
IN
/1.5V
IN
/V
x I
IN
OUT
)
IN
OUT
)
) = V
3123.2003.09.1.1
) = V
OUT
OUT
/1.5V
/V
IN
IN
IN