AAT2801
Multi-Mode Charge Pump for
White LED Backlight and Flash Applications
Load Switch Mode Efficiency
The AAT2801 load switch mode is operational at all
Applications Information—
White LED Charge Pump Section
times and functions alone to enhance device power
conversion efficiency when the condition exists
where VIN is greater than voltage across the load
connected to the constant current source outputs.
When in load switch mode, the voltage conversion
efficiency is defined as output power divided by
input power:
Power Efficiency and Device
Evaluation
The charge pump efficiency discussion in the fol-
lowing sections only accounts for efficiency of the
charge pump section itself. Due to the unique circuit
architecture and design of the AAT2801, it is very
difficult to measure efficiency in terms of a percent
value comparing input power over output power.
POUT
η =
PIN
Since the AAT2801 outputs are pure constant cur-
rent sources and typically drive individual loads, it
is difficult to measure the output voltage for a given
output (D1 to D4) to derive an overall output power
measurement. For any given application, white
LED forward voltage levels can differ, yet the out-
put drive current will be maintained as a constant.
The expression to define the ideal efficiency (η)
can be rewritten as:
POUT VOUT × IOUT VOUT
η =
=
=
PIN
VIN × IOUT
VIN
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
components count, reliability, operating range, and
total energy usage...not just "% efficiency."
-or-
V
⎛
⎝
⎞
⎠
OUT
η(%) = 100
VIN
Efficiency of the AAT2801 can be quantified under
very specific conditions and is dependent upon the
input voltage versus the output voltage seen across
the loads applied to outputs D1 through D4 for a
given constant current setting. Depending upon the
case of VIN being greater than the specific voltage
seen across the load on D1 to D4, the device will
operate in load switch mode. If the voltage seen on
any constant current source output is less than VIN,
then the device will operate in 1.5X or 2X charge
pump mode. Each of these modes will yield differ-
ent efficiency values. One should refer to the fol-
lowing two sections for explanations for each opera-
tional mode.
Charge Pump Section Efficiency
The AAT2801 contains a dual mode fractional and
voltage doubling charge pump which will boost the
input supply voltage in the event where VIN is less
than the voltage required on the constant current
source outputs. The efficiency (η) can be simply
defined as a linear voltage regulator with an effec-
tive 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 typically be expressed
as the output power divided by the input power.
POUT
η =
PIN
16
2801.2006.04.1.3
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
