AAT3156
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
Power Efficiency and Device Evaluation
POUT
PIN
η =
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 AAT3156, it is very
difficult to measure efficiency in terms of a percent
value comparing input power over output power.
The expression to define the ideal efficiency (η)
can be rewritten as:
POUT VOUT × IOUT VOUT
η =
=
=
Since the AAT3156 outputs are pure constant cur-
rent sinks and typically drive individual loads, it is dif-
ficult to measure the output voltage for a given out-
put (D1 to D4) 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.
PIN
VIN × IOUT
VIN
-or-
V
⎛
⎝
⎞
⎠
OUT
η(%) = 100
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
component count, reliability, operating range and
total energy usage...not just "% efficiency."
Charge Pump Section Efficiency
The AAT3156 contains a fractional charge pump that
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
effective output voltage that is equal to one and a 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.
The AAT3156 efficiency may 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 on the
combination of VIN and voltages sensed at the cur-
rent sinks, the device will operate in "Load Switch"
mode. When any one of the voltages sensed at the
current sinks nears dropout, the device will operate
in 1.5X or 2X charge pump mode. Each of these
modes will yield different efficiency values. One
should refer to the following two sections for expla-
nations for each operational mode.
POUT
PIN
η =
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:
POUT
PIN
VOUT × IOUT
=
VOUT
η =
=
VIN × 1.5IOUT 1.5VIN
Load Switch Mode Efficiency
The AAT3156 load switch mode is operational at all
times and functions alone to enhance device power
conversion efficiency when VIN is greater than the
voltage across the load. When in "Load Switch"
mode, the voltage conversion efficiency is defined
as output power divided by input power:
-or-
VOUT
⎛
⎝
⎞
⎠
η(%) = 100
1.5V
IN
12
3156.2005.09.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
