AAT3123/23A/24
High Efficiency 1X/1.5X Fractional Charge
Pump for White LED Applications
Power Efficiency and Device
Evaluation
POUT
PIN
η =
The charge pump efficiency discussion in the follow-
ing sections only accounts for efficiency of the
charge pump section itself. Due to the unique circuit
architecture and design of the AAT3123/23A/24, it is
very difficult to measure efficiency in terms of a per-
cent value comparing input power over output power.
The expression to define the ideal efficiency (η)
can be rewritten as:
POUT VOUT × IOUT
=
VOUT
VIN
η =
=
PIN
VIN × IOUT
Since the AAT3123/23A/24 outputs are pure con-
stant 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.
-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 AAT3123/23A/24 contains a fractional 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 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:
The AAT3123/23A/24 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 or D6 for a given constant current setting.
Depending upon the case of VIN being greater than
the specific voltage seen across the load on D1 (or
D5 when the AAT3124 is used) the device will oper-
ate in load switch mode. If the voltage seen on the
constant current source output is less than VIN, then
the device will operate in 1.5X charge pump mode.
Each of these two modes will yield different efficien-
cy values. Refer to the following two sections for
explanations of 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 =
=
Load Switch Mode Efficiency
VIN × 1.5IOUT 1.5VIN
The AAT3123/23A/24 load switch mode is opera-
tional at all times and functions alone to enhance
device power conversion efficiency when the con-
dition 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:
-or-
VOUT
⎛
⎝
⎞
⎠
η(%) = 100
1.5V
IN
10
3123.2005.12.1.4
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
