AAT3142_07 Datasheet, PDF (13/16 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications

AAT3142
High Efficiency 1X/1.5X/2X Charge Pump
for White LED Applications
devices. Back-injected noise appearing on the input
pin of the charge pump is 20mV peak-to-peak, typi-
cally ten times less than inductor-based DC/DC
boost converter white LED backlight solutions. The
AAT3142 soft-start feature prevents noise transient
effects associated with inrush currents during start-
up of the charge pump circuit.
Power Efficiency and Device Evaluation
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 AAT3142, it is very dif-
ficult to measure efficiency in terms of a percent
value comparing input power over output power.
Since the AAT3142 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 D3) 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.
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 AAT3142 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 D3 for a
given constant current setting. Depending upon the
case of V_IN being greater than the specific voltage
seen across the loads on D1 through D3, the device
will operate in load switch mode. If the voltage seen
on the constant current source output is less than
V_IN, then the device will operate in 1.5X or 2X charge
pump mode. Each of these modes will yield differ-
ent efficiency values. Refer to the following two sec-
tions for explanations of each operational mode.
Load Switch Mode Efficiency
The AAT3142 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:
η
=
P_OUT
P_IN
The expression to define the ideal efficiency (η)
can be rewritten as:
η
=
P_OUT
P_IN
=
V_OUT × I_OUT
V_IN × I_OUT
=
V_OUT
V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT⎞
V_IN ⎠
Charge Pump Section Efficiency
The AAT3142 contains a fractional charge pump
which will boost the input supply voltage when V_IN
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 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:
η
=
P_OUT
P_IN
3142.2007.03.1.2
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