AAT3140 Datasheet, PDF (13/16 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications

AAT3140
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
AAT3140 soft-start feature prevents noise transient
effects associated with in-rush 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 AAT3140, it is very dif-
ficult to measure efficiency in terms of a percent
value comparing input power over output power.
Since the AAT3140 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.
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 AAT3140 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 upon the
case of V_IN being greater than the specific voltage
seen across the loads on D1 through D4, 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
different efficiency values. Refer to the following two
sections for explanations of each operational mode.
Load Switch Mode Efficiency
The AAT3140 load switch mode is operational at all
times and functions alone to enhance device power
conversion efficiency when the condition exists
where V_IN 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:
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 AAT3140 contains a fractional charge pump
which will boost the input supply voltage in the
event where V_IN is less than the voltage required
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 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
3140.2005.03.1.1
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