AAT3122_05 Datasheet, PDF (9/12 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications

AAT3122
High Efficiency 1X/1.5X Fractional Charge
Pump for White LED Applications
EN/SET Timing Diagram
EN/SET
Code OFF
t_HI
t_LO
t_OFF
1
2
3
OFF
LED Selection
The AAT3122 is specifically intended for driving white
LEDs. However, the device design will allow the
AAT3122 to drive most types of LEDs with forward
voltage specifications ranging from 2.0V to 4.3V.
LED applications may include main and sub-LCD dis-
play backlighting, camera photo-flash applications,
color (RGB) LEDs, infrared (IR) diodes for remotes,
and other loads benefiting from a controlled output
current generated from a varying input voltage.
Device Switching Noise Performance
The AAT3122 operates at a fixed frequency of
approximately 1MHz to control noise and limit har-
monics that can interfere with the RF operation of
cellular telephone handsets or other communica-
tion devices. Back-injected noise appearing on the
input pin of the charge pump is 20mV peak-to-
peak, typically ten times less than inductor-based
DC/DC boost converter white LED backlight solu-
tions. The AAT3122 soft-start feature prevents
noise transient effects associated with inrush cur-
rents during start-up of the charge pump circuit.
Power Efficiency and Device
Evaluation
Due to the unique charge pump circuit architecture
and design of the AAT3122, it is very difficult to
measure efficiency in terms of a percent value
comparing input power over output power.
Since the AAT3122 output is purely a constant cur-
rent source, it is difficult to measure the output volt-
age to derive an overall output power measure-
ment. For any given application, white LED forward
voltage levels can differ, yet the output drive cur-
rent will be maintained as a constant.
This makes quantifying output power a difficult task
when taken in the context of comparing to other white
3122.2005.11.1.3
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 Efficiency
The AAT3122 is a fractional charge pump. The effi-
ciency (η) can be simply defined as a linear voltage reg-
ulator with an effective output voltage that is equal to
one and a 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.
η
=
P_OUT
P_IN
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:
η
=
P_OUT
P_IN
=
(V_OUT
x
I_OUT)/(V_IN
x
1.5I_OUT)
=
V_OUT
1.5V_IN
η(%)
=
100
⎛ V_OUT ⎞
⎝1.5V_IN⎠
For a charge pump with an output of 5 volts and a nom-
inal input of 3.5 volts, the theoretical efficiency is 95%.
Due to internal switching losses and IC quiescent cur-
rent consumption, the actual efficiency can be meas-
ured at 93%. These figures are in close agreement for
output load conditions from 3.0mA to 120mA. Efficiency
will decrease as load current drops below 0.05mA or
when the level of V_IN approaches V_OUT. Refer to the
Typical Characteristics section for measured plots of
efficiency versus input voltage and output load current
for the given charge pump output voltage options.
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