AAT3153 Datasheet, PDF (11/14 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump For White LED Applications

AAT3153
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
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 AAT3153 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 inputs D1 through D6 for a
given constant current setting. Depending on the
combination of V_IN 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. Refer to
the following two sections for explanations for each
operational mode.
Load Switch Mode Efficiency
The AAT3153 load switch mode is operational at all
times and functions alone to enhance device power
conversion efficiency when V_IN 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:
η
=
P_OUT
P_IN
The expression to define the ideal efficiency (η)
can be rewritten as:
-or-
η
=
P_OUT
P_IN
=
V_OUT × I_OUT
V_IN × I_OUT
=
V_OUT
V_IN
η(%)
=
100
⎛
⎝
V_{OUT ⎞}
V_IN ⎠
Charge Pump Section Efficiency
The AAT3153 contains a fractional charge pump that
will boost the input supply voltage when V_IN is less
than the voltage required on the constant current
sink inputs. 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 out-
put 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 = ^POUT =
P_IN
V_OUT × I_OUT
V_IN × 1.5I_OUT
=
V_OUT
1.5V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT ⎞
1.5V_IN⎠
Capacitor Selection
Careful selection of the four external capacitors
C_IN, C₁, C₂, and C_OUT is important because they will
affect turn-on time, output ripple, and transient per-
formance. Optimum performance will be obtained
when low equivalent series resistance (ESR)
ceramic capacitors are used. In general, low ESR
may be defined as less than 100mΩ. A value of
1µF for all four capacitors is a good starting point
when choosing capacitors. If the LED current
sources are only programmed for light current lev-
els, then the capacitor size may be decreased.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3153. Ceramic capacitors offer many
advantages over their tantalum and aluminum elec-
3153.2005.12.0.93
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