AAT3151B Datasheet, PDF (12/15 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X Charge Pump For White LED Applications

AAT3151B
High Efficiency 1X/1.5X Charge Pump
For White LED Applications
Since the AAT3151B inputs are pure constant cur-
rent sinks and typically drive individual loads, it is dif-
ficult to measure the output voltage for a given input
(D1 to D4) to derive an overall output power meas-
urement. For any given application, white LED for-
ward voltage levels can differ, yet the load 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
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 AAT3151B 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 D4 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 charge pump mode. Each of these modes
will yield different efficiency values. Refer to the fol-
lowing two sections for explanations for each oper-
ational mode.
Load Switch Mode Efficiency
The AAT3151B 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:
η = ^POUT = ^{VOUT · IOUT} = ^VOUT
P_IN
V_IN · I_OUT
V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT⎞
V_IN ⎠
Charge Pump Section Efficiency
The AAT3151B contains a fractional charge pump
that will boost the input supply voltage when V_IN is
less than the voltage required on the constant cur-
rent sink inputs. The efficiency (η) can be simply
defined as a linear voltage regulator with an effec-
tive 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
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 = ^{VOUT · IOUT} = ^VOUT
P_IN V_IN · 1.5I_OUT 1.5V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT ⎞
1.5V_IN⎠
12
3151B.2007.09.1.0