AAT3151
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
DC/DC boost converter white LED backlight solu-
Load Switch Mode Efficiency
tions. The AAT3151 soft-start feature prevents
noise transient effects associated with inrush cur-
rents during start-up of the charge pump circuit.
The AAT3151 load switch mode is operational at all
times and functions alone to enhance device power
conversion efficiency when VIN 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:
Power Efficiency and Device Evaluation
The charge pump efficiency discussion in the follow-
ing sections only accounts for the efficiency of the
charge pump section itself. Due to the unique circuit
architecture and design of the AAT3151, it is very dif-
ficult to measure efficiency in terms of a percent
value comparing input power over output power.
POUT
PIN
η =
The expression to define the ideal efficiency (η)
can be rewritten as:
Since the AAT3151 inputs are pure constant current
sinks and typically drive individual loads, it is difficult
to measure the output voltage for a given input (D1
to D4) to derive an overall output power measure-
ment. For any given application, white LED forward
voltage levels can differ, yet the load drive current
will be maintained as a constant.
POUT VOUT · IOUT VOUT
η =
=
=
PIN
VIN · IOUT
VIN
-or-
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.
V
⎛
⎝
⎞
⎠
OUT
η(%) = 100
VIN
Charge Pump Section Efficiency
The AAT3151 contains a fractional charge pump
that will boost the input supply voltage when VIN 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.
The AAT3151 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 VIN 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.
POUT
PIN
η =
In addition, with an ideal 1.5X charge pump, the
output current may be expressed as 2/3 of the
12
3151.2007.03.1.10
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
