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AAT3124
(Rev.:2003)

High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications

Advanced Analogic Technologies 

AAT3123/4

High Efficiency 1X/1.5X Fractional Charge

Pump for White LED Applications

Power Efficiency and Device

Evaluation

The charge pump efficiency discussion in the fol-

lowing sections only account for the efficiency of

the charge pump section itself. Due to the unique

circuit architecture and design of the AAT3123/24,

it is very difficult to measure efficiency in terms of a

percent value comparing input power over output

power.

Since the AAT3123/24 outputs are pure constant

current sources and typically drive individual loads, it

is difficult to measure the output voltage for a given

output (D1 to D6) to derive an overall output power

measurement. For any given application, white LED

forward voltage levels can differ, yet the output 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 AAT3123/24 efficiency may be quantified

under very specific conditions and is dependant

upon the input voltage versus the output voltage

seen across the loads applied to outputs D1

through D4 or D6 for a given constant current set-

ting. Depending upon the case of VIN being greater

than the specific voltage seen across the load on

D1 (or D5 when the AAT3124 is used) the device

will operate in "Load Switch" mode. If the voltage

seen on the constant current source output is less

than VIN then the device will operate in 1.5X charge

pump mode. Each of these two modes will yield

different efficiency values. One should refer to the

following two sections for explanations for each

operational mode.

Load Switch Mode Efficiency

The AAT3123/24 load switch mode is operational

at all times and functions alone to enhance device

power conversion efficiency when the condition

exists where VIN is greater then voltage across the

load connected to the constant current source out-

puts. When in "Load Switch" mode, the voltage

conversion efficiency is defined as output power

divided by input power:

η = POUT/PIN

The expression to define the ideal efficiency ( ) can

be rewritten as:

η = POUT/PIN = (VOUT x IOUT)/(VIN x IOUT) = VOUT/VIN

or

η(%) = 100(VOUT/VIN)

Charge Pump Section Efficiency

The AAT3123/24 contains a fractional charge pump

which will boost the input supply voltage in the

event where VIN is less then 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 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 = POUT/PIN

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/PIN = (VOUT x IOUT)/(VIN x 1.5IOUT) = VOUT/1.5VIN

η(%) = 100(VOUT/1.5VIN)

For a charge pump with an output of 5 volts and a

nominal input of 3.5 volts, the theoretical efficiency

is 95%. Due to internal switching losses and IC

quiescent current consumption, the actual efficien-

cy can be measured at 93%. These figures are in

close agreement for output load conditions from

1mA to 100mA. Efficiency will decrease as load

current drops below 0.05mA or when level of VIN

approaches VOUT. Refer to the Typical Character-

istics section for measured plots of efficiency ver-

sus input voltage and output load current for the

given charge pump output voltage options.

10

3123.2003.09.1.1

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