AAT3134ISN-T1(2004) データシートの表示（PDF） - Analog Technology Inc

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AAT3134ISN-T1
(Rev.:2004)

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

Analog Technology Inc 

AAT3134

High Efficiency 1X/1.5X Fractional

Charge Pump for White LED Applications

The AAT3134 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 for a

given constant current setting. Depending upon the

case of VIN being greater than the specific voltage

seen across the load on D1 (or D4) the device will

operate in "Load Switch" mode. If VIN is less than

the voltage required on the constant current source,

the device will operate in 1.5X charge pump mode.

Each of these two modes will yield different efficien-

cy values. One should refer to the following two sec-

tions for explanations for each operational mode

Load Switch Mode Efficiency

The AAT3134 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 outputs.

When in "Load Switch" mode, the voltage conver-

sion efficiency is defined as output power divided

by input power:

η

=

POUT

PIN

The expression to define the ideal efficiency (η)

can be rewritten as:

η

=

POUT

PIN

=

VOUT × IOUT

VIN × IOUT

=

VOUT

VIN

-or-

η(%)

=

100





VOUT 

VIN 

Charge Pump Section Efficiency

The AAT3134 contains a fractional charge pump

which will boost the input supply voltage in the

event where VIN is less than the voltage required on

the constant current source outputs. The efficiency

(η) can be simply defined as a linear voltage regu-

lator 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 × IOUT

VIN × 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 quies-

cent current consumption, the actual efficiency 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 approach-

es VOUT. 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.

Capacitor Selection

Careful selection of the four external capacitors

CIN, C1, C2, COUT is important because they will

affect turn on time, output ripple and transient per-

formance. Optimum performance will be obtained

when low ESR (<100mΩ ) ceramic capacitors are

used. In general, low ESR may be defined as less

than 100m . A capacitor 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 levels, then the

capacitor size may be decreased.

10

3134.2004.04.1.0

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