AAT3131 View Datasheet(PDF) - Advanced Analogic Technologies

Part Name

Description

MFG CO.

AAT3131

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

Advanced Analogic Technologies 

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AAT3131

High Efficiency 1X/1.5X Fractional

Charge Pump for White LED Applications

different efficiency values. One should refer to the

following two sections for explanations for each

operational mode.

Load Switch Mode Efficiency

The AAT3131 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 AAT3131 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 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

3131.2004.08.1.1

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 100mΩ. 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.

Capacitor Characteristics

Ceramic composition capacitors are highly recom-

mended over all other types of capacitors for use with

the AAT3131. Ceramic capacitors offer many advan-

tages over their tantalum and aluminum electrolytic

counterparts. A ceramic capacitor typically has very

low ESR, is lowest cost, has a smaller PCB footprint

and is non-polarized. Low ESR ceramic capacitors

help maximize charge pump transient response.

Since ceramic capacitors are non-polarized, they are

not prone to incorrect connection damage.

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