AAT3132ITP-T1(2004) データシートの表示（PDF） - Analog Technology Inc

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AAT3132ITP-T1
(Rev.:2004)

High Efficiency 1.5X Fractional Charge Pump for White LED Applications

Analog Technology Inc 

AAT3132

High Efficiency 1.5X Fractional Charge

Pump for White LED Applications

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 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 AAT3132. 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.

Equivalent Series Resistance (ESR)

ESR is an important characteristic to consider when

selecting a capacitor. ESR is a resistance internal

to a capacitor, which is caused by the leads, inter-

nal connections, size or area, material composition

and ambient temperature. Capacitor ESR is typi-

cally measured in milliohms for ceramic capacitors

and can range to more than several ohms for tanta-

lum or aluminum electrolytic capacitors.

Ceramic Capacitor Materials

Ceramic capacitors less than 0.1µF are typically

made from NPO or COG materials. NPO and COG

materials typically have tight tolerance and are sta-

ble over temperature. Large capacitor values are

typically composed of X7R, X5R, Z5U or Y5V

dielectric materials. Large ceramic capacitors, typ-

ically greater than 2.2 F are often available in low

cost Y5V and Z5U dielectrics, but capacitors

greater than 1µF are typically not required for

AAT3132 applications.

Capacitor area is another contributor to ESR.

Capacitors that are physically large will have a lower

ESR when compared to an equivalent material

smaller capacitor. These larger devices can improve

circuit transient response when compared to an

equal value capacitor in a smaller package size.

Thermal Protection

The AAT3132 has a thermal protection circuit that

will shut down the charge pump if the die tempera-

ture rises above the thermal limit as is the case

during a short circuit of the OUT pin.

Driving Multiple LEDs, White LED

display module back lights and

individual LEDs connected in parallel

The AAT3132 D1 to D4 outputs are true constant

current sources capable of driving up to 20mA (D1 -

D3) or 25/30mA (D4) each over the operation input

voltage range. Since these outputs are true constant

3132.2004.04.1.0

11

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