AAT3151 datasheet(13/15 Pages) AAT

13 / 15 page

AAT3151

High Efficiency 1X/1.5X/2X Charge Pump

For White LED Applications

3151.2007.03.1.10

input current. The expression to define the ideal

efficiency (

η) can be rewritten as:

-or-

Capacitor Selection

Careful selection of the four external capacitors

IN, C1, C2, and COUT is important because they will

affect turn-on time, output ripple, and transient per-

formance. Optimum performance will be obtained

when low equivalent series resistance (ESR)

ceramic capacitors are used. In general, low ESR

may be defined as less than 100m

Ω. A value of

1µF for all four capacitors is a good starting point

when choosing capacitors. If the constant current

sinks 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 AAT3151. Ceramic capacitors offer many

advantages over their tantalum and aluminum elec-

trolytic counterparts. A ceramic capacitor has very

low ESR, is lowest cost, has a smaller PCB foot-

print, and is non-polarized.

Low ESR ceramic

capacitors help maximize charge pump transient

response. Since ceramic capacitors are non-polar-

ized, they are not prone to incorrect connection

damage.

Equivalent Series Resistance

ESR is an important characteristic to consider when

selecting a capacitor. ESR is a resistance internal

to a capacitor that is caused by the leads, internal

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 C0G materials. NPO and C0G

materials generally have tight tolerance and are

very stable over temperature. Larger capacitor val-

ues are usually composed of X7R, X5R, Z5U, or

Y5V dielectric materials. Large ceramic capacitors

(i.e., greater than 2.2µF) are often available in low-

cost Y5V and Z5U dielectrics, but capacitors

greater than 1µF are not typically required for

AAT3151 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.

η(%) = 100

OUT

1.5V

⎛⎞

⎝⎠

η =

OUT =

OUT · IOUT

OUT

IN · 1.5IOUT

1.5V

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