AAT3172 datasheet(13/15 Pages) AAT

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AAT3172

High Current LED Flash Driver Charge Pump IC

3172.2006.05.1.5

Similarly, when the input falls further, such that

1.5X mode can no longer sustain LED current, the

device will automatically switch to 2X mode. In 2X

mode, the output voltage can be boosted to twice

the input voltage. The doubling conversion ratio

introduces a corresponding doubling of the input

current. For ideal conversion, the 2X mode effi-

ciency is given by:

Capacitor Selection

Careful selection of the four external capacitors

(CIN, C1, C2, and COUT) is important because they

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

performance.

Optimum performance will be

obtained when low equivalent series resistance

(ESR) (<100m

Ω) ceramic capacitors are used. A

value of 1µF for the flying capacitors is a good

starting point when choosing capacitors. If the LED

current sinks are only programmed for light current

levels, then the capacitor size may be decreased.

Ceramic composition capacitors are highly recom-

mended over all other types of capacitors for use

with the AAT3172. Ceramic capacitors offer many

advantages over their tantalum and aluminum elec-

trolytic 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.

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 compo-

sition, and ambient temperature. Capacitor ESR is

typically measured in milliohms for ceramic capac-

itors and can range to more than several ohms for

tantalum 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 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 are

often available in lower-cost dielectrics, but capac-

itors greater than 4.7µF are not typically required

for AAT3172 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.

PCB Layout

To achieve adequate electrical and thermal per-

formance, careful attention must be given to the

PCB layout. In the worst-case operating condition,

the chip must dissipate considerable power at full

load. Adequate heat-sinking must be achieved to

ensure intended operation.

Figure 3 illustrates an example of an adequate

PCB layout. The bottom of the package features an

exposed metal paddle. The exposed paddle acts,

thermally, to transfer heat from the chip and, elec-

trically, as a ground connection.

The junction-to-ambient thermal resistance (

θJA) for

the package can be significantly reduced by follow-

ing a couple of important PCB design guidelines.

The PCB area directly underneath the package

should be plated so that the exposed paddle can

be mated to the top layer PCB copper during the

re-flow process. This area should also be connect-

ed to the top layer ground pour when available.

Further, multiple copper plated thru-holes should

be used to electrically and thermally connect the

top surface paddle area to additional ground

plane(s) and/or the bottom layer ground pour.

η =

F · ILED

IN · 2IIN

2 · V

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