AAT3172 Datasheet, PDF (13/15 Pages) Advanced Analogic Technologies – High Current LED Flash Driver Charge Pump IC

AAT3172
High Current LED Flash Driver Charge Pump IC
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:
η=
V_F · I_LED
V_IN · 2I_IN
=
V_F
2 · V_IN
Capacitor Selection
Careful selection of the four external capacitors
(C_IN, C₁, C₂, and C_OUT) 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) (<100mΩ) 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 (θ_JA) 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.
3172.2006.05.1.5
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