AAT3132 Datasheet, PDF (12/14 Pages) Advanced Analogic Technologies – High Efficiency 1.5X Fractional Charge Pump for White LED Applications

AAT3132/32A
High Efficiency 1.5X Fractional Charge
Pump for White LED Applications
ceramic capacitors are used. In general, low ESR
may be defined as less than 100mΩ. A 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 lev-
els, 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 AAT3132/32A. Ceramic capacitors offer
many advantages over their tantalum and alu-
minum electrolytic counterparts. A ceramic capac-
itor typically has very low ESR, is lowest cost, has
a smaller PCB footprint, and is non-polarized. Low
ESR ceramic capacitors help to maximize charge
pump transient response. Since ceramic capaci-
tors are non-polarized, they are not prone to incor-
rect 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 have tight tolerance and are stable over
temperature. Large capacitor values are typically
composed of X7R, X5R, Z5U, or Y5V dielectric
materials. Large ceramic capacitors, greater than
2.2µF, are often available in low-cost Y5V and Z5U
dielectrics, but capacitors greater than 1µF are
usually not required for AAT3132/32A 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.
12
Test Current/Channel Disable
Each channel of the output is equipped with test cur-
rent function. A small amount of current (~2µA) is
injected into each output current source to detect the
presence of load (LED). Unused channels that are
tied to ground or LED load fail short will be automat-
ically disabled instead of wasting the programmed
output current. The test current in the AAT3132A is
higher (~150µA) to accommodate LEDs with lower
impedance in failure mode.
Thermal Protection
The AAT3132/32A has a thermal protection circuit
that will shut down the charge pump if the die tem-
perature rises above the thermal limit, as is the
case during a short-circuit of the OUT pin.
Driving Multiple LEDs, White LED
Display Module Backlights, and
Individual LEDs Connected in Parallel
The AAT3132/32A D1 to D4 outputs are true con-
stant current sources capable of driving up to 20mA
(D1 to D3) or 25/30mA (D4) each over the operation
input voltage range. Since these outputs are true
constant current sources, they may be connected in
parallel to drive a single power output. Any combina-
tion of outputs (D1 to D4) may be connected in par-
allel. The maximum total output current is a sum of
how many current sources are parallel connected.
This feature is particularly useful to power pre-man-
ufactured display modules which are pre-wired with
white LED backlights connected in a parallel circuit
configuration. Any combination of outputs may be
connected in parallel to drive groups of LEDs. The
AAT3132/32A internal current source reference cir-
cuit bases feedback from current sensed on the D1
and D4 outputs. For best operation, the only require-
ment for this type of application is the outputs D1 and
D4 should always be connected to the load circuit.
The AAT3132/32A may be used to drive multiple
LEDs having differing forward voltages. Using
feedback techniques, the current in D1 to D3 is ref-
erenced to the current in the LED connected to D1.
Current source output D4 is its own reference. If all
LEDs are of similar type, the diodes will be
matched in current, maintaining uniform LED
brightness despite variations in manufacturer, pro-
duction, etc.
3132.2005.12.1.4