AAT3123 ETC, AAT3123 Datasheet - Page 11
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AAT3123
Manufacturer Part Number
AAT3123
Description
High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications
Manufacturer
ETC
Datasheet
1.AAT3123.pdf
(18 pages)
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Capacitor Selection
Careful selection of the four external capacitors
C
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 AAT3123/24.
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 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)
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
AAT3123/24 applications.
3123.2003.09.1.1
IN
, C1, C2, C
OUT
is important because they will
Ceramic capacitors offer
High Efficiency 1X/1.5X Fractional Charge
Pump for White LED 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 AAT3123/24 has a thermal protection circuit
that will shut down the internal LDO and charge
pump if the die temperature rises above the ther-
mal limit as is the case during a short circuit of the
OUT pin.
Driving Multiple LED's, White LED
display module back lights and
individual LED's connected in parallel
The AAT3123/24 D1 to D6 outputs are true con-
stant current sources capable of driving up to
20mA 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 combination of outputs
(D1 to D6) may be connected in parallel. The max-
imum total output current is a sum of how many
current sources are parallel connected. This fea-
ture is particularly useful to power pre-manufac-
tured 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 LED's. The
AAT3123 internal current source reference circuit
bases feedback from current sensed on the D1
output. The AAT3124 internal current source refer-
ence circuit bases feedback from current sensed
on the D1 and D5 outputs. For best operation, the
only requirement for this type of application is the
output D1 (and or D5 for the AAT3124) should
always be connected to the load circuit.
The AAT3124 may be used to drive multiple LED's
having differing forward voltages. Using feedback
techniques, the current in D1 to D4 output currents
sources are referenced to the current in the LED
connected to D1. In AAT3124 (six output version),
the D5 and D6 output current sources are refer-
enced to the current in D5, not to D1. If all six LED's
are of similar type, the diodes will be matched in
current, maintaining uniform LED brightness
despite variations in manufacturer, production, etc.
11