PRODUCT DATASHEET
AAT3123/23A/24
TM
ChargePump
High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications
resistance (ESR) ceramic capacitors are used. In gen-
eral, 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 levels,
then the capacitor size may be decreased.
Test Current/Channel Disable
Each channel of the output is equipped with test current
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 automatically disabled instead
of wasting the programmed output current. The test cur-
rent in the AAT3123A is higher (~150μA) to accommo-
date LEDs with lower impedance in failure mode.
Capacitor Characteristics
Ceramic composition capacitors are highly recommend-
ed over all other types of capacitors for use with the
AAT3123/23A/24. Ceramic capacitors offer many advan-
tages over their tantalum and aluminum electrolytic
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 to max-
imize charge pump transient response. Since ceramic
capacitors are non-polarized, they are not prone to
incorrect connection damage.
Thermal Protection
The AAT3123/23A/24 has a thermal protection circuit
that will shut down the internal LDO and charge pump if
the die temperature 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
Equivalent Series Resistance
The AAT3123/23A/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 maximum total output current is a sum of
how many current sources are parallel connected. This
feature is particularly useful to power pre-manufactured
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 AAT3123 internal current
source reference circuit bases feedback from current
sensed on the D1 output. The AAT3124 internal current
source reference 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.
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 connec-
tions, size or area, material composition, and ambient
temperature. Capacitor ESR is typically measured in mil-
liohms for ceramic capacitors 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 stable over tem-
perature. Large capacitor values are typically composed
of X7R, X5R, Z5U, or Y5V dielectric materials. Large
ceramic capacitors, typically greater than 2.2μF, are
often available in low-cost Y5V and Z5U dielectrics, but
capacitors greater than 1μF are typically not required for
AAT3123/23A/24 applications.
The AAT3124 may be used to drive multiple LEDs having
differing forward voltages. Using feedback techniques,
the current in D1 to D4 output current sources are refer-
enced to the current in the LED connected to D1. In the
AAT3124 (six-output version), the D5 and D6 output cur-
rent sources are referenced to the current in D5, not to
D1. If all six LEDs are of similar type, the diodes will be
matched in current, maintaining uniform LED brightness
despite variations in manufacturer, production, etc.
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.
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