AAT3123/23A/24
High Efficiency 1X/1.5X Fractional Charge
Pump for White LED Applications
For a charge pump with an output of 5 volts and a
Ceramic Capacitor Materials
nominal input of 3.5 volts, the theoretical efficiency
is 95%. Due to internal switching losses and IC
quiescent current consumption, the actual efficien-
cy can be measured at 93%. These figures are in
close agreement for output load conditions from
1mA to 100mA. Efficiency will decrease as load
current drops below 0.05mA or when the level of
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, typ-
ically 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.
VIN approaches VOUT
.
Refer to the Typical
Characteristics section of this datasheet for meas-
ured plots of efficiency versus input voltage and
output load current for the given charge pump out-
put voltage options.
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.
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 per-
formance. Optimum performance will be obtained
when low equivalent series resistance (ESR)
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.
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 pro-
grammed output current. The test current in the
AAT3123A is higher (~150µA) to accommodate
LEDs with lower impedance in failure mode.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3123/23A/24. 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.
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
The AAT3123/23A/24 D1 to D6 outputs are true
constant 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-
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.
3123.2005.12.1.4
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