AAT3131
High Efficiency 1X/1.5X Fractional
Charge Pump for White LED Applications
with white LED backlights connected in a parallel
circuit configuration. Any combination of outputs
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.
may be connected in parallel to drive groups of
LEDs. The AAT3131 internal current source refer-
ence circuit bases feedback from current sensed
on the D1 and D4 outputs. For best operation, the
only requirement for this type of application is the
outputs D1 and D4 should always be connected to
the load circuit.
The AAT3131 may be used to drive multiple LEDs
having differing forward voltages. Using feedback
techniques, the current in D1 to D3 output currents
sources are referenced 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 uni-
form LED brightness despite variations in manu-
facturer, production, etc.
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 AAT3131 applications.
However, if the diodes are dramatically different in
type comprising a mix of high-VF type and low-VF-
type LEDs, the AAT3131 has the capability to opti-
mally and simultaneously drive up to four LEDs of
one type and up to two LEDs of another type. This
feature can be useful for driving different color
LEDs; driving both display backlight and photo-flash
LEDs; or for driving main-LCD and sub-LCD display
LED backlights from a single charge pump IC.
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.
For example, when driving independent RGB
LEDs, the green and blue type LEDs typically
require a high VF to operate (e.g., 3.7V), while the
red LED needs a low forward voltage (e.g., 2V). By
connecting the green and blue diodes to outputs
D1 to D3 and the red diodes to D4, good control
and uniformity in brightness are maintained despite
the 2V difference in the diode forward voltages.
The AAT3131 determines if the 1.5X charge pump
circuit is needed based on the voltage on D1 and
D4, whichever is higher. If adequate voltage is
available to drive the higher voltage LED (of output
D1 or D4) without the charge pump running, the IC
automatically switches into load switch (1X) mode
to maximize efficiency.
Thermal Protection
The AAT3131 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 AAT3131 D1 to D4 outputs are true constant
current sources capable of driving up to 20mA (D1
to D3) or 30mA (D4) each over the operation input
voltage range. Since these outputs are true con-
stant current sources, they may be connected in
parallel to drive a single power output. Any combi-
nation of outputs (D1 to D4) may be connected in
parallel. The maximum total output current is a sum
of how many current sources are parallel connect-
ed. This feature is particularly useful to power pre-
manufactured display modules which are pre-wired
Similarly, if a 4V photo-flash LED array is connect-
ed to outputs D1 through D3 (with the outputs
shorted together) and two 3.3V sub-LCD display
backlight LEDs are connected to output D4, then
the AAT3131 can optimally drive each set of LEDs
at the programmed current level (see Figure 2).
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3131.2005.12.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
