AAT3131 datasheet(12/14 Pages) AAT

12 / 14 page

AAT3131

High Efficiency 1X/1.5X Fractional

Charge Pump for White LED Applications

3131.2005.12.1.2

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 AAT3131 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 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

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 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.

However, if the diodes are dramatically different in

type comprising a mix of high-V

F 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.

For example, when driving independent RGB

LEDs, the green and blue type LEDs typically

require a high V

F 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.

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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