AAT3123(2003) Ver la hoja de datos (PDF) - Advanced Analogic Technologies

Número de pieza

componentes Descripción

Fabricante

AAT3123
(Rev.:2003)

High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications

Advanced Analogic Technologies 

AAT3123/4

High Efficiency 1X/1.5X Fractional Charge

Pump for White LED Applications

Capacitor Selection

Careful selection of the four external capacitors

CIN, C1, C2, COUT is important because they will

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. 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 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

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

Share Link: