AAT3128_08 datasheet(9/12 Pages) ANALOGICTECH

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AAT3128

RGB LED Fashion Lighting Controller

ChargePumpTM

PRODUCT DATASHEET

3128.2008.02.1.1

www .analogictech.com

Application Information

RSET Selection

The maximum output current for current sources ISRC1

and ISRC2 is programmed by the RSET resistor. A nomi-

nal value of 38.3kΩ will set each current source output

to 60mA each. Lower maximum current levels may be

programmed by reducing the value of RSET. Refer to

Table 3 for RSET resistor values to set the maximum

brightness constant current rounded off to the closest

1% value.

Current Source Output (mA)

RSET Value (kΩ)

38.3

46.6

57.6

76.8

115

232

Table 3: RSET Programming Values.

If a desired current source output level is not listed in

Table 3, the RSET resistor value may be calculated using

the following formula:

SET =

(1920 · V

REF)

OUT

Where:

VREF = 1.2V

IOUT = Desired output current for ISRC1 and ISRC2

(ISRC1 = ISRC2 ≤ 60mA)

RGB LED Selection

The AAT3128 is specifically intended for driving RGB

color LEDs. However, the device design will allow the

AAT3128 to drive most types of LEDs with forward volt-

age specifications ranging from 1.0V to 5.0V. The two

current sink outputs provide up to 60mA each and may

be paralleled to drive high-current LEDs up to 120mA

without complication.

Most RGB color LEDs have greatly varied forward voltage

specifications for each red, green and blue LED for a

given forward current operating condition.

Refer to

Figure 2 or the datasheet for a given RGB LED to be used

with the AAT3128. Since the ISRC1 and ISRC2 output

current sources are matched with negligible voltage

dependence, RGB LED brightness and color will be

matched regardless of specific LED forward voltage (VF)

variations.

Forward Voltage (VF ) (V)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

1.00

1.50

2.00

2.50

3.00

3.50

4.00

Figure 2: Typical RGB LED Forward Voltage vs.

Forward Current Characteristics.

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 performance.

Optimum performance will be obtained when low equiv-

alent series resistance (ESR) ceramic capacitors are

used. In general, low ESR may be defined as less than

50mΩ. For best overall performance, 1μF capacitors are

recommended.

Depending on ripple requirements, the input and output

capacitor values can be increased or decreased accord-

ingly. If lower ripple is desired, increase the capacitor

value. The peak-to-peak ripple seen at the input or out-

put increases or decreases with capacitor size.

Capacitor Characteristics

Ceramic composition capacitors are highly recommend-

ed over all other types of capacitors for use with the

AAT3128. Ceramic capacitors offer many advantages

over their tantalum and aluminum electrolytic counter-

parts. 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 maximize

charge pump transient response. Since ceramic capaci-

tors are non-polarized, they are not prone to incorrect

connection damage.

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