PRODUCT DATASHEET
AAT3128
ChargePump
TM
Application Information
R
SET
Selection
The maximum output current for current sources ISRC1
and ISRC2 is programmed by the R
SET
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 R
SET
. Refer to
Table 3 for R
SET
resistor values to set the maximum
brightness constant current rounded off to the closest
1% value.
Current Source Output (mA)
60
50
40
30
20
10
RGB LED Fashion Lighting Controller
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 (V
F
)
variations.
0.08
Forward Current (I
F
) (A)
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
1.00
1.50
2.00
2.50
3.00
3.50
4.00
R
SET
Value (kΩ)
38.3
46.6
57.6
76.8
115
232
Forward Voltage (V
F
) (V)
Figure 2: Typical RGB LED Forward Voltage vs.
Forward Current Characteristics.
Table 3: R
SET
Programming Values.
If a desired current source output level is not listed in
Table 3, the R
SET
resistor value may be calculated using
the following formula:
Capacitor Selection
Careful selection of the four external capacitors C
IN
, C1,
C2, and C
OUT
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.
R
SET
=
Where:
(1920 · V
REF
)
I
OUT
V
REF
= 1.2V
I
OUT
= 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
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.
3128.2008.02.1.1
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