PRODUCT DATASHEET
AAT1281
TM
SwitchReg
2A Driver IC for High Intensity LED Camera Flash
When selecting a super capacitor, it is highly recom-
mended to select a capacitor with an operating voltage
Selecting the Boost Capacitors
In general, it is good design practice to place a decou-
pling capacitor (input capacitor) between the IN and GND
pins. An input capacitor in the range of 2.2ꢀF to 10ꢀF is
recommended. A larger input capacitor in this application
may be required for stability, transient response, and/or
ripple performance. The high output ripple inherent in
the boost converter necessitates the use of low imped-
ance output filtering. Multi-layer ceramic (MLC) capaci-
tors provide small size and adequate capacitance, low
parasitic equivalent series resistance (ESR) and equiva-
lent series inductance (ESL), and are well suited for use
with the AAT1281 boost regulator. MLC capacitors of type
X7R or X5R are recommended to ensure good capaci-
tance stability over the full operating temperature range.
The output capacitor is selected to maintain the output
load without significant voltage droop (ꢅVOUT) during the
power switch ON interval. A 2.2ꢀF ceramic output capac-
itor is recommended (see Table 6). Typically, 6.3V or 10V
rated capacitors are required for this flash LED boost
output. Ceramic capacitors selected as small as 0603 are
available which meet these requirements. MLC capacitors
exhibit significant capacitance reduction with applied
voltage. Output ripple measurements should confirm that
output voltage droop and operating stability are within
acceptable limits. Voltage derating can minimize this fac-
tor, but results may vary with package size and among
specific manufacturers. To maintain stable operation at
full load, the output capacitor should be selected to
maintain ꢅVOUT between 100mV and 200mV. The boost
converter input current flows during both ON and OFF
switching intervals. The input ripple current is less than
the output ripple and, as a result, less input capacitance
is required.
of 5.5V and 550mF DC capacitance for the best results
in AAT1281 camera flash applications. Low profile, low
impedance (low ESR), and a wide environmental operat-
ing range (-40°C to +85°C) should also be considered
when selecting a capacitor for portable power manage-
ment systems. Please contact the super capacitor manu-
facturer for detailed information.
PCB Layout Guidelines
Boost converter performance can be adversely affected
by poor layout. Possible impact includes high input and
output voltage ripple, poor EMI performance, and reduced
operating efficiency. Every attempt should be made to
optimize the layout in order to minimize parasitic PCB
effects (stray resistance, capacitance, and inductance)
and EMI coupling from the high frequency SW node. A
suggested PCB layout for the AAT1281 High Power 2A
Flash LED Driver is shown in Figures 5 and 6. The follow-
ing PCB layout guidelines should be considered:
1. Minimize the distance from capacitor CIN and COUT’s
negative terminals to the PGND pins. This is espe-
cially true with output capacitor COUT, which conducts
high ripple current from the output to the PGND
pins.
2. Minimize the distance between IN and switching pin
SW; minimize the size of the PCB area connected to
the SW pin.
3. Maintain a ground plane and connect to the IC PGND
pin(s) as well as the PGND connections of CIN and
COUT
.
4. Consider additional PCB exposed area for the flash
LEDs to maximize heatsinking capability. This may
be necessary when using high current application
and long flash duration application.
Super Capacitor
5. Connect the exposed paddle (bottom of the die) to
either PGND or GND. Connect AGND, FLGND to GND
as close as possible to the package.
The AAT1281 requires a super capacitor connected to
the output and the LEDs. After the super capacitor is
fully charged, sufficient energy is stored and ready to
provide large power surges such as flash peak current.
Manufacturer
Part Number
Capacitance (ꢂF) Voltage Rating (V) Temp Co.
Case Size
Murata
Murata
Murata
Murata
Murata
Murata
GRM185R60J225KE26
GRM188R71A225KE15
GRM21BR70J225KA01
GRM21BR71A225KA01
GRM219R61A475KE19
GRM21BR71A106KE51
2.2
2.2
2.2
2.2
4.7
10
6.3
10
6.3
10
10
10
X5R
X7R
X7R
X7R
X5R
X7R
0603
0603
0805
0805
0805
0805
Table 6: Typical Suggested Surface Mount Capacitors.
w w w . a n a l o g i c t e c h . c o m
14
1281.2008.11.1.1
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