AAT1278
DATA SHEET
1.5A Single Flash LED Driver
13
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Selecting the Boost Capacitors
In general, it is a good design practice to place a decou-
pling capacitor (input capacitor) between the IN and
ground. An input capacitor in the range of 2.2F to 10F
is recommended. A larger input capacitor in this applica-
tion may be required for stability, transient response,
and/or ripple performance. The high output ripple inher-
ent in the boost converter necessitates the use of low
impedance output filtering. Multi-layer ceramic (MLC)
capacitors provide small size and adequate capacitance,
low parasitic equivalent series resistance (ESR) and
equivalent series inductance (ESL), and are well suited for
use with the AAT1278 boost regulator. MLC capacitors of
type X7R or X5R are recommended to ensure good
capacitance stability over the full operating temperature
range. The output capacitor is selected to maintain the
output load without significant voltage droop (VOUT) dur-
ing the power switch ON interval. A 2.2F ceramic output
capacitor is recommended (see Table 4). Typically, 6.3V
or 10V rated capacitors are required for this flash LED
boost output application. Ceramic capacitors selected as
small as 0603 are available which meet these require-
ments. MLC capacitors exhibit significant capacitance
reduction with applied voltage. Output ripple measure-
ments should confirm that output voltage droop and
operating stability are within acceptable limits. Voltage
de-rating can minimize this factor, but results may vary
with package size among specific manufacturers. To
maintain stable operation at full load, the output capaci-
tor should be selected to maintain VOUT between 100mV
and 200mV. The boost converter input current flows dur-
ing 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.
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 para-
sitic PCB effects (stray resistance, capacitance, and
inductance) and EMI coupling from the high frequency
SW node. A suggested PCB layout for the AAT1278 1.5A
step-up regulator is shown in Figures 3 and 4. The fol-
lowing 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 under the inductor 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
LED to maximize heatsinking capability. This may be
necessary when using high current application and
long flash duration application.
Manufacturer Part Number
Inductance
(H)
Saturated Rated
Current (A)
DCR
(m)
Size (mm)
LxWxH Type
Cooper Bussmann SD3812-1R0-R 1 2.69 48 4.0 x 4.0 x 1.2 shielded drum core
Cooper Bussmann SDH3812-1R0-R 1 3 45 3.8 x 3.8 x 1.2 shielded drum core
Cooper Bussmann SD10-1R0-R 1 2.25 44.8 5.2 x 5.2 x 1.0 shielded drum core
Sumida CDH38D11/S 1 2.69 48 4.0 x 4.0 x 1.2 shielded drum core
Coilcraft LPS4012-102NLC 1 2.5 60 4.1 x 4.1 x 1.2 shielded drum core
Table 4: Typical Suggested Surface Mount Inductors.
Manufacturer Part Number Capacitance (F) Voltage Rating (V) Temp Co. Case Size
Murata GRM185R60J225KE26 2.2 6.3 X5R 0603
Murata GRM188R71A225KE15 2.2 10 X7R 0603
Murata GRM21BR70J225KA01 2.2 6.3 X7R 0805
Murata GRM21BR71A225KA01 2.2 10 X7R 0805
Murata GRM219R61A475KE19 4.7 10 X5R 0805
Murata GRM21BR71A106KE51 10 10 X7R 0805
Table 5: Typical Suggested Surface Mount Capacitors.