AAT1274

1.5A Single Flash LED Driver IC

DATA SHEET

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 2.2μF to 10μF

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 AAT1274 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

ing the power switch ON interval. A 2.2μF ceramic output

capacitor is recommended (see Table TBD). 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-

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 AAT1274 1.5A

step-up regulator is shown in Figures 3 and 4. The fol-

lowing PCB layout guidelines should be considered:

negative terminals to the PGND pins. This is espe-

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

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.

5. Connect the exposed paddle (bottom of the die) to

PGND or GND. Connect AGND, FLGND to GND as

close as possible to the package.

Manufacturer

Part Number

Inductance

(μH)

Saturated Rated

Current (A)

DCR

(mΩ)

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 (μF)

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.