8

LTC3490

3490fa

The output ripple voltage has two primary components.

The first is due to the value of the capacitor and is given by:

VR

IV

CV

f

CAP

LPK

IN

OUT

=

•

••

The second is due to the capacitor ESR:

The LED current ripple and peak pulsed current are calcu-

lated by:

IR

VR

VR

RR

II

IR

LED

CAP

ESR

SENSE

LED

PPFC

OUT

LE

=

+

=+

•

D

D

2

where:

Low ESR capacitors should be used to minimize output

ripple. Ceramic X5R or X7R type capacitors are recom-

mended. See Table 2 for a list of component suppliers.

Table 2. Capacitor Information

CAPACITOR PART NUMBER

DESCRIPTION

TDK C2012X5R0J475K

4.7

µF, 6.3V, X5R in 0805

AVX 1210ZC475MAT

4.7

µF, 10V, X7R in 1210

Taiyo Yuden CELMK316BJ475ML

4.7

µF, 10V, X7R in 1206

Input Capacitor Selection

Most battery-powered applications do not need an input

capacitor. In supply-powered applications or battery ap-

plications with long leads to the battery, a low ESR 3.3

µF

capacitor reduces switching noise and peak currents.

Design Example

The example will use a Lumileds DS25 white LED. The key

specifications are:

Peak Pulsed Forward Current = 0.5A

Component values will be calculated for 1 or 2 NiMH cells

and assumes the end-of-charge voltage to be 0.9V per cell.

The operating frequency is assumed to be 1MHz, the

worst-case low frequency. The allowed inductor ripple

current is 0.31A. Table 3 shows a summary of the key

parameters.

Table 3. Summary of Key Parameters

PARAMETER

1-CELL

2-CELL

UNITS

2.2

3.2

µH

Choose L

3.3

3.3

µH

1.56

0.78

A

1.93

0.96

A

Choose C

4.7

4.7

µF

Cap ESR

5

5

m

Ω

0.09

0.09

V

0.01

0.005

V

0.10

0.09

A

0.40

0.39

A

where:

value

ESR

PC Board Layout Checklist

Keep the inductor and output capacitor as close to the IC

as possible. Make traces as short and wide as is feasible.

Parasitic resistance and inductance reduce efficiency and

increase ripple.

Keep resistance in the battery connections as low as

possible. In single cell applications, only 0.1

Ω in the

battery connections will have a dramatic effect in effi-

the converter operates lower on the efficiency curve.

APPLICATIO S I FOR ATIO