PAM2842

Document number: DSxxxxx Rev. 1 - 2

9 of 17

www.diodes.com

October 2012

© Diodes Incorporated

PAM2842

A Product Line of

Diodes Incorporated

Inductor Selection (cont.)

For the sepic topology, L1 = L2

D

1

I

I

I

O

O

1

L

−

=

I

I

O

2

V

V

V

D

O

IN

O

+

=

)

V

V

(

LF

V

V

I

O

IN

O

IN

L

+

=

Δ

Chose

1

L

L

I

4

.

0

I

=

Δ

so

)

V

V

(

FI

V

5

.

2

L

O

IN

O

2

IN

+

=

Capacitor Selection

An input capacitor is required to reduce the input ripple and noise for proper operation of the PAM2842. For good input decoupling, Low ESR

(equivalent series resistance) capacitors should be used at the input. At least 10µF input capacitor is recommended for most applications. And

A minimum output capacitor value of 10µF is recommended under normal operating conditions, while a 22µF or higher capacitor may be

required for higher power LED current. A reasonable value of the output capacitor depends on the LED current. The total output voltage ripple

has two components: the capacitive ripple caused by the charging and discharging on the output capacitor, and the ohmic ripple due to the

capacitor's equivalent series resistance. The ESR of the output capacitor is the important parameter to determine the output voltage ripple of the

converter, so low ESR capacitors should be used at the output to reduce the output voltage ripple. The voltage rating and temperature

characteristics of the Output capacitor must also be considered. So a value of 10µF, 50V voltage rating capacitor is chosen.

Consider from discharge aspect: I x Δt = C x ΔV

In boost and sepic topology,

RIPPLE

O

O

FV

D

I

C =

In buck topology,

RIPPLE

O

O

FV

)

D

1

(

I

C

−

=

Consider from equivalent series resistance:

In sepic topology, there is a series capacitor Cs between L1 and L2 (see application schematic), it flows the current:

V

V

I

I

IN

O

O

)

RMS

(

CS

=

The ripple voltage is

S

O

CS

FC

D

I

V

=

Δ

The voltage rating must be higher than input voltage.

Because the Cs capacitor will flow the large RMS current, so this topology is suitable for small power application.