12

Applications Information (continued)

PCB Layout Considerations

Poor layout can degrade the performance of the DC-DC

converter and can be a contributory factor in EMI prob-

lems, ground bounce, thermal issues, and resistive voltage

losses. Poor regulation and instability can result. A typical

layout is shown in Figure 3.

The following design rules are recommended:

Place the inductor and filter capacitors as close

to the device as possible and use short, wide

traces between the power components.

Route the output voltage feedback path away

from the inductor and LX node to minimize

noise and magnetic interference.

Use a ground plane to further reduce noise

interference on sensitive circuit nodes.

Figure 3– Layout

•

•

•

SC4539

Diode Selection

A Schottky diode with a reverse voltage of 40V and a

forward current rating of 1A should be used with this

device for optimum per formance. The Central

Semiconductor Corporation CMOSH-4E is a suitable diode

for this application.

Selection of Other Components

R

ISET

sets the maximum load current for the SC4539. Use

the following equation to select the proper value:

R

ISET

= 230×V

ISET

/I

LOAD

where

V

ISET

= 0.5V (typ).

Refer to Figure 2 for selecting values for other current set-

tings. Notice that the error increases as the desired I

BL

current decreases.

1

10

100

1000

1

10

100

I

Figure 2 – Set Resistor Value Selection Graph