Microsemi

Linfinity Microelectronics Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 7

Copyright

© 2000

Rev. 1.1b,2005-03-09

LX1681/1682

Voltage-Mode PWM Controllers

TM

®

APPLI CATI ON I N FORMATI O N

FET SELECTION (continued)

Synchronous Rectification – Lower MOSFET

The lower pass element can be either a MOSFET or a Schottky

diode. The use of a MOSFET (synchronous rectification) will

result in higher efficiency, but at higher cost than using a Schottky

diode (non-synchronous). Power dissipated in the bottom

MOSFET will be:

[]

W

Cycle

Duty

R

I

P

ON

DS

D

51

.

3

1

)

(

2

=

−

×

×

=

[IRL3303 or 1.76W for the IRL3102]

Non-Synchronous Operation - Schottky Diode

A typical Schottky diode, with a forward drop of 0.6V will

dissipate 0.6 * 15 * [1 – 2/5] = 5.4W (compared to the 1.8 to 3.5W

dissipated by a MOSFET under the same conditions). This power

loss becomes much more significant at lower duty cycles. The use

of a dual Schottky diode in a single TO-220 package (e.g. the

MBR2535) helps improve thermal dissipation.

Operation From A Single Power Supply

provide sufficient drive to the upper MOSFET. In many

a bootstrap (charge pump) circuit, as shown in Figure 4 (Typical

This voltage will provide sufficient gate drive to the external

bootstrap circuit in synchronous rectification mode is likely to

result in faster turn-on than in non-synchronous mode.

LAYOUT GUIDELINES - THERMAL DESIGN

A great deal of time and effort were spent optimizing the

thermal design of the demonstration boards. Any user who intends

to implement an embedded motherboard would be well advised to

carefully read and follow these guidelines. If the FET switches

have been carefully selected, external heatsinking is generally not

required. However, this means that copper trace on the PC board

must now be used. This is a potential trouble spot; as much copper

area as possible must be dedicated to heatsinking the FET

switches, and the diode as well if a non-synchronous solution is

used. In our VRM module, heatsink area was taken from internal

with VIAS to the power device tabs. The TO-220 and TO-263

cases are well suited for this application, and are the preferred

packages. Remember to remove any conformal coating from all

exposed PC traces which are involved in heatsinking.

LX168x

5V Input

O utput

GND

FIGURE 2 — Enabling Linear Regulator

General Notes

As always, be sure to provide local capacitive decoupling close

to the chip. Be sure use ground plane construction for all high-

frequency work. Use low ESR capacitors where justified, but be

alert for damping and ringing problems. High-frequency designs

demand careful routing and layout, and may require several

iterations to achieve desired performance levels.

Power Traces

To reduce power losses due to ohmic resistance, careful consid-

eration should be given to the layout of traces that carry high

currents. The main paths to consider are:

Input power from 5V supply to drain of top MOSFET.

Trace between top MOSFET and lower MOSFET or

Schottky diode.

Trace between lower MOSFET or Schottky diode and

ground.

Trace between source of top MOSFET and inductor and

load.

All of these traces should be made as wide and thick as possible,

in order to minimize resistance and hence power losses. It is also

recommended that, whenever possible, the ground, input and

output power signals should be on separate planes (PCB layers).

See Figure 2 – bold traces are power traces.

Layout Assistance

Please contact Linfinity’s Applications Engineers for assistance

with any layout or component selection issues. A Gerber file with

layout for the most popular devices is available upon request.

Evaluation boards are also available upon request. Please check

Linfinity's web site for further application notes.