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LX1660

ADVANCED PWM CONTROLLER

Microsemi Corporation 

PRODUCT DATABOOK 1996/1997

LX1660/1661

ADVANCED PWM CONTROLLER

PRODUCTION DATA SHEET

USING THE LX1660/61 DEVICES

USING THE LX1660/1661 IN PROGRAMMABLE APPLICATIONS

The LX1660/61 device may be used in conjunction with the

LX1670 Programmable Reference to implement a high-perfor-

mance, digitally-controlled switched-mode power supply suitable

for Pentium Pro Processors and other advanced microprocessor-

based designs. The LX1670 incorporates a 5-bit equivalent DAC,

which can be programmed by the microprocessor’s Voltage

Identification Code (VID). The LX1670 then commands the

LX1660/61 to provide the required output voltage. System

protection functions such as over voltage, SCR drive, and power-

good detection are embedded within the LX1670 device. See

Figure 5.

LAYOUT GUIDELINES - THERMAL DESIGN (continued)

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.

General Notes

As always, be sure to provide local capacitive de-coupling 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.

PROGRAMMING THE OUTPUT VOLTAGE

Select the voltage divider R14 and R15 values as shown in the table

below, using 1% metal film resistors:

TABLE 5

Desired Converter VOUT R14 Value

2.8

806Ω

2.9

909Ω

3.0

1.0kΩ

3.1

1.10kΩ

3.2

1.21kΩ

3.3

1.30kΩ

3.4

1.40kΩ

3.5

1.50kΩ

R15 Value

2kΩ

2kΩ

2kΩ

2kΩ

2kΩ

2kΩ

2kΩ

2kΩ

If other V values are needed, the divider values may be

OUT

calculated as follows:

V = V (1 + R14/R15)

OUT

REF

where V = 2.0V. Note that resistor R4 is part of a filter

REF

element, and does not enter into the calculations.

Please refer to the Application Information schematic for

the reference designators and part locations.

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:

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

s Trace between top MOSFET and lower MOSFET or Schottky

diode.

s Trace between lower MOSFET or Schottky diode and

ground.

s Trace between source of top MOSFET and inductor, sense

resistor and load.

All of these traces should be made as wide and thick as pos-

sible, 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 10 – bold traces are power traces.

5V Input

LAYOUT GUIDELINES - THERMAL DESIGN

A great deal of time and effort were spent optimizing the thermal

design of the demo 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

ground and VCC planes which were actually split and connected

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

LX166x

Output

FIGURE 10 — Power Traces

14

Copyright © 1998

Rev. 1.1 7/98

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