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LX1660

ADVANCED PWM CONTROLLER

Microsemi Corporation 

PRODUCT DATABOOK 1996/1997

ADVANCED PWM CONTROLLER

PR O D U C T I O N D ATA S H E E T

LX1660/1661

USING THE LX1660/61 DEVICES

INPUT CAPACITOR

The input capacitor and the input inductor are to filter the

pulsating current generated by the buck converter to reduce

interference to other circuits connected to the same 5V rail. In

addition, the input capacitor provides local de-coupling of the

buck converter. The capacitor should be rated to handle the RMS

current requirement. The RMS current is:

IRMS = IL √ d(1-d)

CURRENT LIMIT (continued)

parasitic resistance of the inductor. One should include an RC

filter at the CS+ and CS- inputs, as shown in the Application

Information section, to eliminate jitter and noise.

For most applications, the resistors R5, R6 can be set at 1kΩ,

and C1 can be in the 300-500pF range as a starting point. If a fine

trim or adjustment of the current trip level is required, C1 may be

shunted by a resistor. C1 will introduce a small delay into the

current limit trip point, which effectively raises the threshold.

where I is the inductor current and d is the duty cycle. The

L

maximum value, when d = 50%, IRMS = 0.5IL . For 5V input and

output in the range of 2 to 3V, the required RMS current is very

close to 0.5I .

L

A high-frequency (ceramic) capacitor should be placed across

the drain of the top MOSFET and the source of the bottom one to

avoid ringing due to the parasitic inductor being switched ON and

OFF. See capacitor C7 in the Product Highlight on the first page

of this data sheet.

TIMING CAPACITOR SELECTION

The frequency of operation of the LX1660 / 1661 is a function of

the duty cycle and OFF-time. The OFF-time is proportional to the

timing capacitor (connected to Pin 8, CT ), and is modulated to

minimize frequency variations with duty cycle. The frequency is

constant, during steady-state operation, due to the modulation of

the OFF-time.

The timing capacitor (C ) should be selected using the follow-

T

ing equation:

C = (1 - VOUT / VIN ) * IDIS

T fS (1.52 - 0.29 * VOUT )

where IDIS is fixed at 200µA and fS is the switching frequency

(recommended to be around 200kHz for optimal operation and

component selection).

When using a 5V input voltage, the switching frequency (fS) can

be approximated as follows:

CT = 0.621 *

IDIS

fS

Choosing a 680pF timing capacitor will result in an operating

frequency of 183kHz at VOUT = 2.8V. When a 12V power input is

used, the capacitor value must be changed (the optimal timing

capacitor for 12V input will be in the range of 1000 - 1500pF).

Sense Resistor

The current sense resistor (R1) is selected according to the for-

mula:

R1 = V / I

TRIP TRIP

Where V is the current sense comparator threshold (100mV)

TRIP

and ITRIP is the desired current limit. Typical choices are shown

below.

TABLE 2 - Current Sense Resistor Selection Guide

Load

Sense Resistor Value

Pentium-Class Processor (<10A)

Pentium II Class (>10A)

5mΩ

2.5mΩ

A smaller sense resistor will result in lower heat dissipation (I²R)

and also a smaller output voltage droop at higher currents.

There are several alternative types of sense resistor. The sur-

face-mount metal “staple” form of resistor has the advantage of

exposure to free air to dissipate heat and its value can be con-

trolled very tightly. Its main drawback, however, is cost. An alter-

native is to construct the sense resistor using a copper PCB trace.

Although the resistance cannot be controlled as tightly, the PCB

trace is very low cost.

PCB Sense Resistor

A PCB sense resistor should be constructed as shown in Figure

7. By attaching directly to the large pads for the capacitor and

inductor, heat is dissipated efficiently by the larger copper masses.

Connect the current sense lines as shown to avoid any errors.

Inductor

2.5mΩ Sense Resistor

100mil Wide, 850mil Long

2.5mm x 22mm (2 oz/ft2 copper)

CURRENT LIMIT

Current limiting occurs when a sensed voltage, proportional to

load current, exceeds the current-sense comparator threshold

value (90mV). The current can be sensed either by using a fixed

sense resistor in series with the inductor to cause a voltage drop

proportional to current, or by using a resistor and capacitor in

parallel with the inductor to sense the voltage drop across the

Output

Capacitor Pad

Sense Lines

FIGURE 7 — Sense Resistor Construction Diagram

Copyright © 1998

Rev. 1.1 7/98

11

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