LX1662/62A, LX1663/63A

PRODUCT DA T ABOOK 1996/1997

Copyright © 1999

Rev. 1.1 11/99

12

USING THE LX1662/63 DEVICES

INPUT INDUCTOR SELECTION

In order to cope with faster transient load changes, a smaller

output inductor is needed. However, reducing the size of the

output inductor will result in a higher ripple voltage on the input

supply. This noise on the 5V rail can affect other loads, such as

graphics cards. It is recommended that a smaller input inductor,

L

that this inductor has the same current rating as the output

inductor.

C

The capacitors on the output of the PWM section are used to filter

the output current ripple, as well as help during transient load

conditions, and the capacitor bank should be sized to meet ripple

and transient performance specifications.

When a transient (step) load current change occurs, the output

voltage will have a step which equals the product of the Effective

Series Resistance (ESR) of the capacitor and the current step (

∆I).

when current increases from low (in sleep mode) to high, the

output voltage will drop below its steady state value. In the

advanced microprocessor power supply, the capacitor should

usually be selected on the basis of its ESR value, rather than the

capacitance or RMS current capability. Capacitors that satisfy the

ESR requirement usually have a larger capacitance and current

capability than needed for the application. The allowable ESR can

be found by:

ESR * (I

Where V

transient and I

as the LX166x series, have adaptive output voltage positioning,

which adds 40mV to the DC set-point voltage — V

the difference between the low load voltage and the minimum

dynamic voltage allowed for the microprocessor.

Ripple current is a function of the output inductor value (L

and can be approximated as follows:

I

*

Where f

Electrolytic capacitors can be used for the output filter capaci-

tor bank, but are less stable with age than tantalum capacitors. As

they age, their ESR degrades, reducing the system performance

and increasing the risk of failure. It is recommended that multiple

parallel capacitors are used so that, as ESR increases with age,

overall performance will still meet the processor's requirements.

There is frequently strong pressure to use the least expensive

components possible, however, this could lead to degraded long-

term reliability, especially in the case of filter capacitors. Linfinity's

demo boards use Sanyo MV-GX filter capacitors, which are

C

(continued)

aluminum electrolytic, and have demonstrated reliability. The

Oscon series from Sanyo generally provides the very best

performance in terms of long term ESR stability and general

reliability, but at a substantial cost penalty. The MV-GX series

provides excellent ESR performance, meeting all Intel transient

specifications, at a reasonable cost. Beware of off-brand, very-low

cost filter capacitors, which have been shown to degrade in both

ESR and general electrolyte characteristics over time.

CURRENT LIMIT

Current limiting occurs when a sensed voltage, proportional to

load current, exceeds the current-sense comparator threshold

value. 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

parasitic resistance of the inductor.

The LX166x family offers two different comparator thresholds.

The LX1662 & 1663 have a threshold of 100mV, while the LX1662A

and LX1663A have a threshold of 60mV. The 60mV threshold is

better suited to higher current loads, such as a Pentium II or

Deschutes processor.

Sense Resistor

The current sense resistor, R

R

Where V

for LX1662/63 and 60mV for LX1662A/63A) and I

current limit. Typical choices are shown below.

Sense Resistor

Recommended

Load

Value

Controller

Pentium-Class Processor (<10A)

5mΩ

LX1662 or LX1663

Pentium II Class (>10A)

2.5mΩ

LX1662A or LX1663A

TABLE 2 - Current Sense Resistor Selection Guide

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

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

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

controlled very tightly. Its main drawback, however, is cost. An

alternative 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.

V

f

V

OUT

V

IN