LX1660 Ver la hoja de datos (PDF) - Microsemi Corporation

Número de pieza

componentes Descripción

Lista de partido

LX1660

ADVANCED PWM CONTROLLER

Microsemi Corporation 

PRODUCT DATABOOK 1996/1997

LX1660/1661

ADVANCED PWM CONTROLLER

PRODUCTION DATA SHEET

USING THE LX1660/61 DEVICES

The LX1660/61 devices are very easy to design with, requiring

only a few simple calculations to implement a given design. The

following procedures and considerations should provide effective

operation for virtually all applications. Refer to the Application

Information section for component reference designators.

SELECTING BETWEEN THE LX1660 AND THE LX1661

In order to provide maximum user versatility, the Advanced PWM

Controller is offered in two versions: the LX1660 and the LX1661.

The LX1661 has a 40mV offset built-in which compensates for

the current sense resistor voltage drop. This allows optimal

transient response for high-speed systems, such as Pentium Pro

processor power supplies. Overall system design could be more

economical with the LX1661, since output capacitance require-

ments could be eased.

The LX1660 provides a very accurate DC set-point, since the

40mV offset is not included in the device. This device is good for

critical DC applications, such as core power in slower micropro-

cessors and related systems.

See "Theory Of Operation" section earlier in this data sheet.

OUTPUT INDUCTOR

The output inductor should be selected to meet the requirements

of the output voltage ripple in steady-state operation and the

inductor current slew-rate during transient.

The peak-to-peak output voltage ripple is:

V

RIPPLE

=

ESR

*

I

RIPPLE

where,

IRIPPLE =

VIN - VOUT

fSW * L

*

VOUT

VIN

I is the inductor ripple current, L is the output inductor

RIPPLE

value and ESR is the Effective Series Resistance of the output

capacitor.

I should typically be in the range of 20% to 40% of the

RIPPLE

maximum output current. Higher inductance results in lower

output voltage ripple, allowing slightly higher ESR to satisfy the

transient specification. Higher inductance also slows the inductor

current slew rate in response to the load-current step change, ∆I,

resulting in more output-capacitor voltage droop. The inductor-

current slew rates at rise and fall edges are:

TRISE = L * ∆I / (VIN - VOUT )

and,

T

FALL

=

L

*

∆I

/

V

OUT

When using electrolytic capacitors, the capacitor voltage

droop is usually negligible, due to the large capacitance.

INPUT INDUCTOR

In order to supply 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 system components, such

as graphic cards. In this case, it is recommended that a 1 - 1.5µH

inductor is used on the input to the regulator, to filter the ripple

on the 5V supply. Ensure that this inductor has the same current

rating as the output inductor.

OUTPUT CAPACITOR

The output capacitor is sized to meet ripple and transient

performance specifications. Effective Series Resistance (ESR) is a

critical parameter. When a step load current occurs, the output

voltage will have a step that equals the product of the ESR and the

current step,∆I. In an advance microprocessor power supply, the

output capacitor is usually selected for ESR instead of capacitance

or RMS current capability. A capacitor that satisfies the ESR

requirement usually has a larger capacitance and current capabil-

ity than strictly needed. The allowed ESR can be found by:

ESR

*

(I

RIPPLE

+

∆I)

<

V

EX

where IRIPPLE is the inductor ripple current, ∆I is the maximum

load current step change, and V is the allowed output voltage

EX

excursion in the transient. Adaptive voltage positioning increases

the value of VEX, allowing a higher ESR value and reducing the cost

of the output capacitor.

Typically, the positioning voltage is 40mV, using the LX1661,

and the transient tolerance is 100mV, resulting in a VEX of 140mV

(See Figure 3). The LX1660 does not have the positioning voltage

offset, so VEX is 100mV maximum.

Electrolytic capacitors can be used for the output capacitor, 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 be 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

demonstration boards use Sanyo MV-GX filter capacitors, which

are 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 at a reasonable cost. Beware

of off-brand, very low-cost filter capacitors, which have been

shown to degrade in both ESR and general electrolytic character-

istics over time.

10

Copyright © 1998

Rev. 1.1 7/98

Share Link: