Microchip LX7176A Manual

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- 6 Voltage-Mode, Current-Mode (and Hysteretic Control) TN 20

by Sanjaya Maniktala

Rev 0.5 - -12 Analog Mixed Signal Group / 07 30

1 Enterprise, Aliso Viejo, CA 92656, USA; Within the USA: (800) 713-4113, Outside the USA: (949) 221-7100 Fax: (949) 756- 0308

Introduction

Switching regulators have been with us for many years. They were considered tricky to design –

and still are. In 1976 Silicon General (later LinFinity, then Microsemi), introduced the irst

monolithic (IC-based) switching controller, the SG1524 “Pulse Width Integrated Circuit”. A little

later, this chip was improved and became the “SG3524” historic industry workhorse. And very soon

thereafter, it was available from multiple chip vendors. Keep in mind that switching stages based on

discrete designs were already gaining ground, particularly in military applications. In fact, some

resourceful engineers had even made “switch mode power supplies” by adding related circuitry -

around one of the highest-selling chips in history: the “555 timer” (sometimes called the “IC Time

Machine”), introduced in 1971 by Signetics (later Philips, then NXP). The SG1524 was however the

irst all IC in which the required control functionality was present on a single chip/die. With the

rapidly escalating concurrent interest in switching power supplies at the time, it is no surprise that

as early as 1977 the very irst book on the subject, written by the late Abraham Pressman, appeared

on the scene. Together, these events spurred interest in an area well beyond most people’s

expectations, and ushered in the world of switching power conversion as we know it today.

The SG1524/3524 drove a pair of (bipolar) switching transistors with a “duty cycle” (ratio of switch

ON-time to the total time period) which was proportional to the “control voltage”. By using switching

transistors to switch the input voltage source ON and OFF into an LC low pass ilter, a relatively

eficient voltage regulator was produced. At the heart of this regulator was the “PWM (pulse width

modulator) comparator”. The output pulse-train to drive the transistors with was a result of

applying a (relatively) smooth control voltage on one of this comparator’s input terminals, along

with a “sawtooth” or “PWM ramp” a generated from the clock Figure 1, on its other input. See . This

technique is known as "voltage-mode programming”, or “voltage mode control” (“VMC”) –- since the

duty cycle is proportional to the control voltage. The control voltage is in effect the difference

between the actual output voltage and the “reference” value (the value we want to fix the output at,

i.e. the “setpoint”). We will discuss this igure in more detail shortly.

Another well-known technique today, which has also been around since the 80s, senses the peak

current in the power switch or inductor, and turns the switch OFF at a programmed level of

current. This technique is called current-mode control (“CMC”). Keep in mind it was not brand-“

new at the time. In fact it had been discovered years ago, but few had realized its signiicance till ”

Unitrode Corp. (now Texas Instruments) came along. It received a huge boost in popularity in the

form of the world’s first current-mode control (CMC) chip, the Flyback controller UC1842 from

Unitrode In CMC, there is in effect a (fast-. acting) “inner” current loop along with the “outer”

(slower) “voltage loop” which carries out the output regulation. See the note on ramp generation

within , indicating how this particular aspect is different between VMC and CMC. Prima Figure 1

- 6 Voltage-Mode, Current-Mode (and Hysteretic Control) TN 20

by Sanjaya Maniktala

Rev 0.5 - -12 Analog Mixed Signal Group / 07 30

1 Enterprise, Aliso Viejo, CA 92656, USA; Within the USA: (800) 713-4113, Outside the USA: (949) 221-7100 Fax: (949) 756- 0308

facie, CMC seems to be better. “Pulse- -by pulse” became synonymous with CMC. It was once even

thought to be the silver bullet, or magic wand, to ix that everything voltage-mode control was not.

The UC1842 was later improved to UC3842, and shortly thereafter, following the success story of

the 3524, the UC3842 was soon available from innumerable chip vendors. But a few years into SG

this success story, expectations got somewhat blunted.

The disadvantages of CMC surfaced slowly. That growing realization was succinctly summed up in a

well-known Design Note - –DN 62 – from Unitrode, which said: “there is no single topology which is

optimum for all applications. Moreover, voltage-mode control If updated with modern circuit and –

process developments – has much to offer designers of today's high-performance supplies and is a

viable contender for the power supply designer's attention. it is reasonable to expect ” It also says: “

some confusion to be generated with the introduction of the UCC3570 a new voltage-mode controller –

introduced almost 10 years after we told the world that current-mode was such a superior approach.”

To put things in perspective, the above-mentioned design note was written by “the father of the

PWM controller IC industry,” Bob Mammano, who developed the first voltage-mode control IC, the

SG1524. Later, as Staff Technologist in the Power IC division of Unitrode (a division that p2-he had

jointly created with two others from Silicon General), Mammano led the development of the first

current-mode control IC, the UC1842.

NP:NS

VCOMP

EA-output

{

VIN

POWER STAGE

CONTROL SECTION

Transformer

(optional)

Feedback trace

Driver

not shown

REF

VFB

EA-input

ERROR

AMP

{

Divider

(Sensor)

{

optocoupler

(optional)

CONTROL

OUTPUT

“LINE”

REFERENCE

Rf2

Rf1

Control voltage

OUTPUT

∑

INPUT

(LINE)

REFERENCE

FB pin

CONTROL

EA OUT/

Control Voltage/

COMP pin

DividerError Amp

PWM

Comparator Switch LC Post Filter

Equivalent

PLANT

FEEDBACK

PLANT

FEEDBACK

Rf2

Rf1

VIN

In Current Mode Control, the “RAMP” is

generated from the Inductor Current

RAMP RAMP

G(s)

H(s)

PLANT

FEEDBACK (COMPENSATOR)

Figure 1: Voltage and Current Mode Regulators with their shared Functional Blocks

Produkt Specifikationer

Mærke:	Microchip
Kategori:	Ikke kategoriseret
Model:	LX7176A

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