SP7661 datasheet(12/20 Pages) SIPEX

12 / 20 page

Mar -07 Rev N

SP766 Wide Input Voltage Range 3A, 600kHz Buck Regulator

SP7661 Voltage Mode Control Loop with Loop Dynamic

(SRz2Cz2+1)(SR1Cz3+1)

(SRESRCOUT+1)

[S^2LC

OUT+S(RESR+RDC) COUT+1]

VIN

SR1Cz2(SRz3Cz3+1)(SRz2Cp1+1)

VRAMP_PP

VOUT

(Volts)

VREF

(Volts)

Notes:

ESR = Output Capacitor Equivalent Series Resistance.

DC = Output Inductor DC Resistance.

RAMP_PP = SP7662 Internal Ramp Amplitude Peak-to-Peak Voltage.

Condition:

Cz2>>Cp1&R1>>Rz3

OutputLoadResistance>>R

ESR & RDC

VREF

(R1+R2)

VOUT

VFBK

(Volts)

TypeIIIVoltageLoop

Compensation

AMP (s) Gain Block

PWMStage

PWM Gain

Block

OutputStage

OUT (s) Gain

Block

VoltageFeedback

FBK Gain Block

capacitor ESR and capacitance. Ignoring

theinductorripplecurrent,theinputvoltage

ripplecanbedeterminedby:

∆ViN =

IOut(MAx)•reSr(CiN) + IOut(MAx)•VOut•(ViN - VOut)

V2iN • ƒs•CiN

Thecapacitortypesuitablefortheoutput

capacitorscanalsobeusedfortheinput

capacitors.However,exerciseextracaution

whentantalumcapacitorsareused.Tantalum

capacitorsareknownforcatastrophicfailure

whenexposedtosurgecurrent,andinput

capacitorsarepronetosuchsurgecurrent

whenpowersuppliesareconnected“live”

tolowimpedancepowersources.Although

tantalumcapacitorshavebeensuccessfully

employed at the input, it is generally not

recommended.

Loop Compensation Design

Theopenloopgainofthewholesystemcan

bedividedintothegainoftheerrorampli-

fier,PWMmodulator,buckconverteroutput

stage,andfeedbackresistordivider.Inorder

tocrossoveratthedesiredfrequencycut-off

(FCO),thegainoftheerroramplifiermust

compensatefortheattenuationcausedby

therestoftheloopatthisfrequency.The

goal of loop compensation is to manipu-

lateloopfrequencyresponsesuchthatits

crossovergainat0db,resultsinaslopeof

-20db/decade.

The first step of compensation design is

topicktheloopcrossoverfrequency.High

crossover frequency is desirable for fast

transientresponse,butoftenjeopardizesthe

powersupplystability.Crossoverfrequency

should be higher than the ESR zero but

lessthan1/5oftheswitchingfrequencyor

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