SP7662_07 datasheet(12/19 Pages) SIPEX

12 / 19 page

Mar -07 Rev AJ

SP7662 Wide Input Voltage Range 2A, 300kHz Buck Regulator

SP7662 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

the inductor ripple current, the input voltage

ripplecanbedeterminedby:

∆ViN =

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

V2iN • ƒs • CiN

The capacitor type suitable for the output

capacitors can also be used for the input

capacitors. However, exercise extra caution

whentantalumcapacitorsareused.Tantalum

capacitors are known for catastrophic failure

when exposed to surge current, and input

capacitors are prone to such surge current

when power supplies are connected “live”

to low impedance power sources. Although

tantalum capacitors have been successfully

employed at the input, it is generally not

recommended.

Loop Compensation Design

The open loop gain of the whole system can

be divided into the gain of the error ampli-

fier,PWMmodulator,buckconverteroutput

stage, and feedback resistor divider. In order

to cross over at the desired frequency cut-off

(FCO),thegainoftheerroramplifiermust

compensate for the attenuation caused by

the rest of the loop at this frequency. The

goal of loop compensation is to manipu-

late loop frequency response such that its

crossover gain at 0db, results in a slope of

-20db/decade.

The first step of compensation design is

to pick the loop crossover frequency. High

crossover frequency is desirable for fast

transientresponse,butoftenjeopardizesthe

power supply stability. Crossover frequency

should be higher than the ESR zero but

less than /5 of the switching frequency or

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