9

Rev F: /0/06

SP7652 Wide Input Voltage Range 6A, 600kHz, Buck Regulator

© Copyright 2006 Sipex Corporation

APPLICATIONS INFORMATION

tors. However, exercise additional 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.

Loop Compensation Design

The open loop gain of the whole system can

be divided into the gain of the error ampli-

fier, PWM modulator, buck converter output

stage,andfeedbackresistordivider.Inorder

tocrossoverattheselectedfrequencyFCO,

the gain of the error amplifier compensates

for the attenuation caused by the rest of the

loop at this frequency.

Thegoalofloopcompensationistomanipu-

late loop frequency response such that its

gain crosses over 0db at a slope of -20db/

dec. The first step of compensation design

is to pick the loop crossover frequency.

High crossover frequency is desirable for

fasttransientresponse,butoftenjeopardizes

the system stability. Crossover frequency

should be higher than the ESR zero but

less than /5 of the switching frequency.

The ESR zero is contributed by the ESR

associated with the output capacitors and

can be determined by:



.

The next step is to calculate the complex

conjugate poles contributed by the LC

output filter,



When the output capacitors are of a Ceramic

Type,theSP7652EvaluationBoardrequires

aTypeIIIcompensationcircuittogiveaphase

boostof180°inordertocounteracttheeffects

of an underdamped resonance of the output

filter at the double pole frequency.

SP7652 Voltage Mode Control Loop with Loop

(sRz2Cz2+1)(sR1Cz3+1)

[s^2LC

sR1Cz2(sRz3Cz3+1)(sRz2Cp1+1)

(volts)

+

_

(volts)

notes: R

R

v

MP Amplitude Peak to Peak v oltage.

Condition: Cz2 >> Cp1 & R1 >> Rz3

output Load Resistance >> R

or

(volts)

Type iii v oltage Loop

Compensation

G

PWM stage

G

Block

output stage

G

Block

voltage Feedback

G

Definitions:

Conditions:

Output Load Resistance >>

The capacitor type suitable for the output ca-

pacitorscanalsobeusedfortheinputcapaci-