IR3621 datasheet(16/29 Pages) IRF

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IR3621&(PbF)

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Cross Over Frequency:

The stability requirement will be satisfied by placing the

poles and zeros of the compensation network according

to following design rules. The consideration has been

taken to satisfy condition (16) regarding transconduc-

tance error amplifier.

These design rules will give a crossover frequency ap-

proximately one-tenth of the switching frequency. The

higher the band width, the potentially faster the load tran-

sient response. The DC gain will be large enough to pro-

vide high DC-regulation accuracy (typically -5dB to -12dB).

The phase margin should be greater than 45

! for overall

stability.

Based on the frequency of the zero generated by ESR

versus crossover frequency, the compensation type can

be different. The table below shows the compensation

type and location of crossover frequency.

Where:

VIN = Maximum Input Voltage

VOSC = Oscillator Ramp Voltage

Lo = Output Inductor

Co = Total Output Capacitors

FO = R7

×C10×

VIN

VOSC

π×Lo×Co

---(17)

FP1 = 0

π×C10×(R6 + R8)

FZ2 =

≅

π×C10×R6

FZ1 =

π×R7×C11

FP3 =

≅

π×R7×

π×R7×C12

FP2 =

π×R8×C10

(

)

C12

×C11

C12+C11

Details are dicussed in application Note AN-1043 which

can be downloaded from the IR Web-Site.

Compensator

Type

Type II (PI)

Type III (PID)

Method A

Type III (PID)

Method B

Location of Zero

Crossover Frequency

(FO)

FLC < FESR < FO < fS/2

FLC < FO < FESR < fS/2

FLC < FO < fS/2 < FZO

Typical

Output

Capacitor

Electrolytic,

Tantalum

Tantalum,

Ceramic

Table - The compensation type and location of zero

crossover frequency.

The slave error amplifier is a differential-input transcon-

ductance amplifier, in 2-phase configuration the main goal

for the slave feed back loop is to control the inductor

current to match the master's inductor current as well

provides highest bandwidth and adequate phase margin

for overall stability. The following analysis is valid for both

using external current sense resistor and using DCR of

inductors.

Where:

VIN = Input Voltage

L2 = Output Inductor

VOSC = Oscillator Peak Voltage

G(s) =

---(18)

IL2(s)

Ve(s)

VIN

sL2

× VOSC

D(s) =

Ve(s)

RS2

× IL2(s)

1 + sC2R2

sC2

(

)

RS1

RS2

(

)

gm×

---(19)

RS2

RS1

IL2

IL1

Fb2

E/A2

Comp2

Vp2

H(s)=[G(s)

× D(s) × RS2]

1+sR2C2

sC2

(

)

(

)

gm×

RS1

RS2

(

)

VIN

sL2

×VOSC

H(s)=RS2

Compensation for Slave Error Amplfier for 2-Phase

Configuration

The transfer function of power stage is expressed by:

As shown the transfer function is a function of inductor

current.

The transfer function for the compensation network is

given by equation (19), when using a series RC circuit

as shown in Figure 17:

Figure 17 - The PI compensation network

for slave channel.

The loop gain function is:

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