13

 2006 Semtech Corp.

www.semtech.com

SC4602A/B

POWER MANAGEMENT

Applications Information (Cont.)

Where:

R = load resistance and

R

The compensation network will have the characteristic

as follows:

2

P

2

Z

1

P

1

Z

I

COMP

s

1

s

1

s

1

s

1

s

)

s

(

G

ω

+

⋅

ω

+

⋅

ω

+

ω

+

⋅

ω

=

Where:

)

C

C

(

R

1

2

1

7

I

+

⋅

=

ω

2

1

1

Z

C

R

1

⋅

=

ω

9

8

7

2

Z

C

)

R

R

(

1

⋅

+

=

ω

2

1

1

2

1

1

P

C

C

R

C

C

⋅

⋅

+

=

ω

9

8

2

P

C

R

1

⋅

=

ω

After the compensation, the converter will have the fol-

lowing loop gain:

4

1

2

1

4

C

2

P

2

Z

1

P

1

Z

I

I

M

VD

COMP

PWM

C

L

s

R

L

s

1

C

R

1

s

1

s

1

s

1

s

1

s

1

s

V

V

1

)

s

(

G

)

s

(

G

G

)

s

(

T

+

+

⋅

+

⋅

ω

+

⋅

ω

+

⋅

ω

+

ω

+

⋅

⋅

ω

⋅

=

⋅

⋅

=

Where:

G

V

The design guidelines for the SC4602A/B applications

are as following:

1. Set the loop gain crossover corner frequency

ω

given switching corner frequency

ω

2. Place an integrator at the origin to increase DC and

low frequency gains,

3. Select

ω

ω

-20dB/dec rate to go across the 0dB line for

obtaining a wide bandwidth,

4. Cancel the zero from C

pole

ω

5. Place a high frequency compensator pole

ωp

= pf

switching

ripple and high frequency noise with the adequate

phase lag at

ω

The compensated loop gain will be as given in Figure 4:

-20dB/dec

0dB

Gd

T

ω

Z2

p1

p2

c

ESR

ω

ω

ω

ω

Loop gain T(s)

-40dB/dec

Figure 4. Asymptotic diagrams of power stage and its

loop gain