9

 2007 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC4521

Application Information (Cont.)

Thermal Considerations

There are three major power dissipation sources for the

SC4521. The internal switch conduction loss, its switching

loss due to the high frequency switching actions and the

base drive boost circuit loss. These losses can be

estimated as:

)

(V

D

I

1000

10

V

I

10

10.8

D

R

I

P

boost

o

I

o

3

on

2

o

total

⋅

⋅

⋅

+

⋅

⋅

⋅

+

⋅

⋅

=

−

Where:

I

R

V

connection.

The junction temperature of the SC4521 can be

further determined by:

total

JA

A

J

P

θ

T

T

⋅

+

=

θ

Its value is a function of the IC package, the application

layout and the air cooling system.

The freewheeling diode also contributes a significant

portion of the total converter loss. This loss should be

minimized to increase the converter efficiency by using

Schottky diodes with low forward drop (V

D)

(1

I

V

P

o

F

diode

−

⋅

⋅

=

Loop Compensation Design

The SC4521 has an internal error amplifier and requires

a compensation network to connect between the COMP

pin and GND pin as shown in Figure 3. The compensation

network includes C4, C5 and R3. R1 and R2 are used to

program the output voltage according to:

)

R

R

1

(

8

.

0

V

2

1

O

+

•

=

Assuming the power stage ESR (equivalent series

resistance) zero is an order of magnitude higher than

the closed loop bandwidth, which is typically one tenth of

the switching frequency, the power stage control to output

transfer function with the current loop closed (Ridley

model) for the SC4521 will be as follows:

C

R

1

s

1

R

4.3

(s)

G

L

L

VD

⋅

+

⋅

=

Where:

R

C – Output capacitor.

The goal of the compensation design is to shape the loop

to have a high DC gain, high bandwidth, enough phase

margin, and high attenuation for high frequency noises.

Figure 3 gives a typical compensation network which

offers 2 poles and 1 zero to the power stage:

IN

2

SW

3

EN

5

FB

6

COMP

7

SS

8

SC4521

R2

R1

R3

C4

C

D2

L1

C5

Vout

IN

2

SW

3

EN

5

FB

6

COMP

7

SS

8

SC4521

R2

R1

R3

C4

C

D2

L1

C5

Vout

Figure 3. Compensation network provides 2 poles and

1 zero.

The compensation network gives the following

characteristics:

2

1

2

m

P2

Z

1

COMP

R

R

R

g

)

ω

s

(1

s

ω

s

1

ω

(s)

G

+

⋅

⋅

+

⋅

+

⋅

=

Where:

5

4

1

C

C

1

ω

+

=

4

3

Z

C

R

1

ω

⋅

=

5

4

3

5

4

P2

C

C

R

C

C

ω

⋅

⋅

+

=

The loop gain will be given by:

)

ω

s

(1

)

ω

s

(1

ω

s

1

s

1

R

R

R

C

R

10

3.655

(s)

G

(s)

G

T(s)

P2

P1

Z

2

1

2

4

L

3

VD

COMP

+

⋅

+

+

⋅

+

⋅

⋅

⋅

=

⋅

=

−

Where:

C

R

1

ω

L

p1

⋅

=