14

 2007 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC4624

SC4624 regulates the inductor current to a new value dur-

ing a load transient, the output capacitor delivers all the

additional current needed by the load.

The ESR and ESL of the output capacitor, the loop para-

sitic inductance between the output capacitor and the

load combined with inductor ripple current are all major

contributors to the output voltage ripple.

Input Capacitor Selection

The input capacitor selection is based on its ripple cur-

rent level, required capacitance and voltage rating. This

capacitor must be able to provide the ripple current by the

switching actions. For the continuous conduction mode,

the RMS value of the input capacitor can be calculated

from:

This current gives the capacitor’s power loss as follows:

This capacitor’s RMS loss can be a significant part of the

total loss in the converter and reduces the overall convert-

er efficiency. The input ripple voltage mainly depends on

the input capacitor’s ESR and its capacitance for a given

load, input voltage and output voltage. Assuming that the

input current of the converter is constant, the required

input capacitance for a given voltage ripple can be calcu-

lated by:

where

D = V

DV

Loop Compensation Design

For a DC/DC converter, it is usually required that the con-

verter has a loop gain of a high cross-over frequency for

fast load response, high DC and low frequency gain for low

steady state error, and enough phase margin for its oper-

ating stability. Often one can not have all these properties

at the same time. The purpose of the loop compensation

is to arrange the poles and zeros of the compensation

Operation

Application Information (Cont.)

Application Information (Cont.)

network to meet the requirements for a specific applica-

tion.

The SC4624 has an internal error amplifier and requires

the compensation network to connect among the COMP

pin and FB pin, GND, and the output as shown in Figure

17. The compensation network includes C1, C2, R1, R7,

R8 and C8. R9 is used to program the output voltage ac-

cording to:

5

5

4

4

3

3

2

2

1

1

D

D

C

C

B

B

A

A

R7

R1

C4

C1

L1

R

C2

SC4624

COMP

FB

PH

R9

C8

R8

Vout

Figure 17. Compensation Network

Provides 3 Poles and 2 Zeros

For voltage mode step down applications as shown in Fig-

ure 17, the power stage transfer function is:

where

R = load resistance

R

The compensation network will have these characteris-

tics:

IN

∆

2

P

2

Z

1

P

1

Z

I

COMP

s

1

s

1

s

1

s

1

s

)

s

(

G

w

+

⋅

w

+

⋅

w

+

w

+

⋅

w

=