11

 2005 Semtech Corp.

www.semtech.com

SC4607

POWER MANAGEMENT

Application Information (Cont.)

SC4607 application:

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 cal-

culated from:

2

in

out

in

out

OMAX

)

RMS

(

CIN

V

)

V

V

(

V

I

I

−

⋅

⋅

=

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

)

ESR

(

CIN

)

RMS

(

CIN

2

CIN

R

I

P

⋅

=

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

total loss in the converter and reduce the overall con-

verter 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 calculated by:

)

R

I

V

(

fs

)

D

1

(

D

I

C

)

ESR

(

CIN

OMAX

I

OMAX

IN

⋅

−

∆

⋅

−

⋅

⋅

=

Where:

D = V

∆V

Because the input capacitor is exposed to the large surge

current, attention is needed for the input capacitor. If

tantalum capacitors are used at the input side of the

converter, one needs to ensure that the RMS and surge

ratings are not exceeded. For generic tantalum capaci-

tors, it is wise to derate their voltage ratings at a ratio of

2 to protect these input capacitors.

Boost Capacitor Selection:

The boost capacitor selection is based on its discharge

ripple voltage, worst case conduction time and boost

current. The worst case conduction time T

mated as follows:

max

s

D

f

1

Tw

⋅

=

Where:

f

Dmax = maximum duty ratio, 0.97 for the SC4607.

The required minimum capacitance for boost capacitor

will be:

W

D

B

boost

T

V

I

C

⋅

=

Where:

I

V

With f

capacitance for the boost capacitor is:

nF

540

97

.

0

k

300

1

3

.

0

05

.

0

D

f

1

V

I

C

max

s

D

B

boost

=

⋅

⋅

=

⋅

⋅

=

Power MOSFET Selection:

The SC4607 can drive an N-MOSFET at the high side

and an N-MOSFET synchronous rectifier at the low side.

The use of the high side N-MOSFET will significantly re-

duce its conduction loss for high current. For the top

MOSFET, its total power loss includes its conduction loss,

switching loss, gate charge loss, output capacitance loss

and the loss related to the reverse recovery of the bot-

tom diode, shown as follows:

s

in

rr

OSS

s

GATE

GT

2

GS

GD

G

GATE

s

in

PEAK

_

TOP

ON

_

TOP

RMS

_

TOP

2

TOTAL

_

TOP

f

V

)

Q

Q

(

f

V

Q

)

Q

Q

(

R

V

f

V

I

R

I

P

⋅

⋅

+

+

⋅

⋅

+

+

⋅

⋅

⋅

+

⋅

=

Where:

R

Q

Q

Q

Q

Q

For the top MOSFET, it experiences high current and high

voltage overlap during each on/off transition. But for the