5

 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2618

Component Selection

General design guideline of switching power supplies can

be applied to the component selection for SC2618.

Output Filter Components

Output Filter Components

Output Filter Components

Output Filter Components

Output Filter Components

The purpose of soft start is to control inductor current

during start up, this in turn controls the amount of ex-

cess charge the output capacitor will receive and there-

fore the output voltage overshoot that will occur at the

end of soft start.

GND

L

VIN

Q2

2618-02

Co

VO

Q1

GND

Fig. 2. Output filter components

v

C

i

L

V

O

0

0

t

SS

t

P

I

L

V

P

t

V

REF

Fig. 3. Soft start, inductor current and overshoot

Normally, controllers with an external soft start capaci-

tor, have soft start set so long that overshoot is unde-

tectable. Since the soft start for the SC2618 is internal,

it is necessarily limited and the maximum values of L and

Co should meet the constraints of:

50

2

S

SS

O

X

t

C

L

⋅

=

⋅

percentage overshoot allowable.

Induct

Induct

Induct

Induct

Inductor and MOSFET

or and MOSFET

or and MOSFET

or and MOSFET

or and MOSFETsssss

The selection of inductor and MOSFETs should meet ther-

mal requirement because they are power loss dominant

components. Pick an inductor with as high inductance

as possible if without adding extra cost and size. Higher

inductance, lower ripple current, smaller core loss and

higher efficiency. However, too high inductance slows

down output transient response. It is recommended to

choose the inductance that gives the inductor ripple cur-

rent to be approximate 20% of maximum load current.

So choose inductor value from:

)

1

(

5

IN

O

O

osc

O

V

V

V

f

I

L

−

⋅

⋅

⋅

=

and package. The SC2618 provides 0.5A gate drive cur-

rent. To drive a 25nC gate charge MOSFET gives 25nC/

0.5A=50ns switching time. The switching time ts contrib-

utes to the top MOSFET switching loss:

OSC

S

IN

O

S

f

t

V

I

P

⋅

⋅

⋅

=

There is no switching loss for the bottom MOSFET be-

cause of its zero voltage switching. The conduction losses

of the top and bottom MOSFETs are given by:

D

R

I

P

dson

O

TOP

C

⋅

⋅

_

)

1

(

2

_

D

R

I

P

dson

O

BOT

C

−

⋅

⋅

=

If the requirement of total power losses for each MOSFET

is given, the above equations can be used to calculate

using above equations, then the devices can be deter-

mined accordingly. The solution should ensure the

MOSFET is within its maximum junction temperature at

highest ambient temperature.

Output Capacitor(s)

Output Capacitor(s)

Output Capacitor(s)

Output Capacitor(s)

Output Capacitor(s)

The output capacitors should be selected to meet both

output ripple and transient response criteria. The output

inductor ripple current flowing in. To meet output ripple

criteria, the ESR value should be:

)

1

(

IN

O

O

RIPPLE

OSC

ESR

V

V

V

V

f

L

R

−

⋅

⋅

⋅

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