7

 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC4508

Application Information (Cont.)

In Buck converter, the worst case input RMS ripple current

occurs at 50% duty cycle, or at V

condition, the input capacitors RMS ripple current is about

half of the load current.

MOSFET Selection

The selection criteria for the power MOSFET is its

operating junction temperature not exceeding the

maximum junction temperature. Therefore, from a

specified ambient temperature, the maximum junction

to ambient temperature rise has to be determined, which

is related to the MOSFET power dissipation or power loss.

The power loss includes conduction loss and switching

loss. The conduction loss is given by:

)

ON

(

DS

2

O

D

IN

D

O

ON

R

I)

V

V

V

V

(

P

+

+

=

for Buck

)

ON

(

DS

2

O

D

O

IN

D

O

IN

ON

R

I)

V

V

)(

V

V

V

V

(

P

+

+

+

=

for Buck-Boost

Freewheeling Diode Selection

The Schottky diode is recommended as freewheeling

diode in the both Buck and Buck-Boost applications. The

diode conducts during the off-time. The diode voltage

and current ratings are selected based upon the peak

reverse voltage, the peak current and average power

dissipation.

Buck

for

V

V

V

V

I

I

,

2

I

I

I

,

V

V

D

IN

O

IN

O

)

AVG

(

D

L

O

)

PEAK

(

D

IN

)

REV

(

D

+

−

=

∆

+

=

=

Boost

Buck

for

I

I,

2

I

)

V

V

V

V

(

I

I

,

V

V

V

O

)

AVG

(

D

L

IN

D

O

IN

O

)

PEAK

(

D

O

IN

)

REV

(

D

−

=

∆

+

+

+

=

+

=

The most stressful condition for the diode occurs when

the output is shorted. Under this condition, due to the

V

Therefore, attention should be paid to the thermal

condition when laying out a board.

LAYOUT GUIDELINES

Careful attention to layout requirements are necessary

for successful implementation of the SC4508 PWM con-

troller. High switching currents are present in the appli-

cation and their effect on ground plane voltage differen-

tials must be understood and minimized.

1). The high power parts of the circuit should be laid out

first. A ground plane should be used, the number and

position of ground plane interruptions should be such as

to not unnecessarily compromise ground plane integrity.

Isolated or semi-isolated areas of the ground plane may

be deliberately introduced to constrain ground currents

to particular areas, for example, the input capacitor and

output capacitor ground.

2). The loop formed by the Input Capacitor(s) (Cin), the

MOSFET and the Schottky diode or inductor must be kept

as small as possible. This loop contains all the high cur-

rent, fast transition switching. Connections should be as

wide and as short as possible to minimize loop induc-

tance

3). The connection between the junction of MOSFET,

Schottky diode and the output inductor should be a wide

trace or copper region. It should be as short as practical.

Since this connection has fast voltage transitions, keep-

ing this connection short will minimize EMI.

4) The Output Capacitor(s) (Cout) should be located as

close to the load as possible, fast transient load cur-

rents are supplied by Cout only, and connections between

Cout and the load must be short, wide copper areas to

minimize inductance and resistance.

5) A separate analog ground plane connects to the

SC4508 AGND pin. All analog grounding path including

decoupling capacitors, feedback resistors, compensation

components, and current-limit setting resistors should

be connected to this plane.