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 2003 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2621

Applications Information (Cont.)

150

175

200

225

250

275

300

325

350

0

100

200

300

400

500

600

Rset (k-ohm)

150

175

200

225

250

275

300

325

350

0

100

200

300

400

500

600

Rset (k-ohm)

Fig. 2. Pull down resistor for current limit setting

The manufacture data and bench tested results show

the optimum gate drive voltage is around 8.2V, where

the total power losses of power MOSFETs are minimized.

COMPONENT SELECTION

COMPONENT SELECTION

COMPONENT SELECTION

COMPONENT SELECTION

COMPONENT SELECTION

General design guideline of switching power supplies can

be applied to the component selection for the SC2621.

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 without adding extra cost and size. The higher

inductance, the lower ripple current, the smaller core loss

and the higher efficiency will be. However, too high in-

ductance slows down output transient response. It is rec-

ommended to choose the inductance that gives the in-

ductor ripple current to be approximate 20% of maxi-

mum load current. So choose inductor value from:

)

1

(

5

IN

O

O

osc

O

V

V

V

f

I

L

-

×

×

×

=

and package. The SC2621 provides 1.5A gate drive cur-

rent. To drive a 50nC gate charge MOSFET gives 50nC/

1.5A=33ns 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 significant switching loss for the bottom

MOSFET because of its zero voltage switching. The con-

duction losses of the top and bottom MOSFETs are given

by:

D

R

I

P

dson

O

TOP

C

×

×

=

2

_

)

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

Output Capacitor

Output Capacitor

Output Capacitor

Output Capacitor

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

-

×

×

×

<

The output capacitor ESR also causes output voltage tran-

meet output transient criteria, the ESR value should be:

T

T

ESR

I

V

R

<

To meet both criteria, the smaller one of above two ESRs

is required.

The output capacitor value also contributes to load tran-

sient response. Based on a worst case where the induc-

tor energy 100% dumps to the output capacitor during

the load transient, the capacitance then can be calcu-

lated by:

2

2

T

T

V

I

L

C

×

>