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 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC4614

Applications Information (Cont.)

termined by the output inductance and output capaci-

tance. The values of output inductor and output bulk

capacitors have to be properly selected so that the soft

start peak current does not exceed the load trip point of

the short circuit protection.

Internal LDO for Gate Drive

Internal LDO for Gate Drive

Internal LDO for Gate Drive

Internal LDO for Gate Drive

Internal LDO for Gate Drive

An internal LDO is designed in the SC4614 to lower the

12V supply voltage for gate drive. A 1uF external ce-

ramic capacitor connected in between DRV pin to the

ground is needed to support the LDO. The LDO output is

connected to the low gate drive internally, and has to be

connected to the high gate drive through an external

bootstrap circuit. The LDO output voltage is set at 8.2V.

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 SC4614.

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 requirements 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 creates an

inductor ripple current of 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 specifications. The SC4614 provides 1.5A

gate drive current and gives 50nC/1.5A=33ns switching

time for driving a 50nC gate charge MOSFET. The switch-

ing time ts contributes 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

can be determined accordingly. The solution should en-

sure the MOSFET is within its maximum junction tem-

perature 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

×

>

Input Capacitor

Input Capacitor

Input Capacitor

Input Capacitor

Input Capacitor

The input capacitor should be chosen to handle the RMS

ripple current of a synchronous buck converter. This value