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www.semtech.com

SC4603

POWER MANAGEMENT

Applications Information - (Cont.)

Power MOSFET Drivers

The SC4603 has two drivers for external complemen-

tary power MOSFETs. The driver block consists of one

high side P-MOSFET driver, PDRV, and one low side N-

MOSFET driver, NDRV, which are optimized for driving

external power MOSFETs in a synchronous buck con-

verter. The output drivers also have gate drive non-over-

lap mechanism that gives a dead time between PDRV

and NDRV transitions to avoid potential shoot through

problems in the external MOSFETs. By using the proper

design and the appropriate MOSFETs, a 6A converter can

be achieved. As shown in Figure 2, t

the P-MOSFET off to the N-MOSFET on is adaptive by

detecting the voltage of the phase node. t

from the N-MOSFET off to the P-MOSFET on is fixed, is

50ns for the SC4603. This control scheme guarantees

avoiding the cross conduction or shoot through between

two MOSFETs and minimizes the conduction loss in the

bottom diode for high efficiency applications.

NMOSFET Gate Drive

PMOSFET Gate Drive

Phase node

Ground

Figure 2. Timing Waveforms for Gate Drives and Phase Node

Inductor Selection

The factors for selecting the inductor include its cost,

efficiency, size and EMI. For a typical SC4603 applica-

tion, the inductor selection is mainly based on its value,

saturation current and DC resistance. Increasing the in-

ductor value will decrease the ripple level of the output

voltage while the output transient response will be de-

graded. Low value inductors offer small size and fast tran-

sient responses while they cause large ripple currents,

poor efficiencies and more output capacitance to smooth

out the large ripple currents. The inductor should be able

to handle the peak current without saturating and its

copper resistance in the winding should be as low as

possible to minimize its resistive power loss. A good trade-

off among its size, loss and cost is to set the inductor

ripple current to be within 15% to 30% of the maximum

output current.

The inductor value can be determined according to its

operating point and the switching frequency as follows:

OMAX

s

in

out

in

out

I

I

f

V

)

V

V

(

V

L

⋅

∆

⋅

⋅

−

⋅

=

Where:

f

∆I = ratio of the peak to peak inductor current to the

maximum output load current.

The peak to peak inductor current is:

OMAX

p

p

I

I

I

•

∆

=

−

After the required inductor value is selected, the proper

selection of the core material is based on the peak in-

ductor current and efficiency requirements. The core

must be able to handle the peak inductor current I

PEAK

without saturation and produce low core loss during the

high frequency operation.

2

I

I

I

p

p

OMAX

PEAK

−

+

=

The power loss for the inductor includes its core loss and

copper loss. If possible, the winding resistance should

be minimized to reduce inductor’s copper loss. The core

loss can be found in the manufacturer’s datasheet. The

inductor’s copper loss can be estimated as follows:

WINDING

LRMS

2

COPPER

R

I

P

⋅

=

Where:

I

be calculated as follows:

2

OMAX

LRMS

I

3

1

1

I

I

∆

⋅

+

⋅

=

Output Capacitor Selection

Basically there are two major factors to consider in se-

lecting the type and quantity of the output capacitors.

The first one is the required ESR (Equivalent Series Re-

sistance) which should be low enough to reduce the volt-

age deviation from its nominal one during its load changes.

The second one is the required capacitance, which should

be high enough to hold up the output voltage. Before the

SC4603 regulates the inductor current to a new value