SC4608 datasheet(10/19 Pages) SEMTECH

10 / 19 page

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SC4608

POWER MANAGEMENT

between the upper and lower MOSFETs and also mini-

mizes the conduction loss in the body diode of the bot-

tom MOSFET for high efficiency applications.

BOTTOM MOSFET Gate Drive

TOP MOSFET Gate Drive

Phase node

Ground

BOTTOM MOSFET Gate Drive

TOP MOSFET Gate Drive

Phase node

Ground

Figure 3. 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 SC4608 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

OUT

)

(

⋅

∆

⋅

−

⋅

Where:

s = switching frequency and

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

maximum output load current.

The peak to peak inductor current is:

OMAX

•

∆

−

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 is:

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’ copper loss can be estimated as follows:

WINDING

LRMS

COPPER

⋅

Where:

LRMS is the RMS current in the inductor. This current can

be calculated as follow is:

OMAX

LRMS

∆

⋅

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

SC4608 regulates the inductor current to a new value

during a load transient, the output capacitor delivers all

the additional current needed by the load. The ESR and

ESL of the output capacitor, the loop parasitic inductance

between the output capacitor and the load combined

with inductor ripple current are all major contributors to

the output voltage ripple. Surface mount speciality poly-

mer aluminum electrolytic chip capacitors in UE series

from Panasonic provide low ESR and reduce the total

capacitance required for a fast transient response.

POSCAP from Sanyo is a solid electrolytic chip capacitor

that has a low ESR and good performance for high fre-

quency with a low profile and high capacitance. Above

mentioned capacitors are recommended to use in

SC4608 application.

Input Capacitor Selection

The input capacitor selection is based on its ripple cur-

rent level, required capacitance and voltage rating. This

capacitor must be able to provide the ripple current by

the switching actions. For the continuous conduction

Application Information (Cont.)

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