SC4518HEVB datasheet(9/15 Pages) SEMTECH

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POWER MANAGEMENT

SC4518H

Input Capacitor Selection

The input capacitor selection is based on its ripple current

level, required capacitance and voltage rating. This

capacitor must be able to provide the ripple current by

the switching actions. For the continuous conduction

mode, the RMS value of the input capacitor current

CIN(RMS) can be calculated from:

OMAX

)

RMS

(

CIN

)

(

−

⋅

This current gives the capacitor’s power loss through its

CIN(ESR) as follows:

)

ESR

(

CIN

)

RMS

(

CIN

•

The input ripple voltage mainly depends on the input

capacitor’s ESR and its capacitance for a given load, input

voltage and output voltage. Assuming that the input

current of the converter is constant, the required input

capacitance for a given voltage ripple can be calculated

by:

)

(

)

(

)

ESR

(

CIN

OMAX

⋅

−

∆

⋅

−

⋅

Where:

∆V

I = the given input voltage ripple.

Because the input capacitor is exposed to the large surge

current, attention is needed for the input capacitor. If

tantalum capacitors are used at the input side of the

converter, one needs to ensure that the RMS and surge

ratings are not exceeded. For generic tantalum

capacitors, it is suggested to derate their voltage ratings

at a ratio of about two to protect these input capacitors.

Boost Capacitor and its Supply Source Selection

The boost capacitor selection is based on its discharge

ripple voltage, worst case conduction time and boost

current. The worst case conduction time T

w can be

estimated as follows:

max

⋅

Where:

s = the switching frequency and

Dmax = maximum duty ratio, 0.85 for the SC4518H.

The required minimum capacitance for the boost

capacitor will be:

boost

⋅

Where:

B = the boost current and

D= discharge ripple voltage.

With f

s = 600kHz, VD = 0.5V and IB =0.045A, the required

minimum capacitance for the boost capacitor is:

128

600

045

max

boost

⋅

The internal driver of the switch requires a minimum 2.7V

to fully turn on that switch to reduce its conduction loss.

If the output voltage is less than 2.7V, the boost capacitor

can be connected to either the input side or an

independent supply with a decoupling capacitor. But the

Pin BST should not see a voltage higher than its maximum

rating.

Freewheeling Diode Selection

This diode conducts during the switch’s off-time. The diode

should have enough current capability for full load and

short circuit conditions without any thermal concerns.

Its maximum repetitive reverse block voltage has to be

higher than the input voltage of the SC4518H. A low

forward conduction drop is also required to increase the

overall efficiency. The freewheeling diode should be

turned on and off fast with minimum reverse recovery

because the SC4518H is designed for high frequency

applications. SS13 Schottky rectifier is recommended

for certain applications. The average current of the diode,

D_AVG can be calculated by:

)

(

max

AVG

−

⋅

Thermal Considerations

There are three major power dissipation sources for the

SC4518H. The internal switch conduction loss, its

switching loss due to the high frequency switching actions

and the base drive boost circuit loss. These losses can

be estimated as:

)

(

500

boost

total

⋅

−

Application Information (Cont.)

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