SC1109BSTR datasheet(11/19 Pages) SEMTECH

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POWER MANAGEMENT

SC1109

or more generally,

)

(

OSC

⋅

c) Body diode recovery losses are more difficult to esti-

mate, but to a first approximation, it is reasonable to

assume that the stored charge on the bottom FET body

diode will be moved through the top FET as it starts to

turn on. The resulting power dissipation in the top FET

will be:

OSC

⋅

To a first order approximation, it is convenient to only

consider conduction losses to determine FET suitability.

For a 5V in; 2.8V out at 14.2A requirement, typical FET

losses would be:

Using 1.5X Room temp R

DS(ON) to allow for temperature

rise.

)

(

Ω))

(

1D2

BOTTOM FET - Bottom FET losses are almost entirely

due to conduction. The body diode is forced into conduc-

tion at the beginning and end of the bottom switch con-

duction period, so when the FET turns on and off, there

is very little voltage across it, resulting in low switching

losses. Conduction losses for the FET can be determined

by:

)

(

)

(

COND

−

⋅

For the example above:

)

(

Ω)P

)

1D2

0D2

Each of the package types has a characteristic thermal

impedance, for the TO-220 package, thermal impedance

is mostly determined by the heatsink used. For the sur-

face mount packages on double sided FR4, 2 oz printed

circuit board material, thermal impedances of 40oC/W

for the D2PAK and 80oC/W for the SO-8 are readily achiev-

able. The corresponding temperature rise is detailed be-

low:

(

)

It is apparent that single SO-8 Si4410 are not adequate

for this application, but by using parallel pairs in each

position, power dissipation will be approximately halved

and temperature rise reduced by a factor of 4.

INPUT CAPACITORS - since the RMS ripple current in the

input capacitors may be as high as 50% of the output

current, suitable capacitors must be chosen accordingly.

Also, during fast load transients, there may be restric-

tions on input di/dt. These restrictions require useable

energy storage within the converter circuitry, either as

extra output capacitance or, more usually, additional in-

put capacitors. Choosing low ESR input capacitors will

help maximize ripple rating for a given size.

Application Information (Cont.)

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