SC1109ACSTR SEMTECH [Semtech Corporation], SC1109ACSTR Datasheet - Page 11

• Manufacturer Part Number: SC1109ACSTR
• Description: Synchronous PWM Controller with Dual Low Dropout Regulator Controllers
• Manufacturer: SEMTECH [Semtech Corporation]

or more generally,
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:
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
rise.
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:
For the example above:
POWER MANAGEMENT
Application Information (Cont.)
F
R I
R I
S
E
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11
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 40
for the D
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.
F
R I
R I
F
R I
R I
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4 i
4 i
L
L
L
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T
T
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y T
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p
2
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p
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C/W for the SO-8 are readily achiev-
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