10 2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC1185 & SC1185A
Layout Guidelines
sider 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.
epytTEFR
)no(SD
m( Ω)P
D
)W(egakcaP
52043LRI5196.1D
2
kaP
3022LRI5.0191.1D
2
kaP
0144iS0262.28-0S
BOBO
BOBO
BO
TTTT
TTTT
TT
OM FET OM FET
OM FET OM FET
OM FET - Bottom FET losses are almost entirely due
to conduction. The body diode is forced into conduction at
the beginning and end of the bottom switch conduction
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:
)1(RIP
)on(DS
2
OCOND
δ−⋅⋅=
For the example above:
epytTEFR
)no(SD
m( Ω)P
D
)W(egakcaP
52043LRI5133.1D
2
kaP
3022LRI5.0139.0D
2
kaP
0144iS0277.18-0S
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
o
C/W for
the D
2
PAK and 80
o
C/W for the SO-8 are readily achiev-
able. The corresponding temperature rise is detailed be-
low:
(esirerutarepmeT
o
)C
epytTEFTEFpoTTEFmottoB
52043LRI6.762.35
3022LRI6.742.73
0144iS8.0816.141
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 CAPINPUT CAP
INPUT CAPINPUT CAP
INPUT CAP
AA
AA
A
CITCIT
CITCIT
CIT
ORSORS
ORSORS
ORS - 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 restrictions
on input di/dt. These restrictions require useable energy
storage within the converter circuitry, either as extra out-
put capacitance or, more usually, additional input capaci-
tors. Choosing low ESR input capacitors will help maximize
ripple rating for a given size.
