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LT3757 Datasheet(PDF) 23 Page - Linear Technology

Part # LT3757
Description  Boost, Flyback, SEPIC and Inverting Controller
PDF  36 Pages
Scroll/Zoom Zoom In 100%  Zoom Out
Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT3757 Datasheet(HTML) 23 Page - Linear Technology

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LT3757

3757fb
Givenanoperatinginputvoltagerange,andhavingchosen
theoperatingfrequencyandripplecurrentintheinduc-
tor,theinductorvalue(L1andL2areindependent)ofthe
SEPICconvertercanbedeterminedusingthefollowing
equation:
L L
V
I
f
D
IN MIN
SW
MAX
1 2
0 5
= =
(
)
. •
•
•
∆
FormostSEPICapplications,theequalinductorvalues
willfallintherangeof1µHto100µH.
BymakingL1=L2,andwindingthemonthesamecore,the
valueofinductanceintheprecedingequationisreplaced
by2L,duetomutualinductance:
L
V
I
f
D
IN MIN
SW
MAX
=
(
)
•
•
∆
Thismaintainsthesameripplecurrentandenergystorage
intheinductors.Thepeakinductorcurrentsare:
IL1(PEAK)=IL1(MAX)+0.5•∆IL1
IL2(PEAK)=IL2(MAX)+0.5•∆IL2
TheRMSinductorcurrentsare:
I
I
L RMS
L MAX
L
1
1
2
1
1
12
(
)
(
) •
=
+
c
where:
cL
L
L MAX
I
I
1
1
1
=
∆
(
)
I
I
L RMS
L MAX
L
2
2
2
2
1
12
(
)
(
) •
=
+
c
where:
cL
L
L MAX
I
I
2
2
2
=
∆
(
)
Basedontheprecedingequations,theusershouldchoose
theinductorshavingsufficientsaturationandRMScur-
rentratings.
InaSEPICconverter,whenthepowerswitchisturnedon,
thecurrentflowingthroughthesenseresistor(ISENSE)is
theswitchcurrent.
SetthesensevoltageatISENSE(PEAK)tobetheminimum
oftheSENSEcurrentlimitthresholdwitha20%margin.
Thesenseresistorvaluecanthenbecalculatedtobe:
R
mV
I
SENSE
SW PEAK
=
80
(
)
SEPIC Converter: Power MOSFET Selection
FortheSEPICconfiguration,chooseaMOSFETwitha
VDCratinghigherthanthesumoftheoutputvoltageand
inputvoltagebyasafetymargin(a10Vsafetymarginis
usuallysufficient).
ThepowerdissipatedbytheMOSFETinaSEPICcon-
verteris:
PFET=I2SW(MAX)•RDS(ON)•DMAX
+2•(VIN(MIN)+VOUT)2•IL(MAX)•CRSS•f/1A
Thefirstterminthisequationrepresentstheconduction
lossesinthedevice,andthesecondterm,theswitching
loss.CRSSisthereversetransfercapacitance,whichis
usuallyspecifiedintheMOSFETcharacteristics.
For maximum efficiency, RDS(ON) and CRSS should be
minimized.Fromaknownpowerdissipatedinthepower
MOSFET,itsjunctiontemperaturecanbeobtainedusing
thefollowingequation:
TJ=TA+PFET•θJA=TA+PFET•(θJC+θCA)
TJ must not exceed the MOSFET maximum junction
temperaturerating.Itisrecommendedtomeasurethe
MOSFETtemperatureinsteadystatetoensurethatabsolute
maximumratingsarenotexceeded.
applicaTions inForMaTion



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