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LT3757 Datasheet(PDF) 22 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) 22 Page - Linear Technology

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LT3757

3757fb
applicaTions inForMaTion
Figure 9. The Switch Current Waveform of the SEPIC Converter
3757 F08
ISW =
ISW(MAX)
ISW
t
DTS
ISW(MAX)
TS
that,intheSEPICconverter,theinductorL1isinseries
withtheinput,andtheripplecurrentflowingthroughthe
inputcapacitoriscontinuous.
SEPIC Converter: Switch Duty Cycle and Frequency
ForaSEPICconverteroperatinginCCM,thedutycycle
ofthemainswitchcanbecalculatedbasedontheoutput
voltage (VOUT), the input voltage (VIN) and the diode
forwardvoltage(VD).
Themaximumdutycycle(DMAX)occurswhentheconverter
hastheminimuminputvoltage:
D
V
V
V
V
V
MAX
OUT
D
IN MIN
OUT
D
=
+
+
+
(
)
SEPIC Converter: Inductor and Sense Resistor Selection
AsshowninFigure1,theSEPICconvertercontainstwo
inductors:L1andL2.L1andL2canbeindependent,but
canalsobewoundonthesamecore,sinceidenticalvolt-
agesareappliedtoL1andL2throughouttheswitching
cycle.
For the SEPIC topology, the current through L1 is the
converterinputcurrent.Basedonthefactthat,ideally,the
outputpowerisequaltotheinputpower,themaximum
averageinductorcurrentsofL1andL2are:
I
I
I
D
D
I
L MAX
IN MAX
O MAX
MAX
MAX
L MAX
1
2
1
(
)
(
)
(
)
(
•
=
=
−
))
(
)
=IO MAX
InaSEPICconverter,theswitchcurrentisequaltoIL1+
IL2whenthepowerswitchison,therefore,themaximum
averageswitchcurrentisdefinedas:
I
I
I
I
D
SW MAX
L MAX
L MAX
O MAX
MAX
(
)
(
)
(
)
(
) •
=
+
=
−
1
2
1
1
andthepeakswitchcurrentis:
I
I
D
SW PEAK
O MAX
MAX
(
)
(
)
•
•
= +


−
1
2
1
1
c
Theconstantcintheprecedingequationsrepresentsthe
percentagepeak-to-peakripplecurrentintheswitch,rela-
tivetoISW(MAX),asshowninFigure9.Then,theswitch
ripplecurrent∆ISWcanbecalculatedby:
∆ISW=c•ISW(MAX)
Theinductorripplecurrents∆IL1and∆IL2areidentical:
∆IL1=∆IL2=0.5•∆ISW
The inductor ripple current has a direct effect on the
choiceoftheinductorvalue.Choosingsmallervaluesof
∆ILrequireslargeinductancesandreducesthecurrent
loop gain (the converter will approach voltage mode).
Acceptinglargervaluesof∆ILallowstheuseoflowin-
ductances,butresultsinhigherinputcurrentrippleand
greatercorelosses.Itisrecommendedthatcfallsinthe
rangeof0.2to0.4.



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