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LT3757 Datasheet(PDF) 25 Page - Linear Technology

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

LT3757 Datasheet(HTML) 25 Page - Linear Technology

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LT3757

3757fb
Inverting Converter: Switch Duty Cycle and Frequency
ForaninvertingconverteroperatinginCCM,thedutycycle
ofthemainswitchcanbecalculatedbasedonthenegative
outputvoltage(VOUT)andtheinputvoltage(VIN).
Themaximumdutycycle(DMAX)occurswhentheconverter
hastheminimuminputvoltage:
D
V
V
V
V
V
MAX
OUT
D
OUT
D
IN MIN
=
−
−
−
(
)
Inverting Converter: Inductor, Sense Resistor, Power
MOSFET, Output Diode and Input Capacitor Selections
The selections of the inductor, sense resistor, power
MOSFET,outputdiodeandinputcapacitorofaninverting
converteraresimilartothoseoftheSEPICconverter.Please
refertothecorrespondingSEPICconvertersections.
Inverting Converter: Output Capacitor Selection
The inverting converter requires much smaller output
capacitorsthanthoseoftheboost,flybackandSEPIC
convertersforsimilaroutputripples.Thisisduetothefact
that,intheinvertingconverter,theinductorL2isinseries
withtheoutput,andtheripplecurrentflowingthroughthe
outputcapacitorsarecontinuous.Theoutputripplevoltage
isproducedbytheripplecurrentofL2flowingthroughthe
ESRandbulkcapacitanceoftheoutputcapacitor:
∆
∆
V
I
ESR
f C
OUT P P
L
COUT
OUT
( – )
•
• •
=
+


2
1
8
Afterspecifyingthemaximumoutputripple,theusercan
selecttheoutputcapacitorsaccordingtothepreceding
equation.
TheESRcanbeminimizedbyusinghighqualityX5Ror
X7Rdielectricceramiccapacitors.Inmanyapplications,
ceramiccapacitorsaresufficienttolimittheoutputvolt-
ageripple.
The RMS ripple current rating of the output capacitor
needstobegreaterthan:
IRMS(COUT)>0.3•∆IL2
Inverting Converter: Selecting the DC Coupling Capacitor
The DC voltage rating of the DC coupling capacitor
(CDC,asshowninFigure10)shouldbelargerthanthe
maximuminputvoltageminustheoutputvoltage(nega-
tivevoltage):
VCDC>VIN(MAX)–VOUT
CDChasnearlyarectangularcurrentwaveform.During
theswitchoff-time,thecurrentthroughCDCisIIN,while
approximately–IOflowsduringtheon-time.TheRMS
ratingofthecouplingcapacitorisdeterminedbythefol-
lowingequation:
I
I
D
D
RMS CDC
O MAX
MAX
MAX
(
)
(
) •
>
−
1
AlowESRandESL,X5RorX7Rceramiccapacitorworks
wellforCDC.
applicaTions inForMaTion



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