
LTC4099
4099fd
CLPROG Resistor and Capacitor
AsdescribedintheBat-TrackInputCurrentLimitedStep-
DownSwitchingRegulatorsection,theresistoronthe
CLPROGpindeterminestheaverageinputcurrentlimit
ineachofthecurrentlimitmodes.Theinputcurrentwill
becomprisedoftwocomponents,thecurrentthatisused
todeliverpowertoVOUT,andthequiescentcurrentofthe
switchingregulator.ToensurethattheUSBspecification
isstrictlymet,bothcomponentsofinputcurrentshould
beconsidered.TheElectricalCharacteristicstablegives
thetypicalvaluesforquiescentcurrentsinallsettings,
aswellascurrentlimitprogrammingaccuracy.Togetas
closetothe500mAor100mAspecificationsaspossible,
aprecisionresistorshouldbeused.
An averaging capacitor is required in parallel with the
resistorsothattheswitchingregulatorcandeterminethe
averageinputcurrent.Thiscapacitoralsoprovidesthe
dominantpoleforthefeedbackloopwhencurrentlimit
isreached.Toensurestability,thecapacitoronCLPROG
shouldbe0.1Forlarger.
Choosing the Inductor
Becausetheinputvoltagerangeandoutputvoltagerange
ofthepowerpathswitchingregulatorarebothfairlynar-
row,theLTC4099wasdesignedforaspecificinductance
valueof3.3H.Someinductorswhichmaybesuitable
forthisapplicationarelistedinTable9.
Table 9. Recommended Inductors for the LTC4099
INDUCTOR
TYPE
L
(H)
MAX
IDC
(A)
MAX
DCR
(
Ω)
SIZE IN mm
(L
× W × H)
MANUFACTURER
LPS4018
3.3
2.2
0.08 3.9×3.9×1.7 Coilcraft
www.coilcraft.com
D53LC
DB318C
3.3
2.26
1.55
0.034
0.070
5
×5×3
3.8
×3.8×1.8
Toko
www.toko.com
WE-TPC
TypeM1
3.3 1.95 0.065 4.8×4.8×1.8 WürthElektronik
www.we-online.com
CDRH6D12
CDRH6D38
3.3
2.2
3.5
0.063
0.020
6.7
×6.7×1.5
7
×7×4
Sumida
www.sumida.com
applicaTions inForMaTion
VBUS and VOUT Bypass Capacitors
The style and value of the capacitors used with the
LTC4099determineseveralimportantparameterssuchas
regulatorcontrolloopstabilityandinputvoltageripple.
BecausetheLTC4099usesastep-downswitchingpower
supplyfromVBUStoVOUT,itsinputcurrentwaveform
containshighfrequencycomponents.Itisstronglyrec-
ommendedthatalowequivalentseriesresistance(ESR)
multilayerceramiccapacitorbeusedtobypassVBUS.
Tantalumandaluminumcapacitorsarenotrecommended
becauseoftheirhighESR.Thevalueofthecapacitoron
VBUSdirectlycontrolstheamountofinputripplefora
givenloadcurrent.Increasingthesizeofthiscapacitor
willreducetheinputripple.TheUSBspecificationallows
amaximumof10Ftobeconnecteddirectlyacrossthe
USBpowerbus.Iftheovervoltageprotectioncircuitis
usedtoprotectVBUS,thenitssoft-startingnaturecan
beexploitedandalargerVBUScapacitorcanbeused
ifdesired.
TopreventlargeVOUTvoltagestepsduringtransientload
conditions,itisalsorecommendedthataceramicca-
pacitorbeusedtobypassVOUT.Theoutputcapacitoris
usedinthecompensationoftheswitchingregulator.At
least10FwithlowESRarerequiredonVOUT.Additional
capacitance will improve load transient performance
andstability.
Multilayerceramicchipcapacitorstypicallyhaveexcep-
tionalESRperformance.MLCCscombinedwithatight
boardlayoutandanunbrokengroundplanewillyield
verygoodperformanceandlowEMIemissions.
Thereareseveraltypesofceramiccapacitorsavailable,
eachhavingconsiderablydifferentcharacteristics.For
example,X7Rceramiccapacitorshavethebestvoltage
andtemperaturestability.X5Rceramiccapacitorshave
apparently higher packing density but poorer perfor-
manceovertheirratedvoltageandtemperatureranges.
Y5Vceramiccapacitorshavethehighestpackingdensity,
butmustbeusedwithcautionbecauseoftheirextreme