SC4524D Ver la hoja de datos (PDF) - Semtech Corporation

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Fabricante

SC4524D

18V 2A Step-Down Switching Regulator

Semtech Corporation 

Applications Information (Cont.)

Closed-loop measurement shows that the SC4524D

minimum on time is about 120ns at room temperature

(Figure 4). If the required switch on time is shorter than

the minimum on time, the regulator will either skip cycles

or it will jitter.

To allow for transient headroom, the minimum operating

switch on time should be at least 20% to 30% higher than

the worst-case minimum on time.

Minimum Off Time Limitation

The PWM latch in Figure 2 is reset every cycle by the

clock. The clock also turns off the power transistor to

refresh the bootstrap capacitor. This minimum off time

limits the attainable duty cycle of the regulator at a given

switching frequency. The measured minimum off time is

100ns typically. If the required duty cycle is higher than

the attainable maximum, then the output voltage will not

be abRle4

mode.

t=oRr6ea1chV.0OiVts−s1et

value

in

continuous-conduction

InductDo=r

SelecVtOio+nVD

VIN + VD − VCES

AT

The inductor ripple current for a non-synchronous step-

down converter in continuous-conduction mode is

RD 4IL==R(6V O1V+.0FOSVVWD −)⋅⋅L1(11 − D )

(3)

winhdeurcetLDa1nF=Sc=WeV(.INVis2O+V0+tVOh%DVe+D⋅−I)VOsV⋅Dw(⋅C1FEiSt−WAcTDhi)ng frequency and L1 is the

An inductor ripple current between 20% to 50% of the

maximum load current gives a good compromise among

easffsiucimenIDiRncMIgLyS,_=3cCI5o(NV%sO=t

given by

I+aOnV⋅dD )Ds⋅iz(⋅1e(1.−R−DeD)-a) rranging Equation (3) and

iFnSdWu⋅cLto1 r ripple current, the inductor is

LD 1VO==(VDO3 IL5+%⋅ VED⋅ IS)O ⋅R⋅(1F+S−W8D ⋅)FS1W ⋅ C O 

(4)

If the input voltage varies over a wide range, then choose

Lco1 nbvaesrIeRteMdrS

CIN

The peak

>_ooCnpINetr=haIteOiIoO⋅nnoaDmt⋅ti(nh1ael−iniDnp)puuttvovlotaltgaegee.xtArelwmaeyss. verify

cu4rr⋅eDnVtINlim⋅ FitSWof SC4524D power transistor is at

DVO

=

DIL

⋅ ESR

+

8

1

⋅ FSW

⋅CO



R4

=

R

6



1

O

.0 V

− 1 

R4

D

=

=

R6

VIN

 1VOV.0O+VV−D1 

+ VD − VCESAT

SC4524D

D

=

VIN

VO + VD

+ VD − VCESAT

lScIenuCapr4sru5tet2n24DDLt.D6.D1IeIALLc=.i==osT(uh((V2VVpe2O.O6Ol0m+iAn++%FFVagSVmDSVx⋅WWIDCD)iOmi⋅)n)a⋅⋅(⋅⋅L⋅1uLpuF((11Ss11ma−Wc−o−Didtn)DDeoe)l)rivhearlaf bolef

load current for the

the inductor ripple

TrihpeplienpLcIR1uuMrtS=r_ce(CanVIpNt2Oao=0+cf%IitVaOoD⋅b⋅rI)Ou⋅s(⋅Dch1FkoS⋅−Wuc(1oDldn−) vbDee)rtcehro. Tshenis

to handle the

value is given

RMS

by

AC =

AC =

AC =

R7 =

RC75 ==

CC58 ==

C8 =

Vo =

Vc

I RMS _ CIN = IO ⋅ D ⋅ (1 − D)

(5)

TiiinnnhpperuueittdnrcupDDiapcuVVpiptnOOalcgec==aivtptDDaohaInIleLcLtcai⋅⋅tecgaoEceEnnaSScwdneRRuibtm+c+hetiui8n8evses⋅st⋅tFpFaiESmSel11WMWscoaiI⋅⋅ftiCbeCfcrdeaOOothmfiroiogntmh.hTeehnirsoeiugsguimhlatptooor.rkteTaehnpet

Vo =

Vc

GPWM

GPWM

RA7C

=

=

RC 4IN=>R64 ⋅1DV.V0OIIOVN ⋅ −F1SW

(6)

AwMC hu=eltr−ie-l2CDaD0yINV=e⋅INr>lVoicIsN4get+r⋅haVGDeVmOCVD1IaA+IiONRc−llVo⋅ScVDFwaC⋅SEpaW2SabAπTclFei1tCoiCnrpsO,u⋅wtVVrhFiOBipcphlehvaoveltavgeery. low ESR

A(aCare=fewt−h2em0Wid⋅)leoaagnldc2ch8aon⋅ic6ee.a11sf⋅oil1yr0hin−a3pnu⋅d2tleπfhi⋅lti8ge0hrin⋅R1gM0. S13Ar⋅ips2ip2nlge⋅1lce0u4−rr6.e7⋅nµ13tF,..03

RC75 ==

AC

CC58

=

==

CR=871==5

RCCAAXsfvpsO758CCowaol5uart=====lRiitcttsapc20e22f−−cgyh0du.eππ222eLIiD0b1trRni⋅⋅8100akMnaIoC016gLmHpS=t⋅⋅⋅a60p1=_h1ll2pfz5ipoo00ra.Cc(9⋅0letIgg(Va1riNthc⋅Vqac2−Oco01ae3Ola=ui0G2l+t0t3ep1eEiI+%8o=51ColOF3aVn⋅S1A0rwnDc22SV⋅⋅RRc⋅⋅0WI6iDs)222yOitSka,t⋅.o)..2⋅Dh1a1(⋅Hna13⋅1⋅rLF.p(dszk⋅2⋅1aS11i⋅−m1(sWp1π1bl⋅s−D0lo0aaF1uiw1−cw)dCl−D0l3la3kiCDect)3tEc=qceOi⋅)oShar=u20naR⋅ipanπ1.mVsa4Vteg,2F⋅ecO5ilBa8pceintfnfc0Foa(rF1dtenr⋅rµqo1c51Fuel000yoe1r0µt3enrikFc⋅q2cH2cyu.i2sz2a.iµrpaoe⋅FFa1drmo)ce0rhcieqta−iongh6unrthiasg⋅btet.13hoeer..03

C5 =

C8 =



= 15

Vo =

Vc

RCVVTe oc75hx==p=er(0o2e1u.sπ2Ds+ t1⋅e8pV01sduO⋅6/11t2a5=G0.ω9⋅r0s1iPDpp −W0)3IpM(L131l(=e⋅1⋅+22vE+so22S sl/..tR1R3ωakEg+⋅nS1eQR80CD+ ⋅3OVFs)OS=21Wo0/f⋅.ωCa4 n25Ob)nuFck

converter

(7)

can

be

GPWM

RGCVVwSdtA7Poc8hiWenh=1e==Mcce0rr1ee(re2≈µ1egf0aFoπCGm+tsCAr2⋅hteC0IeCO6NosesARit0/s>4⋅h(G0Rωi7tEen4hPSµqp⋅dWhe1)⋅F,uuM(iD01ogaXc(1Vtu1h3t+5IiIoOtoNeR⋅+psrsn2⋅utcs/F2reRtwSω(i.3rpWc1EhanSω)ap)mQRe⋅p,pl1Cnea+it≈0cOcVhsc)3icRIetN2uaa1C=irp/nsorO1aωecau,cen2tn2ti.pptti)toFusDrtmIiLsraifpxionipumcωlnreueZdmav=sao.edRlsteEaqS1gaRuCesaOtDies,

CCGfiP58noWr==Mtoh22≈ueππtGpFFoC11uPZuA11Rtt⋅RRpfRiu77lStte, rcinapgaicnitmorωopisst≈anRpo1CptOliac, actoionncse.rnRωibpZep=cleaRucEsuSe1rRrCethO1ne1,t

R7 =

C5 =

C8 =

AC

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