SC4525AEVB View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC4525AEVB

28V 3A Step-Down Switching Regulator

Semtech Corporation 

SC4525A

Applications Information (Cont.)

CONTROLLER AND SCHOTTKY DIODE

Io

CA

Rs

Including the voltage divider (R4 and R6), the control to

feedback transfer function is found and plotted in Figure

REF

+

EA

FB

-

Vc

Vramp

PWM

MODULATOR

COMP

C5

C8

R7

SW

L1

Co

Resr

Vo

R4

R6

8 as the converter gain.

Since the converter gain has only one dominant pole at

low frequency, a simple Type-2 compensation network

is sufficient for voltage loop compensation. As shown in

Figure 8, the voltage compensator has a low frequency

integrator pole, a zero at FZ1, and a high frequency pole

at FP1. The integrator is used to boost the gain at low

frequency. The zero is introduced to compensate the

excessive phase lag at the loop gain crossover due to the

Figure 7. Block diagram of control loops

integrator pole (-90deg) and the dominant pole (-90deg).

TbhueckbcloAAocnCCkv==deir−−ateg22rr00awm⋅⋅illtoohinggtFhGGiegCCuS11AArCRRe4SS75⋅⋅2s22h5ππoAFFw.11CCTsCChteOOhei⋅⋅nVcVnVVoFFOOeBBnrtlrooolplo(ocpusrroefnat

The high frequency pole nulls the ESR zero and attenuates

high frequency noise.

O

⋅

VFB

VO



laloonoodppa) (vccAAuooCCrlntr==aseigns−−etts22al00omoof⋅⋅pplloaol)iggficceuo2r2rn(88rCsei⋅⋅Ans66tt)s..w1111soei⋅⋅tfn11has00ignn−−a3g3einr⋅⋅rr2(o2eGsrππCisaA⋅⋅t=m88o200rp8⋅⋅l()i11R.fiTs00e=h11r334e(.⋅⋅1Eo22Amu22)tW,⋅e⋅ 1a1r) 00

PWM modulator, and a LC filter.

−−66

⋅⋅ 1133....003360 == 1155..99ddBB

30

Fz1

Fp1

=⋅⋅⋅1ll52oo0.9ggπ.4⋅5G28n80C1FAStcFicg⋅n⋅Raoooii61nvdsSmnr.k0ecu11t1e⋅npar3cfo2⋅otetbRRCC⋅1CCalchrπn2y0n77(o5858Fest1V2:chnCa−======Cce3C⋅tve)uo100e2222OL⋅trlr0r.o.oπ1πr2ππ22t(,⋅e11−oeCπ⋅⋅⋅⋅o88on6VV01pr06165uuF⋅t,O⋅⋅60⋅60B118tRw1t1c22l5513p0o00p0..o09⋅97⋅00i,.u.11outm03⋅⋅a⋅−−htp01t01133np(00c33011idVse==s11a13w3⋅⋅3OnpCi12222)n⋅⋅is⋅a8t522222t2a)t2ccr.e.t..2.2ahi291313itron..iandkk1n1⋅⋅⋅nsn1aB11gf⋅⋅cle0i0l011esyrf−0033r6ftcCeuo33l==Oqo⋅n=d=13us0a0ceene.t.11..0344idsndo22i,55cgnpptlnnyn=oFhFiFFane1tFdh5SFriWeeni.g,9mgvudooarRBueiln,ttap7tinghuigeest

013⋅612=0⋅011−023p31=F⋅ T22h22is..13trk⋅aVVVV 1nococ0s==f3er((=11f u0++n.4ssc5t//inGGoωωFnP PppWW))hMM((11a((11s++a++ss fss//inRRωωiEEtnnSSeQQRRDCC ++COOss))g22a//inωωnn22 ))

(8)

0

-30

-60

1K

Fp

COMPENSATOR GAIN

CONVERTER GAIN Fc

LOOP GAIN

Fz Fsw/2

10K

100K

1M

10M

FREQUENCY (Hz)

Figure 8. Bode plots for voltage loop design

Therefore, the procedure of the voltage loop design for

1

6200/ ω⋅ 1n20) 3

⋅ 22.1GG⋅PP1WW0MM3≈≈=GG1CCA2ARRp⋅⋅RRFSS

,,

ωωpp

≈≈

11

RRCCOO

,,

ωωZZ == RREESS11RRCCt(1hOO)e,,PSloCt4t5h2e5cAocnavnerbteersguaminm, ia.eri.zceodntarso: l to feedback transfer

ARR/⋅1CGRωOPSpW,),M(1(1+aan+sdsEo/RSmωREnRωRCCCCSiωnQzR775Z858peaC+rn====≈===Oots)11RR2222Fl2ggo001CEZππππ/Smwm1aAOAF2FF2FRωCC001111t-CPZPZ,f11n112rO)eRRRR,q7777uencyωpZo=leRFEPS1aRtCO ,

and double poles at half the switching frequency.

R4

=

R6





VO

1.0V

− 1



function.

(2) Select the open loop crossover frequency, FC, between

10% and 20% of the switching frequency. At FC, find the

required compensator gain, AC. In typical applications with

ceramic output capacitors, the ESR zero is neglected and

the required compensator gain at FC can be estimated by

A C = − 20 ⋅ log  GC1AR S ⋅ 2πF 1CC O ⋅ VVFOB 

(9)

AC

=

−

20

⋅

log



28

⋅

1

6.1

⋅

10

−3

⋅

1

2π ⋅ 80 ⋅ 103

13

⋅ 22 ⋅ 10−6

⋅

1.0

3.

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