SC4525C View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC4525C

28V 3A Step-Down Switching Regulator

Semtech Corporation 

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SC4525C

Applications Information (Cont.)

The high frequency pole nulls the ESR zero and attenuates

high frequency noise.

60

Example: Determine the voltage compensator for an

800kHz, 12V to 3.3V/3A converter with 47uF ceramic

output capacitor.

30

Fz1

Fp1

0

Fp

COMPENSATOR GAIN

CONVERTER

Fc

GAIN

LOOP GAIN

-30

Fz Fsw/2

Choose a loop gain crossover frequency of 80kHz, and

place voltage compensator zero and pole at FZ1=16kHz

(20% of FC), and FP1=600kHz. From Equation (9), the

required compensator gain at FC is

AC

20

log

1

18.5 3. 53 10

3

2

1

80 103 47 10 6

1.0

3.3

14.1dB

Then the compensator parameters are

-60

1K

10K

100K

1M

10M

AC

=

− 20

⋅

log

1FREQ⋅UENC1Y (Hz)

GCAR S 2πFCC O

⋅

VFB

VO



Figure 8. Bode plots for voltage loop design

14.1

R7 =

10 20

0.3⋅ 10−3

= 16.9 k

C5

=

2π

⋅

16

⋅

1

103 ⋅

16.9

⋅103

= 0.589 nF

TthheerSeCfo4A5rCe2,=5tCh−ec2ap0nr⋅obloceegsduu2mr8em⋅o6afr.11itzhe⋅e1d0vao−s3:lta⋅ g2eπ

loop

⋅80 ⋅

1

1d0es3ig⋅ 2n2fo⋅ 1r

0

−6

⋅

1.0

3.3





=

15C.89=dB2

π⋅

600

⋅

1

103 ⋅16.9

⋅

103

=15.7 pF

Select R7=16.9k, C5=0.68nF, and C8=22pF for the design.

(1) Plot the conver1t5e.9 r gain, i.e. control to feedback transfer

functioRn7.

(2) Select

=

the0

10 20

.o2p8e⋅n1l0oo−3p

= 22.3k

crossover

frequency,

FC,

between

1re0q%uiarenCdd5c2=o0m2%pπeo⋅n1f s6tah⋅te1ors0wg31ai⋅ticn2h,2Ain.1Cg. I⋅n1frt0eyq3pui=cean0lc.a4yp.5pAnltiFcFaCt,iofinnsdwtihthe

Compensator parameters for various typical applications

are listed in Table 5. A MathCAD program is also available

upon request for detailed calculation of the compensator

parameters.

ctheerarmeqiCcu8oirue=dtp2cuπot⋅m6cap0pe0an⋅cs1iat0ot1o3rsr⋅,g2tah2ien.1Ea⋅St1RF0Cz3ecra=on1ibs2enpeeFsgtliemctaeteddabnyd

(2(340))%UPsloaefAAc VVtteCCochh==e=ethcc−−(e1roo 22mc+s00ospsom⋅⋅e/llvoonpGeωggse rPpanWf)trG2Ms(oe1a8C(rq11tA+op⋅uR+6r esoS.nls/z11e⋅Rceω,2⋅yrEF1no,SπPQRF0,1F 1,CCFC−+.t3OCZo1s),O⋅2c2ba⋅/nπeVωVtcF⋅wOn2eB8)le0teh⋅n1e

(9)

10% and

1

0E3SR⋅ 2z2er⋅o1,

0

−6

Thermal Considerations

For the power transistor inside the SC4525C, the

conduction loss PC, the switching loss PSW, and bootstrap

c⋅ir1c.u0itPl=oTOs1Ts5APL.B9=STdP, BcCan+ PbSeWe+stiPmBSaTte+dPaQs follows:

3.3 

FZ.

(5)

TheGnP,WtMhe≈

pGaCrAR1a5⋅m.9R Set,ers

of

theωpco≈mRp1CeOn,sation

neωtwZ o=rkR

E

1

SRC

O

,

PC = D ⋅ VCESAT ⋅ IO

PQ = VIN ⋅ 2mA

can

beRCRc757al===cu012lg.a0π2t1m⋅Ae28C001d⋅612b0⋅0y1−03

=

1

3⋅

2

2

2

2

.

.

3k

1⋅

1

0

3

= 0.45nF

CC85

=

=

1

2

2

ππ⋅F6Z10

R

0

7⋅

1

1

03

⋅ 22.1 ⋅10 3

= 12pF

C8

=

1

2 πFP1

R7

PSW

=

1

2

⋅

tS

⋅ VIN

⋅IO

⋅ FSW

(10)

PBST

=D⋅

VBST

⋅

IO

40

wswhietcrehPiVnDBgST=tisi(m1the−eoDBf)St⋅ThVseDuN⋅pIPpONlytvroanltsaigsteoarn(sdeteS

is the equivalent

Table 4).

where

VVgocm==0(.13m+ As //VGωiPpsW)tM(1h(e1+E+sAs/RgωaEnSiQnR Co+Ofs)t2he/

SC4525C.

ωn2 )

PIND = (1.1 ~ 1.3) ⋅ I2O ⋅ R DC

14

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