SC4525C Просмотр технического описания (PDF) - Semtech Corporation

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SC4525C

28V 3A Step-Down Switching Regulator

Semtech Corporation 

SC4525C

Applications Information (Cont.)

Table 3: Resistor for Typical Switching Frequency

becomes armed. As the load draws more current from

the regulator, the current-limit comparator ILIM (Figure

2) will eventually limit the switch current on a cycle-by-

cycle basis. The over-current signal OC goes high, setting

the latch B3. The soft-start capacitor is discharged with

(ID - IC) (Figure 3). If the inductor current falls below the

current limit and the PWM comparator instead turns off

the switch, then latch B3 will be reset and IC will recharge

the soft-start capacitor. If over-current condition persists

or OC becomes asserted more often than PWM over

a period of time, then the soft-start capacitor will be

discharged below 1.9V. At this juncture, comparator B4

sets the overload latch B2. The soft-start capacitor will be

continuously discharged with (ID - IC). The COMP pin is

immediately pulled to ground. The switching regulator is

shut off until the soft-start capacitor is discharged below

1.0V. At this moment, the overload latch is reset. The

soft-start capacitor is recharged and the converter again

undergoes soft-start. The regulator will go through soft-

start, overload shutdown and restart until it is no longer

overloaded.

If the FB voltage falls below 0.8V because of output

overload, then the switching frequency will be reduced.

Fig.4 Frequency foldback helps to limit the inductor current

when the output is hard shorted to ground.

During normal operation, the soft-start capacitor is

charged to 2.4V.

Setting the Output Voltage

The regulator output voltage, VO, is set with an external

resistive divider (Figure 1) with its center tap tied to the FB

pin. For a given R6 value, R4 can be found by

R 4 = R6  1V.0OV − 1 

(1)

SettingDt=he SwVitOc+hiVnDg Frequency

VIN + VD − VCESAT

The switching frequency of the SC4525C is set with an

external resistor from the ROSC pin to ground. Table 3 lists

the

stanDdILar=d(rVeOsi+sFtSVoWDr

)v⋅a(l1ue−sDfo) r

⋅L1

typical

frequency

setting.

L1

=

(VO + VD ) ⋅ (1 − D)

20%⋅IO ⋅FSW

Freq. (k)

200

250

300

350

400

500

600

ROSC (k)

110

84.5

69.8

57.6

49.9

38.3

30.9

Freq. (k)

700

800

900

1000

1100

1200

1300

ROSC (k)

25.5

21.5

18.2

15.8

14.0

12.4

11.0

Freq. (k)

1400

1500

1600

1700

1800

1900

2000

ROSC (k)

9.76

8.87

8.06

7.15

6.34

5.62

5.23

Minimum On Time Consideration

AC =

TdhoewnoRp4sewr=aitRticn6hgin1dgV.0OuVtrye−gc1uylcalteorofina

non-synchronous step-

continuous-conduction

mode (CCM) is given by

AC =

D = VIN +VVOD+−V VDCESAT

(2)

where VIN is the input voltage, VCESAT is the switch saturation

vdoioltdaeg.eD,ILa=nd(VOVD+FiSVsWD

voltage

) ⋅ (1 − D)

⋅L1

drop

across

the

rectifying

In peak current-mode control, the PWM modulating

rTahmispcuiLsr1rte=hne(tVsr2Oae0m+n%sVpeD⋅dIi)Os⋅

c(1ur−reDn) t

⋅aFbSWsent

ramp of the power

unless the switch is

switch.

turned

on. The intersection of this ramp with the output of the

voltage feedback error amplifier determines the switch

pulse IwRMidS _thCIN. T=hIeO ⋅prDop⋅ (a1g−atDio)n delay time required to

immediately turn off the switch after it is turned on is the

SSm27i0nRimEVu6m-7 controllable switch on time (TON(MIN)).

DVO20=0

DMILin⋅umEuSmRO+n



T

8

i⋅mFeS1Wvs⋅TCeOmperature

R7 =

C5 =

C8 =

Vo =

Vc

GPWM

AC

AC

=

=

− 20

CIN

− 20

1⋅8l0og

160 

>

VO1=1.5V⋅,

GCAR S

IO

IO=1A1,

2 πFC

1MHz

⋅

CO

VFB

VO



1⋅4l0o4g⋅DVIN

28 ⋅

⋅ F1SW

6.1 ⋅

1

0

−3

⋅

2π ⋅ 80

⋅

1

10 3

⋅ 22 ⋅10 −6

R7 =

⋅

1.0

C =5

=

15

3.3 

120

C8 =

15.9

R7

=

1 010200

0.28 ⋅ 10-50−3

-=252 2 .03k

25

50

75 100 125

Temperature (OC)

C5

=

2

π

1

F⋅ 1ig6u⋅r1e043. V⋅ a2r2ia.1ti⋅o1n0o3f

= 0.45nF

Minimum

On

Time

C 8 = 2 π⋅ 600 ⋅ 101w3 i⋅t2h2A.1m⋅b1i0e3nt=T1em2ppFerature

10

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