SC4524A Просмотр технического описания (PDF) - Semtech Corporation

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SC4524A

28V 2A Step-Down Switching Regulator

Semtech Corporation 

SC4524A

Applications Information (Cont.)

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

Fig.4 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.

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 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  V.0OV −  

(1)

SettingDt=he SwVitOc+hiVnDg Frequency

VIN + VD − VCESAT

The switching frequency of the SC4524A is set with an

external resistor from the ROSC pin to ground.

MinimuDmIL =On(VTOim+FeSVWDC)⋅o⋅Ln(si−dDer)ation

The

opLera=ti(nVg2O

+duVDty) ⋅

0 % ⋅ IO

(cyc−leD)

⋅ FS W

of

a

non-synchronous

step-

downR4sw=iRtc6hingV.0OVre−gulator in continuous-conduction

mode (CCM) is given by

D = VIN +VVOD+−V VDCESAT

(2)

where VCESAT is the switch saturation voltage and VD is

voltage drop across the rectifying diode.

In

DIL

peak

= (VO + VD ) ⋅ (

currentF-mSWo⋅dLe

− D)

control,

the

PWM

modulating

ramp is the sensed current ramp of the power switch.

Tohni.sTchuLerrine=nte(trVsr2Oae0m+c%tVpioD⋅nIi)Os⋅

(ab−seDn) t unless the switch is turned

o⋅FfStWhis ramp with the output of the

voltage feedback error amplifier determines the switch

pulse width. The propagation delay time required to

immedIRiaMtSe_lCyINtu=rInOo⋅ ff Dth⋅e(sw−iDtc)h after it is turned on is the

minimum controllable switch on time (TON(MIN)).

AC =

AC =

R7 =

C5 =

C8 =

Vo =

Vc

Closed-loop measurement shows that the SC4524A

m(FiignuimreDu4mV)O.

oI=fnDthtILiem⋅reeEqiSsuRiareb+do8us⋅twF1iSt3Wc5h⋅nCosOnattirmoeomis

temperature

shorter than

the minimum on time, the regulator will either skip cycles

or it will jitter.

SS270 REV 6-7

GPWM

R7 =

CIN

>

4

200

IO

M⋅ DinVimINu⋅mFSOWn

Time

vs Temperature

C5 =

190

VO =1.5V

180

1MHz

170

C8 =

160

150

140

130

AC

=

−

20

⋅

l111o012g000 



GCAR

S

⋅

2πFCC O

⋅

VFB

VO



-50 -25 0 25 50 75 100 125

AC

=

−

20

⋅

log



28

⋅

6.Te⋅ m0pe−r3atu⋅r2e

(OC)

π ⋅ 80

⋅



0 3

⋅ 22 ⋅0 −6

⋅ .0  = 5

3.3 

Figure 4. Variation of Minimum On Time

5.9 with Ambient Temperature

RTo7

=all0o.w28f0o⋅r20t0ra−n3 s=ie2n2t

.3k

headroom,

the

minimum

operating

Csthw5ei=twc2hoπros⋅nt-tc6iam⋅see0msh3ion⋅ 2uiml2du.bme⋅oa0tn3letia=ms0te2..405%nFto 30% higher than

C8

=

2

π⋅

600

⋅





03

⋅ 22. ⋅0 3

= 2pF

10

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