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SC4525D

18V, 3A, 350kHz Step-Down Switching Regulator

Semtech Corporation 

SC4525D

Applications Information (Cont.)

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

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

voltage, and VD is voltage drop across the rectifying

diode.

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

In peak current-mode control, the PWM modulating

ramp is the sensed current ramp of the power switch.

the switch, then latch B3 will be reset and IC will recharge This current ramp is absent unless the switch is turned

the soft-start capacitor. If over-current condition persists on. The intersection of this ramp with the output of the

or OC becomes asserted more often than PWM over voltage feedback error amplifier determines the switch

a period of time, then the soft-start capacitor will be pulse width. The propagation delay time required to

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

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

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

Closed-loop measurement shows that the SC4525D

shut off until the soft-start capacitor is disFcihga.4rg: eMdinbeOlon-wTimemvsinTimemump on time is about 120ns at room temperature

1.0V. At this moment, the overload latch is reset. The (Figure 4). If the required switch on time is shorter than

soft-start capacitor is recharged and the converter again the minimum on time, the regulator will either skip cycles

undergoes soft-start. The regulator will go through soft- SoSr2i7t0wRiEllVji6t-7ter.

start, overload shutdown and restart until it is no longer

overloaded.

Minumum On Time vs Temperature

200

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.

180

VO=1.5V, IO=1A

160

140

During normal operation, the soft-start capacitor is

charged to 2.4V.

120

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)

MinimDum= On TViOm+eVCDonsideration

VIN + VD − VCESAT

TdhoewnoRp4sewr=aiRtticn6hgi1ndgV.0OuVtrye−gc1uylcalteorofina

non-synchronous step-

continuous-conduction

m ode D( DC=ICLMV=I)N(i Vs+VOgVOi+DvF+e−SVnVWD VD)bC⋅⋅ELyS(11AT− D)

(2)

©

2011

LSe1 m=te(Vch2O

+ VD ) ⋅

0C%orp⋅ I.O

(1 − D)

⋅ FS W

DI = (VO + VD ) ⋅ (1 − D)

100

-50 -25

0 25 50 75

Temperature (OC)

100 125

AC = − 2F0ig⋅ luorge4G.CVw1AaRitrShia⋅Ati2omπnbF1oCieCfnMOt iT⋅neVVimFmOB puemraOtunrTeime

ATswoC ia=tlcl−how2on0fot⋅rilmotgreans2hs8ioe⋅un6ltd.h11be⋅ea1da0rto−l3eoam⋅s2t,

πt2h0⋅e8%m0to⋅in13i0m013%u⋅m2h2iog⋅ph1ee0rra−tth6ina⋅gn13..03





=

1

5

AARCtMTcCChl7h5oie==en=c=wki−−Pm02.oW.22πTu2r1s00hM⋅m8t01e-⋅⋅⋅c6ll1Oloo12a5ca0.9⋅gg0slfto1fec−cT0hG23mki381mCi=i1Anan⋅⋅Re22li6smFS22Lo.i11giu..⋅m13tum2⋅uk1⋅riπret10onFa10nC−st23i3CotoiiO⋅s=nmf2f⋅0reπetVV..h4sF⋅OeBe85t0npeF⋅ov1we0re1y3r

⋅c2y2cl⋅e10by−6 th⋅ 13e..03

transistor to





=

1

5

refresh the bootstrap capacitor. This minimum off time

RCl7im8 ==it0s2.t2hπ18⋅e06⋅a10125t0.9t00a−⋅in31a0=b13l2e⋅22d.23uk.t1y

cycle

⋅10 3

o=f1t2hpeFregulator at a given

www.semtech.com 10

C=

1

= 0.45nF

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