SC4525AEVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC4525AEVB Datasheet PDF : 19 Pages
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SC4525A
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
the converter again undergoes soft-start. The regulator
Fig. 4 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, 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 SC4525A is set with an
external resistor from the ROSC pin to ground.
MinimuDmIL =On(VTOim+FeSVWDC)⋅o⋅Ln(11si−dDer)ation
The
opLe1ra=ti(nVg2O 0+d%uVDt⋅yI)O⋅
(c1yc−leD)of
⋅ FSW
a
non-synchronous
step-
AC =
downR4sw=iRtc6hin1gV.0OVre−g1ulator in 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
minimum controllable switch on time (TON(MIN)).
R7 =
C5 =
C8 =
Vo =
Vc
CmloinsiemdDu-lmVoOoop=nDmtILime⋅aesEuiSsrReamb+oe8nut⋅tF1Ss13Wh0o⋅nCwsOsatthroaot mthteemSpCe4ra5t2u5rAe
(Figure 4). If the required switch on time is shorter than
the minimum on time, the regulator will either skip cycles
or it will jitter.
SS270
REV 6-7
C IN
>
IO
4M⋅inDimVuINm⋅ OFSnWTime
vs
Temperature
GPWM
R7 =
C5 =
200
190
VO =1.5V
180
1MHz
C8 =
170
160
150
140
AC
=
−
130
2011⋅12l00og
1
GCAR
S
⋅1
2πFCC O
⋅
VFB
VO
100
AC
=
−
20
⋅
lo-5g0
-25
28 ⋅
6.T011em⋅ 1p2e05ra−t3u5r0⋅e
75
2(oπC)⋅
100 1215
80 ⋅ 103
⋅
22
⋅
10
−6
⋅ 1.0 = 15
3.3
Fig1u5.9re 4. Variation of Minimum On Time
R7
=
10 20
0.28 ⋅ 10 −3
=w2it2h.3Akmbient Temperature
CTswo5 ia=tlcl2hoπwo⋅nf1ot6rimt⋅r1ea0nsh3s1ioe⋅ 2unl2tdh.1bee⋅a1da0rto3leoa=ms0t, t2.4h0e5%nmFtoin3im0%umhiogpheerratthinagn
the worst-case minimum on time.
C8
=
1
2 π⋅ 600 ⋅ 103
⋅ 22.1 ⋅ 103
= 12pF
10
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