SC4524C Ver la hoja de datos (PDF) - Semtech Corporation
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SC4524C Datasheet PDF : 22 Pages
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SC4524C
Applications Information (Cont.)
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.
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
R4
=
R6
VO
.0V
−
Setting the Switching Frequency
The switching frequency of the SC4524C is set with an
external resistor from the ROSC pin to ground. Table 3
lists standard resistor values for typical frequency setting.
Table 3 — Resistor for Typical Switching Frequency
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
The operating duty cycle of a non-synchronous step-
down switching regulator in continuous-conduction
mode (CCM) is given by
D
=
VIN
VO + VD
+ VD − VCESAT
where VCESAT is the switch saturation voltage and VD is
voltage drop across the rectifying diode.
In peak current-mode control, the PWM modulating
ramp is the sensed current ramp of the power switch.
This current ramp is absent unless the switch is turned
on. The intersection of this ramp with the output of the
voltage feedback error amplifier determines the switch
pulse width. The propagation delay time required to
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 SC4524C
minimum on time is about 120ns at room temperature
(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.
10
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