SC4524SETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC4524SETRT Datasheet PDF : 21 Pages
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SC4524
POWER MANAGEMENT
Applications Information
longer blanked. If VFB is still below 0.7V, then the regulator
will undergo shutdown and restart. The soft-start process
should allow the output voltage to reach 70% of its final
value before CSS is charged above 2V. Figures 9(a) and
9(b) show the timing diagrams of successful and failed
start-up waveforms respectively. The soft-start interval
should also be made sufficiently long so that the output
voltage rises monotonically and it does not overshoot
its final voltage by more than 5%.
During normal soft-start, both the COMP voltage and the
switch current limit gradually increase until the converter
becomes regulated. If the regulator output is shorted to
ground, then the COMP voltage will continue to rise to
its 2.4V upper limit. The SC4524 will reach its cycle-by-
cycle current limit sometime during the soft-start charging
phase. As described previously, the switches in the
SC4524 either do not turn on at all or for at least 105ns.
With the output shorted, the error amplifier will command
the regulator to operate at full duty cycle. The current
limit comparator will turn off the switch if the switch
current exceeds 3.2A. However, this happens only after
the switch is turned on for 105ns. During switch off time,
the inductor current ramps down at a slow rate
determined by the forward voltage of the freewheeling
diode and the resistance of the short. If the resulting
reverse volt-second is insufficient to reset the inductor
before the start of the next cycle, then the inductor
current will keep increasing until the diode forward
voltage becomes high enough to achieve volt-second
balance. This makes the current limit comparator
ineffective. Setting the switching frequency below
500kHz at high VIN (> 20V) will make the off time
sufficiently long to keep the inductor current within
bounds under short circuit condition.Shortening the soft-
start interval from the onset of switching to hiccup enable
also makes short circuit operation more robust. A 22-
47nF soft-start capacitor is found adequate for most
applications.
Loop Compensation
proportional to its controlling input VCOMP. Its
transconductance GMP is 8W-1. With the current loop
closed, the control-to-output transfer function
Y 287
Y&203
has
a dominant-pole p2 located at a frequency slightly higher
than that of the output filter pole.
wS
- Q,287 = - Q
9287& 5287&
(8)
where C1 is the output capacitor, ROUT is the equivalent
load resistance and n (depending on duty ratio, slope
compensation, frequency and passive components) is
usually between 1 and 2.
If C1 is ceramic, then its ESR zero can be neglected as it
situates well beyond half the switching frequency. The
low frequency gain of the control-to-output transfer
function is simply the product of power stage
transconductance and the equivalent load resistance
(Figure 11).
The transfer functions of the feedback network and the
error amplifier are:
( ) Y)%
Y287
=
ÌÌÍË
5
5 + 5
ÜÜÝÛ
Î
ÏÐ
+ V&5
+ V 5ÔÏ5 &
Þ
ß
à
(9)
and
Y&203
*0$52 ( + V&5 )
Y)% ( + V&52 )¼ ( + V&5 )
(10)
provided that & >> & and 52 >> 5 .
In Equation (10), C5 forms a low frequency pole p1 with
the output resistance RO of the error amplifier and C6
forms a high frequency pole p3 with R5:
Figure 10 shows a simplified equivalent circuit of a step-
down converter. The power stage, which consists of the
current-mode PWM comparator, the power switch, the
freewheeling diode and the inductor, feeds the output
network. The power stage can be modeled as a voltage-
controlled current source, producing an output current
52
=
$PSOLILHU2SHQ/RRS *DLQ
7UDQVFRQGXF WDQFH
=
G%
mW-
= 0W
wS
=-
52&
ã 2006 Semtech Corp.
16
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