SC4525A Semtech Corporation, SC4525A Datasheet - Page 10

• Manufacturer Part Number: SC4525A
• Description: 28V 3A Step-Down Switching Regulator
• Manufacturer: Semtech Corporation

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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 B
soft-start capacitor is discharged with (I
the inductor current falls below the current limit and the
PWM comparator instead turns off the switch, then latch
B
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 .9V. At this
juncture, comparator B
soft-start capacitor will be continuously discharged with
(I
switching regulator is shut off until the soft-start capacitor
is discharged below .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, V
resistive divider (Figure ) with its center tap tied to the FB
pin. For a given R
Setting the Switching Frequency
The switching frequency of the SC4525A is set with an
external resistor from the ROSC pin to ground.
Minimum On Time Consideration
The operating duty cycle of a non-synchronous step-
D
3
will be reset and I
- I
C
). The COMP pin is immediately pulled to ground. The
R
R
D
D
L
L
D
D
4
4
1
1
I
I
=
=
L
L
=
=
=
=
=
=
V
V
R
R
V (
V (
IN
IN
6
6
V (
V (






O
O
20
20
+
+
O
O
1
1
V
V
+
+
V
V
O
O
V
V
0 .
0 .
+
+
%
%
6
D
D
F
F
V
V
O
O
+
+
value, R
V
V
SW
SW
D
D
V
V
⋅
⋅
−
−
I
I
C
D
D
)
)
V
V
O
O
−
−
V
V
⋅
⋅
D
D
will recharge the soft-start capacitor.
)
)
⋅
⋅
1 (
1 (
⋅
⋅
CESAT
CESAT
⋅
⋅
1
1
L
L
F
F
1 (
1 (
1
1
SW
SW
4






−
−
sets the overload latch B
−
−
) D
) D
4
can be found by
) D
) D
O
, is set with an external
Fig. 4
Fig. 4
D
- I
C
) (Figure 3). If
()
2
3
. The
. The
SS270 REV 6-7
SS270 REV 6-7
R
R
C
C
C
C
A
A
A
A
down switching regulator in continuous-conduction
mode (CCM) is given by
where V
voltage, and V
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 (T
Closed-loop measurement shows that the SC4525A
minimum on time is about 30ns 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.
To allow for transient headroom, the minimum operating
switch on time should be at least 20% to 30% higher than
the worst-case minimum on time.
7
7
C
C
C
C
5
5
8
8
=
=
=
=
=
=
=
=
=
=
. 0
. 0
2
2
2
2
−
−
−
−
R
R
D
D
L
L
I
I
D
D
D
D
C
C
⋅ π
⋅ π
RMS
RMS
20
20
20
20
28
28
⋅ π
⋅ π
4
4
1
1
10
10
IN
I
I
V
V
IN
IN
Figure 4. Variation of Minimum On Time
=
=
L
L
=
=
16
16
O
O
600
600
=
=
is the input voltage, V
200
200
190
190
180
180
170
170
160
160
150
150
140
140
130
130
120
120
110
110
100
100
⋅
⋅
⋅
⋅
=
=
_
_
>
>
⋅
⋅
V
V
R
R
15
15
=
=
log
log
log
log
10
10
CIN
CIN
20
20
V (
V (
IN
IN
9 .
9 .
6
6
V (
V (
4
4
-50
-50
⋅
⋅
Minimum On Time vs Temperature
Minimum On Time vs Temperature
D
D
10
10
20
20






O
O
+
+
⋅
⋅
D ⋅
D ⋅


 
 








−
−
O
O
=
=
I
I
1
1
V
V
10
10
L
L
+
+
3
3
V
V
G
G
28
28
D
O
O
V
V
0 .
0 .
+
+
%
%
I
I
V
V
3
3
1
1
⋅
⋅
D
D
V
V
-25
-25
F
F
O
O
V
V
=
=
O
O
O
O
CA
CA
1
1
I
I
1MHz
1MHz
+
+


 
 


with Ambient Temperature
3
3
IN
IN
is voltage drop across the rectifying
O
O
1
1
⋅
⋅
V
V
SW
SW
D
D
V
V
⋅
⋅
−
−
⋅
⋅
ESR
ESR
=1.5V
=1.5V
⋅
⋅
22
22
22
22
I
I
R
R
D
D
V
V
)
)
⋅
⋅
O
O
6
6
⋅
⋅
−
−
V
V
⋅
⋅
22
22
D
D
)
)
S
S
F
F
⋅
⋅
D
D
1 (
1 (
⋅
⋅
1 .
1 .
CESAT
CESAT
Temperature (
Temperature (
0
0
1
1
⋅
⋅
.
.
1 .
1 .
L
L
F
F
1
1
SW
SW
k 3
k 3
⋅
⋅
1 (
1 (
⋅
⋅
SW
SW
1
1
+
+
1 .
1 .






−
−
2
2
⋅
⋅
1 (
1 (
⋅
⋅
10
10
10
10
−
−
) D
) D
π
π
25
25
8
8
⋅
⋅
−
−
F
F
10
10
) D
) D
1
1
⋅
⋅
C
C
−
−
F
F
) D
) D
3
3
3
3
C
C
SW
SW
50
50
3
3
1
1
=
=
O
O
⋅
⋅
CESAT
2
2
=
=
. 0
. 0
⋅
⋅
⋅
⋅
o
o
C
C
C)
C)
⋅ π
⋅ π
12
12
V
V
75
75
V
V
45
45
is the switch saturation
FB
FB
O
O
O
O
80
80
pF
pF


 
 


nF
nF


 
 


100 125
100 125
⋅
⋅
10
10
ON(MIN)
SC4525A
1
1
3
3
⋅
⋅
).
22
22
(2)
⋅
⋅
10
10
−
−
6
6
0
⋅
⋅
1
1
3
3
0 .
0 .
3 .
3 .
R
R
C
C
C
C
G
G
R =
R =
A
A
A
A
C =
C =
C =
C =






V
V
V
V
7
7
o
o
c
c
PWM
PWM
7
7
=
=
C
C
C
C
5
5
8
8
5
5
8
8
=
=
=
=
15
15
=
=
=
=
=
=
=
=