sc4525d Semtech Corporation, sc4525d Datasheet - Page 10

• Manufacturer Part Number: sc4525d
• Description: 18v, 3a, 350khz Step-down Switching Regulator
• Manufacturer: Semtech Corporation

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
the latch B
(I
current limit and the PWM comparator instead turns off
the switch, then latch B
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 B
sets the overload latch B
continuously discharged with (I
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, V
resistive divider (Figure 1) with its center tap tied to the FB
pin. For a given R
Minimum On Time Consideration
The operating duty cycle of a non-synchronous step-
down switching regulator in continuous-conduction
mode (CCM) is given by
© 2011 Semtech Corp.
D
- I
C
R
R
) (Figure 3). If the inductor current falls below the
R
R
D
D
D
D
D
D
L
L
D
D
4
4
4
4
I
I
1
1
=
=
I
I
=
=
=
=
L
L
=
=
=
=
=
=
V
V
R
R
=
=
V
V
R
R
3
IN
IN
V (
V (
. The soft-start capacitor is discharged with
IN
IN
6
6
V (
V (
6
6
V (
V (






+
+
20
20






O
O
+
+
O
O
1
1
V
V
O
O
1
1
V
V
V
V
+
+
O
O
V
V
V
V
0 .
0 .
+
+
O
O
V
V
0 .
0 .
+
+
D
D
%
%
O
O
6
D
D
F
F
+
+
V
V
O
O
+
+
V
V
V
V
−
−
value, R
V
V
SW
SW
D
D
V
V
⋅
⋅
−
−
D
D
V
V
I
I
D
D
)
)
V
V
O
O
−
−
V
V
D
D
)
)
−
−
V
V
⋅
⋅
D
D
)
)
CESAT
CESAT
⋅
⋅
1 (
1 (
⋅
⋅
⋅
⋅
1
1
CESAT
CESAT
⋅
⋅
1
1
L
L
F
F
1 (
1 (
1 (
1 (






SW
SW
3
1
1






−
−
2
will be reset and I
−
−
. The soft-start capacitor will be
−
−
) D
) D
4
) D
) D
can be found by
) D
) D
O
D
, is set with an external
- I
C
). The COMP pin is
Fig.4: Min On-Time vs Temp
Fig.4: Min On-Time vs Temp
C
will recharge
(1)
(2)
4
R
R
C
C
A
A
A
A
R
R
C
C
C
C
A
A
A
A
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 SC4525D
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
SS270 REV 6-7
SS270 REV 6-7
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.
Minimum Off Time Limitation
The PWM latch in Figure 2 is reset every cycle by the
clock. The clock also turns off the power transistor to
refresh the bootstrap capacitor. This minimum off time
limits the attainable duty cycle of the regulator at a given
7
7
C
C
C
C
7
7
C
C
C
C
5
5
8
8
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
. 0
. 0
−
−
−
−
. 0
. 0
2
2
2
2
−
−
−
−
20
20
20
20
28
28
⋅ π
⋅ π
⋅ π
⋅ π
20
20
20
20
28
28
10
10
10
10
IN
Figure 4. Variation of Minimum On Time
16
16
600
600
is the input voltage, V
⋅
⋅
⋅
⋅
⋅
⋅
15
15
⋅
⋅
⋅
⋅
⋅
⋅
log
log
log
log
10
10
20
20
15
15
log
log
log
log
10
10
20
20
200
200
180
180
160
160
140
140
120
120
100
100
9 .
9 .
9 .
9 .
⋅
⋅
10
10


 
 








−
−
⋅
⋅
-50
-50


 
 








−
−
3
3
10
10
Minumum On Time vs Temperature
Minumum On Time vs Temperature
G
G
28
28
3
3
G
G
28
28
D
1
1
3
3
1
1
=
=
CA
CA
=
=
CA
CA
1
1
1
1
with Ambient Temperature
3
3
1
1
⋅
⋅
is voltage drop across the rectifying
V
V
⋅
⋅
22
22
-25
-25
R
R
⋅
⋅
22
22
22
22
O
O
R
R
6
6
⋅
⋅
6
6
=1.5V, I
=1.5V, I
S
S
22
22
S
S
1 .
1 .
.
.
1 .
1 .
1
1
.
.
1 .
1 .
k 3
k 3
1
1
⋅
⋅
k 3
k 3
⋅
⋅
Temperature (
Temperature (
2
2
⋅
⋅
1 .
1 .
0
0
2
2
⋅
⋅
10
10
⋅
⋅
10
10
π
π
π
π
10
10
O
O
⋅
⋅
F
F
1
1
F
F
=1A
=1A
10
10
1
1
25
25
C
C
−
−
C
C
−
−
3
3
C
C
3
3
3
3
C
C
3
3
=
=
O
O
⋅
⋅
=
=
O
O
⋅
⋅
CESAT
50
50
2
2
. 0
. 0
2
2
=
=
⋅
⋅
. 0
. 0
⋅
⋅
⋅ π
⋅ π
V
V
⋅ π
⋅ π
12
12
V
V
45
45
V
V
V
V
45
45
O
O
FB
FB
O
O
is the switch saturation
FB
FB
C)
C)
O
O
80
80
75
75
80
80
pF
pF
www.semtech.com
nF
nF


 
 


nF
nF


 
 


⋅
⋅
100 125
100 125
⋅
⋅
10
10
10
10
ON(MIN)
SC4525D
1
1
3
3
1
1
3
3
⋅
⋅
⋅
⋅
22
22
).
22
22
⋅
⋅
⋅
⋅
10
10
10
10
−
−
−
−
6
6
6
6
⋅
⋅
10
⋅
⋅
1
1
3
3
1
1
3
3
0 .
0 .
3 .
3 .
0 .
0 .
3 .
3 .












=
=
=
=
15
15
15
15