SC4525CSETRT Semtech, SC4525CSETRT Datasheet - Page 10

• Manufacturer Part Number: SC4525CSETRT
• Description: IC BUCK ADJ 3A 8SOIC
• Manufacturer: Semtech
• Series: -r
• Type: Step-Down (Buck), PWM - Current Moder

Specifications of SC4525CSETRT

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1 V ~ 26.88 V
• Current - Output: 3A
• Frequency - Switching: 300kHz ~ 1.3MHz
• Voltage - Input: 3 V ~ 28 V
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width) Exposed Pad
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: SC4525CSETR

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 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
Setting the Switching Frequency
The switching frequency of the SC4525C is set with an
external resistor from the ROSC pin to ground. Table 3 lists
the standard resistor values for typical frequency setting.
D
- I
C
) (Figure 3). If the inductor current falls below the
R
R
D
D
L
L
D
D
4
4
1
1
I
I
=
=
L
L
=
=
=
=
=
=
V
V
R
R
3
V (
V (
. The soft-start capacitor is discharged with
IN
IN
6
6
V (
V (
20
20






O
O
+
+
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
D
D
)
)
V
V
O
O
−
−
V
V
⋅
⋅
D
D
)
)
⋅
⋅
1 (
1 (
⋅
⋅
CESAT
CESAT
⋅
⋅
1
1
L
L
F
F
1 (
1 (
SW
SW
3
1
1






−
−
2
will be reset and I
. The soft-start capacitor will be
−
−
) D
) D
4
can be found by
) D
) D
O
D
, is set with an external
- I
Fig.4
Fig.4
C
). The COMP pin is
C
will recharge
(1)
4
SS270 REV 6-7
SS270 REV 6-7
R
R
C
C
C
C
A
A
A
A
Table 3: Resistor for Typical Switching Frequency
Minimum On Time Consideration
The operating duty cycle of a non-synchronous step-
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
7
7
C
C
C
C
5
5
8
8
Freq. (k)
=
=
=
=
=
=
=
=
=
=
200
250
300
350
400
500
600
. 0
. 0
2
2
2
2
−
−
−
−
R
R
D
D
L
L
I
I
D
D
D
D
C
C
⋅ π
⋅ π
⋅ π
⋅ π
RMS
RMS
20
20
20
20
28
28
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
180
180
160
160
140
140
120
120
100
100
=
=
_
_
⋅
⋅
>
>
⋅
⋅
⋅
⋅
V
V
R
R
15
15
=
=
log
log
log
log
10
10
CIN
CIN
20
20
V (
V (
IN
IN
R
9 .
9 .
6
6
V (
V (
4
4
⋅
⋅
-50
-50
OSC
84.5
69.8
57.6
49.9
38.3
30.9
D
D
110
10
10






O
O
20
20
Minumum On Time vs Temperature
Minumum On Time vs Temperature
+
+
⋅
⋅


 
 


D ⋅
D ⋅






−
−
O
O
=
=
I
I
10
10
1
1
V
V
L
L
+
+
3
3
(k)
V
V
G
G
28
28
D
O
O
V
V
0 .
0 .
+
+
I
I
%
%
3
3
⋅
⋅
V
V
1
1
D
D
V
V
O
O
-25
-25
F
F
V
V
=
=
O
O
CA
CA
1
1
I
I
+
+


 
 


O
O
with Ambient Temperature
3
3
1
1
IN
IN
⋅
⋅
is voltage drop across the rectifying
D
D
O
O
V
V
V
V
SW
SW
⋅
⋅
−
−
⋅
⋅
=1.5V, I
=1.5V, I
ESR
ESR
⋅
⋅
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
Freq. (k)
1 (
1 (
⋅
⋅
1 .
1 .
CESAT
CESAT
Temperature (
Temperature (
⋅
⋅
1
1
L
L
.
.
0
0
1 .
1 .
F
F
1
1
SW
SW
k 3
k 3
1000
1100
1200
1300
⋅
⋅
1 (
1 (
700
800
900
⋅
⋅
SW
SW
1
1
+
+
1 .
1 .






−
−
2
2
⋅
⋅
1 (
1 (
O
O
⋅
⋅
10
10
=1A, 1MHz
=1A, 1MHz
π
π
10
10
−
−
) D
) D
8
8
25
25
⋅
⋅
−
−
F
F
10
10
1
1
) D
) D
⋅
⋅
C
C
−
−
F
F
3
3
) D
) D
3
3
C
C
SW
SW
50
50
3
3
1
1
=
=
O
O
⋅
⋅
CESAT
R
2
2
=
=
. 0
. 0
OSC
25.5
21.5
18.2
15.8
14.0
12.4
11.0
⋅
⋅
⋅
⋅
O
O
C
C
⋅ π
⋅ π
12
12
C)
C)
V
V
75
75
V
V
45
45
(k)
is the switch saturation
O
O
FB
FB
O
O
80
80
pF
pF


 
 


nF
nF


 
 


100 125
100 125
⋅
⋅
10
10
Freq. (k)
ON(MIN)
1400
1500
1600
1700
1800
1900
2000
SC4525C
1
1
3
3
⋅
⋅
).
22
22
(2)
⋅
⋅
10
10
R
OSC
9.76
8.87
8.06
7.15
6.34
5.62
5.23
−
−
6
6
(k)
10
⋅
⋅
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
=
=
7
7
o
o
c
c
PWM
PWM
C
C
C
C
5
5
8
8
5
5
8
8
=
=
15
15
=
=
=
=
=
=
=
=
=
=