POWER MANAGEMENT
1
www.semtech.com
SC4521
3A Step-Down Switching Regulator with
Independent Soft Start
Features
Revision: January 25, 2006
The SC4521 is a current mode switching regulator with
an integrated switch, operating at 600kHz with
independent soft start and enable functions. The
integrated switch allows for cost effective low power
solutions (peak switch current 3 amps). High frequency
operation allows for very small passive components.
Current mode operation allows for fast dynamic response
and instantaneous duty cycle adjustment as the input
varies (ideal for CPE applications where the input is a
wall plug power).
The low shutdown current makes it ideal for portable
applications where battery life is important.
The SC4521 is an 600kHz switching regulator with a low
pin count.
The SC4521 allows customers to use large capacitive
loads because its independent soft start limits the dv/dt
of the output at start up.
Integrated 3 Amp switch
600kHz frequency of operation
Independent soft start funchion
Current mode control
Precision enable threshold
SO-8 EDP Lead-free package, fully WEEE and RoHS
compliant
XDSL modems
CPE equipment
DC-DC point of load applications
Portable equipment
Digtial consumer electronics
C1
V
OUT
D2
R3
D1
L1
C2
BST
1
IN
2
SW
3
GND
4
EN
5
FB
6
COMP
7
SS
8
Enable
C4
V
IN
C3
R2
R1
SC4521
C6
C1
V
OUT
D2
R3
D1
L1
C2
BST
1
IN
2
SW
3
GND
4
EN
5
FB
6
COMP
7
SS
8
Enable
C4
V
IN
C3
R2
R1
SC4521
C6
Typical Application Circuit
Applications
Description
2
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
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Electrical Characteristics
Note: (1) Minimum pad size.
Unless specified: V
IN
= 12V, V
COMP
= 0.8V, V
BST
= V
IN
+ 5V, EN = tied to V
IN
, SW = open.
T
A
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J
= -40C to 125C.
Absolute Maximum Ratings
Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters
specified in the Electrical Characteristics section is not implied. Exposure to Absolute Maximum rated conditions for extended periods of time may
affect device reliability.
3
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
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Electrical Characteristics
(Cont.)
Notes:
(1) The required minimum input voltage for a regulated output depends on the output voltage and load condition.
(2) Guaranteed by design.
Unless specified: V
IN
= 12V, V
COMP
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BST
= V
IN
+ 5V, EN = tied to V
IN
, SW = open.
T
A
= T
J
= -40C to 125C.
4
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
Pin Configurations
Ordering Information
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Notes:
(1) Only available in tape and reel packaging. A reel
contains 2500 devices.
(2) Lead free product. This product is WEEE and
RoHS compliant.
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Pin Descriptions
1
2
3
4
SS
BST
TOP VIEW
(SO-8 EDP)
5
6
7
8
COMP
IN
FB
SW
EN
GND
5
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
Block Diagram
FB
COMP
R
Q
S
PWM
-
+
POWER
IN
TRANSISTOR
SLOPE
+
+
+
ISEN
40m
SW
BST
OSCILLATOR
CLK
FREQUENCY
SLOPE COMP
SS
SLOPE
OL
FB
EN
REFERENCE
UVLO
1.6V INTERNAL SUPPLY
EA
HICCUP
Is
Is
GND
FB
COMP
R
Q
S
PWM
-
+
POWER
IN
TRANSISTOR
SLOPE
+
+
+
ISEN
40m
SW
BST
OSCILLATOR
CLK
FREQUENCY
SLOPE COMP
SS
SLOPE
OL
FB
EN
REFERENCE
UVLO
1.6V INTERNAL SUPPLY
EA
HICCUP
Is
Is
GND
Typical Characteristic - OCP Limit
SC4519H OCP Limit vs Duty Cycle
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
0
20
40
60
80
100
Duty Cycle (%)
C
u
r
r
e
nt
Li
m
i
t
(
A
)
ILIM @-40C
ILIM @25C
ILIM @125C
6
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
Application Information
The SC4521 is a current mode buck converter regulator.
The SC4521 uses two feedback loops that control the
duty cycle of the internal power switch. The error amplifier
functions like that of the voltage mode converter. The
output of the error amplifier works as a switch current
reference. This technique effectively removes one of the
double poles in the voltage mode system. With this, it is
much simpler to compensate a current mode converter
to have better performance.
Soft Start
Internally, connected to the SS pin is a 100K pull-up
resistor from an internal 1.6V regulator and the collector
of an NPN pull-down transistor from SS to GND. The NPN
transistor is "on" when the Enable pin is low or when a
fault is detected (Input UVLO, Over-Current, or Over
Temp). When the SC4521 is disabled or when a fault is
detected, the NPN transistor pulls the SS pin low. The
SS charging time is controlled by the internal 100k
resistor and external soft-start cap. This is a closed-loop
soft-start which effectively "ramps the reference". The
SS process completes when the SS pin voltage is
exceeds the internal reference voltage, 0.8V. The SS pin
continues to charge up to 1.6V, well above the reference
voltage to ensure it does not interfere with normal
operation. The governing SS equation is:
SS
SS
C
69000
T
=
For example, a 22nF SS cap would give a SS time of
approximately 1.5mS.
Overcurrent Protection
The current sense amplifier in the SC4521 monitors the
switch current during each cycle. Overcurrent protection
(OCP) is triggered when the current limit exceeds the upper
limit of 3A, detected by a voltage on COMP greater than
about 2V. When an OCP fault is detected, the switch is
turned off and the external COMP capacitor is gradually
discharged at the rate of dv/dt=3
A/Ccomp
. Ccomp is
the total capacitance value attached to COMP. At the
same time, the soft start cap C
SS
is quickly discharged.
Once the COMP voltage has fallen below 250mV, the
part enters a normal startup cycle. In the case of
sustained overcurrent or dead-short, the part will
continually cycle through the retry sequence as described
above.
Enable
Pulling and holding the EN pin below 0.4V activates the
shut down mode of the SC4521 which reduces the input
supply current to 250A. During the shut down mode,
the switch is turned off. The SC4521 is turned on if the
EN pin is pulled high.
Oscillator
Its internal free running oscillator sets the PWM frequency
at 600kHz for the SC4521 without any external
components to program the frequency.
UVLO
When the EN pin is pulled and held above 1.8V, the voltage
on Pin IN determines the operation of the SC4521. As
VIN increases during power up, the internal circuit senses
VIN and keeps the power transistor off until VIN reaches
4.4V.
Load Current
The peak current I
PEAK
in the switch is internally limited.
For a specific application, the allowed load current I
OMAX
will change if the input voltage drifts away from the original
design as given for continuous current mode:
s
O
OMAX
f
L
2
)
D
1
(
V
3
I
-
-
=
Where:
fs = switching frequency,
Vo = output voltage and
D = duty ratio, V
O
/V
I
V
I
= input voltage.
Figure 2 shows the theoretical maximum load current
for the specific cases. In a real application, however, the
allowed maximum load current also depends on the layout
and the air cooling condition. Therefore, the maximum
load current may need to be degraded according to the
thermal situation of the application.
7
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
Maximum Load Current vs Input Voltage
L=10uH
2.700
2.720
2.740
2.760
2.780
2.800
2.820
2.840
2.860
2.880
2.900
4
6
8
10
12
14
16
18
Vi (V)
Io
ma
x
(
A
)
Vo=2.5V
Vo=3.3V
Vo=5V
Figure 2. Theoretical maximum load current curves
Inductor Selection
The factors for selecting the inductor include its cost,
efficiency, size and EMI. For a typical SC4521 application,
the inductor selection is mainly based on its value,
saturation current and DC resistance. Increasing the
inductor value will decrease the ripple level of the output
voltage while the output transient response will be
degraded. Low value inductors offer small size and fast
transient responses while they allow large ripple currents,
poor efficiencies and require more output capacitance
for low output ripple. The inductor should be able to
handle the peak current without saturating and its copper
resistance in the winding should be as low as possible to
minimize its resistive power loss. A good trade-off among
its size, loss and cost is to set the inductor ripple current
to be within 15% to 30% of the maximum output current.
The inductor value can be determined according to its
operating point under its continuous mode and the
switching frequency as follows:
OMAX
s
I
O
I
O
I
f
V
)
V
(V
V
L
-
=
Where:
fs = switching frequency,
= ratio of the peak to peak inductor current to the
output load current and
V
O
= output voltage.
Application Information (Cont.)
The peak to peak inductor current is:
OMAX
p
p
I
I
=
-
After the required inductor value is selected, the proper
selection of the core material is based on the peak
inductor current and efficiency specifications. The core
must be able to handle the peak inductor current I
PEAK
without saturation and produce low core loss during the
high frequency operation.
2
I
I
I
p
p
OMAX
PEAK
-
+
=
The power loss for the inductor includes its core loss and
copper loss. If possible, the winding resistance should
be minimized to reduce inductor's copper loss. The core
must be able to handle the peak inductor current I
PEAK
without saturation and produce low core loss during the
high frequency operation. The core loss can be found in
the manufacturer's datasheet. The inductor's copper loss
can be estimated as follows:
WINDING
LRMS
2
COPPER
R
I
P
=
Where:
I
LRMS
is the RMS current in the inductor. This current can
be calculated as follows:
2
OMAX
LRMS
12
1
1
I
I
+
=
Output Capacitor Selection
Basically there are two major factors to consider in
selecting the type and quantity of the output capacitors.
The first one is the required ESR (Equivalent Series
Resistance) which should be low enough to reduce the
output voltage deviation during load changes. The second
one is the required capacitance, which should be high
enough to hold up the output voltage. Before the SC4521
regulates the inductor current to a new value during a
load transient, the output capacitor delivers all the
additional current needed by the load. The ESR and ESL
of the output capacitor, the loop parasitic inductance
between the output capacitor and the load combined
with inductor ripple current are all major contributors to
the output voltage ripple. Surface mount ceramic
capacitors are recommended.
8
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
Input Capacitor Selection
The input capacitor selection is based on its ripple current
level, required capacitance and voltage rating. This
capacitor must be able to provide the ripple current
drawn by the converter. For the continuous conduction
mode, the RMS value of the input capacitor current
I
CIN(RMS)
can be calculated from:
I
2
O
I
O
OMAX
(RMS)
CIN
V
)
V
(V
V
I
I
-
=
This current gives the capacitor's power loss through its
R
CIN(ESR)
as follows:
CIN(ESR)
(RMS)
CIN
2
CIN
R
I
P
=
The input ripple voltage mainly depends on the input
capacitor's ESR and its capacitance for a given load, input
voltage and output voltage. Assuming that the input
current of the converter is constant, the required input
capacitance for a given voltage ripple can be calculated
by:
)
R
I
V
(
fs
D)
(1
D
I
C
(ESR)
CIN
OMAX
I
OMAX
IN
-
-
=
Where:
V
I
= the given input voltage ripple.
Because the input capacitor is exposed to the large surge
current, attention is needed for the input capacitor. If
tantalum capacitors are used at the input side of the
converter, one needs to ensure that the RMS and surge
ratings are not exceeded. For generic tantalum
capacitors, it is suggested to derate their voltage ratings
at a ratio of about two to protect these input capacitors.
Boost Capacitor and its Supply Source Selection
The boost capacitor selection is based on its discharge
ripple voltage, worst case conduction time and boost
current. The worst case conduction time T
w
can be
estimated as follows:
max
s
W
D
f
1
T
=
Where:
f
s
= the switching frequency and
Dmax = maximum duty ratio, 0.85 for the SC4521.
The required minimum capacitance for the boost
capacitor will be:
W
D
B
boost
T
V
I
C
=
Where:
I
B
= the boost current and
V
D
= discharge ripple voltage.
With f
s
= 600kHz, V
D
= 0.5V and I
B
= 0.045A, the required
minimum capacitance for the boost capacitor is:
128nF
0.85
600k
1
0.5
0.045
D
f
1
V
I
C
max
s
D
B
boost
=
=
=
The internal driver of the switch requires a minimum 2.7V
to fully turn on that switch to reduce its conduction loss.
If the output voltage is less than 2.7V, the boost capacitor
can be connected to either the input side or an
independent supply with a decoupling capacitor. But the
Pin BST should not see a voltage higher than its maximum
rating.
Freewheeling Diode Selection
This diode conducts during the switch's off-time. The diode
should have enough current capability for full load and
short circuit conditions without any thermal concerns.
Its maximum repetitive reverse block voltage has to be
higher than the input voltage of the SC4521. A low
forward conduction drop is also required to increase the
overall efficiency. The freewheeling diode should be
turned on and off fast with minimum reverse recovery
because the SC4521 is designed for high frequency
applications. SS23 Schottky rectifier is recommended
for certain applications. The average current of the diode,
I
D
_
AVG
can be calculated by:
)
D
I
(
I
I
omax
AVG
-
D
-
=
Thermal Considerations
There are three major power dissipation sources for the
SC4521. The internal switch conduction loss, its switching
loss due to the high frequency switching actions and the
base drive boost circuit loss. These losses can be
estimated as:
)
(V
D
I
1000
10
V
I
10
10.8
D
R
I
P
boost
o
I
o
3
on
2
o
total
+
+
=
-
Application Information (Cont.)
9
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
Application Information (Cont.)
Where:
I
O
= load current;
R
on
= on-equivalent resistance of the switch;
V
BOOST
= input voltage or output based on the boost circuit
connection.
The junction temperature of the SC4521 can be
further determined by:
total
JA
A
J
P
T
T
+
=
JA
is the thermal resistance from junction to ambient.
Its value is a function of the IC package, the application
layout and the air cooling system.
The freewheeling diode also contributes a significant
portion of the total converter loss. This loss should be
minimized to increase the converter efficiency by using
Schottky diodes with low forward drop (V
F
).
D)
(1
I
V
P
o
F
diode
-
=
Loop Compensation Design
Assuming the power stage ESR (equivalent series
resistance) zero is an order of magnitude higher than
the closed loop bandwidth, which is typically one tenth of
the switching frequency, the power stage control to output
transfer function with the current loop closed (Ridley
model) for the SC4521 will be as follows:
C
R
1
s
1
R
4.3
(s)
G
L
L
VD
+
=
Where:
R
L
Load and
C
Output capacitor.
The goal of the compensation design is to shape the loop
to have a high DC gain, high bandwidth, enough phase
margin, and high attenuation for high frequency noises.
Figure 3 gives a typical compensation network which
offers 2 poles and 1 zero to the power stage:
BST
1
IN
2
SW
3
GN
D
4
EN
5
FB
6
COMP
7
SYNC
8
SC4519H
R2
R1
R3
C4
C
D2
L1
C5
Vout
Figure 3. Compensation network provides 2 poles and
1 zero.
The compensation network gives the following
characteristics:
2
1
2
m
P2
Z
1
COMP
R
R
R
g
)
s
(1
s
s
1
(s)
G
+
+
+
=
Where:
5
4
1
C
C
1
+
=
4
3
Z
C
R
1
=
5
4
3
5
4
P2
C
C
R
C
C
+
=
The loop gain will be given by:
)
s
(1
)
s
(1
s
1
s
1
R
R
R
C
R
10
3.655
(s)
G
(s)
G
T(s)
P2
P1
Z
2
1
2
4
L
3
VD
COMP
+
+
+
+
=
=
-
Where:
C
R
1
L
p1
=
One integrator is added at origin to increase the DC gain.
Z
is used to cancel the power stage pole
P1
so that the
loop gain has 20dB/dec rate when it reaches 0dB line.
P2
is placed at half switching frequency to reject high
frequency switching noises. Figure 4 gives the asymptotic
diagrams of the power stage with current loop closed
and its loop gain.
SC4521
S S
10
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
The design guidelines for the SC4521 applications are
as following:
1. Set the loop gain crossover corner frequency
C
for
given switching corner frequency
C
=
2
f
C
2. Place an integrator at the origin to increase DC and
low frequency gains.
3. Select
Z
such that it is placed at
P1
to obtain a
-20dB/dec rate to go across the 0dB line.
4. Place a high frequency compensator pole
P2
(
P2
=
f
s
) to get the maximum attenuation of
the switching ripple and high frequency noise with
the adequate phase lag at
C.
Layout Guidelines:
In order to achieve optimal electrical and thermal
performance for high frequency converters, special
attention must be paid to the PCB layouts. The goal of
layout optimization is to identify the high di/dt loops and
minimize them. The following guidelines should be used
to ensure proper operation of the converters.
1. A ground plane is suggested to minimize switching
noises and trace losses and maximize heat
transferring.
2. Start the PCB layout by placing the power components
first. Arrange the power circuit to achieve a clean
power flow route. Put all power connections on one
side of the PCB with wide copper filled areas if
possible.
3. The V
IN
bypass capacitor should be placed next to
the V
IN
and GND pins.
4. The trace connecting the feedback resistors to the
output should be short, direct and far away from any
noise sources such as switching node and switching
components.
5. Minimize the loop including input capacitor, the
SC4521 and freewheeling diode D
2
. This loop passes
high di/dt current. Make sure the trace width is wide
enough to reduce copper losses in this loop.
6. Maximize the trace width of the loop connecting the
inductor, freewheeling diode D
2
and the output
capacitor.
7. Connect the ground of the feedback divider and the
compensation components directly to the GND pin
of the SC4521 by using a separate ground trace.
Application Information (Cont.)
8. Connect Pin 4 to a large copper area to remove the
IC heat and increase the power capability of the
SC4521. A few feedthrough holes are required to
connect this large copper area to a ground plane to
further improve the thermal environment of the
SC4521. The traces attached to other pins should
be as wide as possible for the same purpose.
11
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
Application Information (Cont.)
Design Example: 16V to 5V at 2A
s
l
a
i
r
e
t
a
M
f
o
ll
i
B
m
e
t
I
y
t
Q
e
c
n
e
r
e
f
e
R
e
u
l
a
V
r
e
r
u
t
c
a
f
u
n
a
M
/.
o
N
t
r
a
P
1
1
1
C
R
7
X
,
5
0
8
0
,
V
5
2
,
F
u
2
2
.
0
y
a
h
s
i
V
2
2
3
C
,
2
C
V
5
2
,
R
5
X
,
0
1
2
1
,
u
0
1
c
i
n
o
s
a
n
a
P
3
1
4
C
V
5
2
,
R
7
X
,
5
0
8
0
,
n
3
.
3
y
a
h
s
i
V
4
1
5
C
F
p
0
8
1
5
1
6
C
F
n
2
2
6
1
1
D
3
2
3
-
D
O
S
,
S
W
8
4
1
4
N
1
7
1
2
D
3
3
S
S
3
3
S
S
:
N
/
P
d
li
h
c
r
i
a
F
8
1
1
L
H
u
2
.
8
2
R
8
-
5
2
1
R
D
:
N
/
P
R
E
P
O
O
C
9
1
1
R
K
3
.
2
5
0
1
1
2
R
k
0
1
1
1
1
3
R
k
4
.
3
2
1
1
4
R
k
5
7
.
4
3
1
1
1
U
1
2
5
4
C
S
h
c
e
t
m
e
S
.
e
g
a
k
c
a
p
3
0
6
0
h
ti
w
n
o
i
s
i
c
e
r
p
%
1
e
v
a
h
s
r
o
t
s
i
s
e
r
ll
a
,
d
e
if
i
c
e
p
s
s
s
e
l
n
U
%
0
2
-
/
+
e
r
a
s
r
o
ti
c
a
p
a
c
ll
a
d
n
a
%
1
-
/
+
e
r
a
s
r
o
t
s
i
s
e
R
C1
0.22u
SC4521
BS
T
1
IN
2
SW
3
GND
4
EN
5
FB
6
COMP
7
SS
8
D3
R2
10k
R1
52.3k
R3
3.4k
C4
3.3n
C2
10u
D2
L1
8.2uH
R4
4.75k
C5
180p
V
IN
=16V
V
o
=5V
C3
10u
C6
C1
0.22u
SC4521
BS
T
1
IN
2
SW
3
GND
4
EN
5
FB
6
COMP
7
SS
8
D3
R2
10k
R1
52.3k
R3
3.4k
C4
3.3n
C2
10u
D2
L1
8.2uH
R4
4.75k
C5
180p
V
IN
=16V
V
o
=5V
C3
10u
C6
12
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
Outline Drawing - SOIC-8L EDP
SEE DETAIL
DETAIL
A
A
.050 BSC
.236 BSC
8
.010
.150
.189
.154
.193
.012
-
8
0.25
1.27 BSC
6.00 BSC
3.90
4.90
-
.157
.197
3.80
4.80
.020
0.31
4.00
5.00
0.51
bxN
2X N/2 TIPS
SEATING
aaa C
E/2
2X
1
2
N
A
D
A1
E1
bbb
C A-B D
ccc C
e/2
E
A2
(.041)
.004
.008
-
.028
-
-
-
-
0
.016
.007
.049
.000
.053
8
0
0.20
0.10
-
8
0.40
0.17
1.25
0.00
.041
.010
.069
.065
.005
1.35
(1.05)
0.72
-
1.04
0.25
-
-
-
1.75
1.65
0.13
0.25
-
.010
.020
0.50
-
c
L
(L1)
01
0.25
GAGE
PLANE
h
3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS
OR GATE BURRS.
-B-
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
DATUMS AND TO BE DETERMINED AT DATUM PLANE
NOTES:
1.
2.
-A-
-H-
SIDE VIEW
A
B
C
D
e
H
PLANE
EXPOSED PAD
L1
N
01
bbb
aaa
ccc
A
b
A2
A1
D
E
E1
L
h
e
c
DIM
MIN
MILLIMETERS
NOM
DIMENSIONS
INCHES
MIN
MAX
MAX
NOM
F
h
H
F
H
.085
.120
.095
.116
.130
.099
2.95
2.15
3.05
2.41
3.30
2.51
4. REFERENCE JEDEC STD MS-012, VARIATION BA.
13
2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4521
Contact Information
Semtech Corporation
Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805)498-2111 FAX (805)498-3804
Land Pattern - Power SOIC-8L
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
NOTES:
1.
REFERENCE IPC-SM-782A, RLP NO. 300A.
2.
3. THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD
SHALL BE CONNECTED TO A SYSTEM GROUND PLANE.
FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR
FUNCTIONAL PERFORMANCE OF THE DEVICE.
E
Z
(C)
G
P
0.36mm
THERMAL VIA
X
Y
SOLDER MASK
(5.20)
C
(.205)
X
Y
Z
F
G
P
D
E
.024
.087
.291
.101
.118
.050
.201
.134
0.60
2.20
7.40
2.56
3.00
1.27
5.10
3.40
MILLIMETERS
INCHES
DIM
DIMENSIONS
D
F
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