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SC4520SETRT

P1-P3P4-P6P7-P9P10-P12
72006 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4520
Application Information (Cont.)
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.
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
4681012141618
Vi (V)
Iomax (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 SC4520 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:
OMAXsI
OIO
IδfV
)V(VV
L
⋅⋅⋅
−⋅
=
Where:
fs = switching frequency,
δ = ratio of the peak to peak inductor current to the
output load current and
V
O
= output voltage.
The peak to peak inductor current is:
OMAXpp
IδI •=
−
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
II
pp
OMAXPEAK
−
+=
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
RIP ⋅=
Where:
I
LRMS
is the RMS current in the inductor. This current can
be calculated as follows:
2
OMAXLRMS
δ
12
1
1II ⋅+⋅=
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 SC4520
regulates the inductor current to a new value during a
82006 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4520
Application Information (Cont.)
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.
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
OIO
OMAX
(RMS)
CIN
V
)V(VV
II
−⋅
⋅=
This current gives the capacitor’s power loss through its
R
CIN(ESR)
as follows:
CIN(ESR)
(RMS)
CIN
2
CIN
RIP •=
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:
)RIV(fs
D)(1D
IC
(ESR)
CINOMAXI
OMAXIN
⋅−∆⋅
−⋅
⋅=
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 SC4520.
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:
128nF0.85
600k
1
0.5
0.045
D
f
1
V
I
C
max
sD
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 SC4520. 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 SC4520 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:
)DI(II
omaxAVG-D
−⋅=
92006 Semtech Corp. www.semtech.com
POWER MANAGEMENT
SC4520
Application Information (Cont.)
Thermal Considerations
There are three major power dissipation sources for the
SC4520. 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:
)(VDI
1000
10
VI1010.8DRIP
boostoIo
3
on
2
ototal
⋅⋅⋅+⋅⋅⋅+⋅⋅=
−
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 SC4520 can be
further determined by:
totalJAAJ
PθTT ⋅+=
θ
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)(1IVP
oFdiode
−⋅⋅=
Loop Compensation Design
The SC4520 has an internal error amplifier and requires
a compensation network to connect between the COMP
pin and GND pin as shown in Figure 3. The compensation
network includes C4, C5 and R3. R1 and R2 are used to
program the output voltage according to:
)
R
R
1(2.1V
2
1
O
+•=
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 SC4520 will be as follows:
CR
1
s
1
R4.3
(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:
B
S
T
1
IN
2
SW
3
GND
4
EN
5
FB
6
COMP
7
FSET
8
SC4520
R2
R1
R3
C4
C
D2
L1
C5
Vout
B
S
T
IN
2
SW
3
GND
EN FB
COMP
SC452
R2
R1
R3
C4
C
D2
L1
C5
B
S
T
1
IN
2
SW
3
GND
4
EN
5
FB
6
COMP
7
FSET
8
SC4520
R2
R1
R3
C4
C
D2
L1
C5
Vout
B
S
T
IN
2
SW
3
GND
EN FB
COMP
SC452
R2
R1
R3
C4
C
D2
L1
C5
Figure 3. Compensation network provides 2 poles and
1 zero.
The compensation network gives the following
characteristics:
21
2
m
P2
Z
1COMP
RR
R
g
)
ω
s
(1s
ω
s
1
ω(s)G
+
⋅⋅
+⋅
+
⋅=
Where:
54
1
CC
1
ω
+
=
43
Z
CR
1
ω
⋅
=
543
54
P2
CCR
CC
ω
⋅⋅
+
=
The loop gain will be given by:
)
ω
s
(1)
ω
s
(1
ω
s
1
s
1
RR
R
C
R
103.655(s)G(s)GT(s)
P2P1
Z
21
2
4
L
3
VDCOMP
+⋅+
+
⋅
+
⋅⋅⋅=⋅=
−
Where:
CR
1
ω
L
p1
⋅
=
P1-P3P4-P6P7-P9P10-P12

SC4520SETRT

Mfr. #:
Buy SC4520SETRT
Manufacturer:
Semtech
Description:
Switching Voltage Regulators 600KHZ 3A STEP-DOWN SW/REG
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