SC4520 データシートの表示（PDF） - Semtech Corporation
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SC4520 Datasheet PDF : 12 Pages
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SC4520
POWER MANAGEMENT
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:
Ptotal
=
Io2
⋅ Ron
⋅D
+ 10.8 ⋅10−3
⋅ Io
⋅ VI
+
10
1000
⋅ Io
⋅ D ⋅ (Vboost )
Where:
IO = load current;
Ron = on-equivalent resistance of the switch;
VBOOST = input voltage or output based on the boost circuit
connection.
The junction temperature of the SC4520 can be
further determined by:
TJ = TA + θJA ⋅ Ptotal
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.
Where:
RL – 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:
1
2
IN
5
EN
8
FSET
4
SC 4520
3
SW
6
FB
7
COMP
C5
L1
R1
C
Vout
C4
R2
R3
D2
Figure 3. Compensation network provides 2 poles and
1 zero.
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 (VF).
Pdiode = VF ⋅Io ⋅ (1− D)
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:
VO
= 1.2 • (1+
R1
R2
)
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:
G VD
(s)
=
4.3
1+
⋅ RL
s
1
RL ⋅ C
The compensation network gives the following
characteristics:
Where:
GCOMP
(s)
=
ω1
⋅
s
⋅
1+
s
ωZ
(1+
s
ωP2
)
⋅
gm
⋅
R2
R1 + R2
ω1
=
C4
1
+ C5
ωZ
=
1
R3 ⋅ C4
ωP2
=
C4 + C5
R3 ⋅ C4 ⋅ C5
The loop gain will be given by:
T(s)
=
GCOMP(s) ⋅ GVD (s)
=
3.655
⋅ 10 −3
⋅
RL
C4
⋅
R2
R1 + R2
⋅
1
s
(1+
1+
s
ωZ
s
ωP1
)
⋅
(1
+
s
ωP2
)
Where:
ωp1
=
1
RL ⋅C
2006 Semtech Corp.
9
www.semtech.com
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