SC4518HEVB Просмотр технического описания (PDF) - Semtech Corporation
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SC4518HEVB Datasheet PDF : 15 Pages
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SC4518H
POWER MANAGEMENT
Application Information (Cont.)
Where:
IO = load current;
R = on-equivalent resistance of the switch;
V ON
BOOST
=
input
voltage
or
output
based
on
the
boost
circuit
connection.
The junction temperature of the SC4518H can be
further decided 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.
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:
2 IN
5 EN
8 SYNC
SC4518H
SW 3
FB 6
COMP 7
C5
L1
R1
C
Vout
C4
R2
R3
D2
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
Pdiode = VF ⋅ Io ⋅ (1− D)
Loop Compensation Design
The SC4518H 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
=
0.8 • (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 SC4518H will be as follows:
GVD (s)
=
3
1+
⋅
RL
s
1
RL ⋅ C
Where:
R – Load and
L
C – Output capacitor.
Figure 3. Compensation network provides 2 poles and 1
zero.
The compensation network gives the following
characteristics:
1+ s
GCOMP
(s)
=
ω1
⋅
s
⋅
ωZ
(1+ s
)
⋅
gm
⋅
R2
R1 + R2
ωP2
Where:
1
ω1 = C4 + C5
1
ωZ = R3 ⋅ C4
ωP2
=
C4 + C5
R3 ⋅ C4 ⋅ C5
The loop gain will be given by:
T(s)
=
GCOMP (s) ⋅ GVD (s)
=
2.55 ⋅10−3
⋅
RL
C4
⋅
R2
R1 + R2
⋅
1
s
(1+
1+ s
ωZ
s ) ⋅ (1+
s
)
ωP1
ωP2
2007 Semtech Corp.
10
www.semtech.com
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