SC4601EVB データシートの表示（PDF） - Semtech Corporation
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SC4601EVB Datasheet PDF : 18 Pages
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SC4601
POWER MANAGEMENT
Applications Information - (Cont.)
For a low voltage and high output current application such
as the 3.3V/1.5V@6A case, the conduction loss is often
dominant and selecting low RDS(ON) MOSFETs will notice-
ably improve the efficiency of the converter even though
they give higher switching losses.
The gate charge loss portion of the top/bottom MOSFET’s
total power loss is derived from the SC4601. This gate
charge loss is based on certain operating conditions (fs,
VGATE, and IO).
C1
R1
C2
FS
ISET
VC C
PD R V
SY NC/SLEEPPHASE
COMP
NDRV
VSEN SE
GN D
SC4601
L1
C9
R7
R8
Output
C4
R9
The thermal estimations have to be done for both
MOSFETs to make sure that their junction temperatures
do not exceed their thermal ratings according to their
total power losses PTOTAL, ambient temperature Ta and their
thermal resistances Rθja as follows:
Tj(max)
< Ta
+
PTOTAL
R θja
Loop Compensation Design:
For a DC/DC converter, it is usually required that the
converter has a loop gain of a high cross-over frequency
for fast load response, high DC and low frequency gain
for low steady state error, and enough phase margin for
its operating stability. Often one can not have all these
properties at the same time. The purpose of the loop
compensation is to arrange the poles and zeros of the
compensation network to meet the requirements for a
specific application.
The SC4601 has an internal error amplifier and requires
the compensation network to connect among the COMP
pin and VSENSE pin, GND, and the output as shown in
Figure 3. The compensation network includes C1, C2,
R1, R7, R8 and C9. R9 is used to program the output
voltage according to:
VOUT
=
0.8
⋅
(1
+
R
R
7
9
)
Figure 3. Compensation network provides 3 poles and 2
zeros.
For voltage mode step down applications as shown in
Figure 3, the power stage transfer function is:
1+
s
1
GVD (s)
=
VI
1+
s
RC
L1
R
+
⋅ C4
s2L1C4
Where:
R = load resistance and
R = C ’s ESR.
C
4
The compensation network will have the characteristic
as follows:
1+ s 1+ s
GCOMP (s) =
ωI
s
⋅
1+
ωZ1
s
⋅
⋅1+
ωZ2
s
ωP1
ωP2
Where:
1
ωI = R7 ⋅ (C1 + C2 )
1
ωZ1 = R1 ⋅ C2
1
ωZ2 = (R7 + R8 ) ⋅ C9
ωP1
=
C1 + C2
R1 ⋅ C1 ⋅ C2
2006 Semtech Corp.
11
www.semtech.com
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