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參數資料
| 型號： | FAN5018B |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | 6-Bit VID Controller 2-4 Phase VR10.X Controller |
| 中文描述： | 6期西元控制器2-4位的VR10.X控制器 |
| 文件頁數： | 19/30頁 |
| 文件大小： | 509K |
| 代理商： | FAN5018B |
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PRODUCT SPECIFICATION
FAN501
8
B
REV. 1.0.0 Jul/15/05
19
where t
SS
is the desired soft-start time. Assuming an R
DLY
of
301k
Ω
and a desired a soft-start time of 3ms, C
DLY
is 35nF.
A close standard value for C
DLY
is 47nF. Once C
DLY
has
been chosen, R
DLY
can be calculated for the current limit
latch-off time using:
If the result for R
DLY
is less than 200k
Ω
, then a smaller soft-
start time should be considered by recalculating the equation
for C
DLY
or a longer latch-off time should be used. R
DLY
should never be less than 200k
Ω
. In this example, a delay
time of 8ms gives R
DLY
= 334k
Ω
. A close
standard 1% value is 301k
Ω
.
Inductor Selection
The choice of inductance value for the inductor determines
the ripple current in the inductor. Less inductance leads to
more ripple current, which increases the output ripple volt-
age and conduction losses in the MOSFETs, but allows the
use of smaller-size inductors and, for a specified peak-to-
peak transient deviation, less total output capacitance. Con-
versely, a higher inductance means lower ripple current and
reduced conduction losses, but requires larger inductors and
more output capacitance for the same peak-to-peak transient
deviation. In any multi-phase converter, a practical value for
the peak-to-peak inductor ripple current is less than 50% of
the maximum DC current in the same inductor. Equation 4
shows the relationship between the inductance, oscillator
frequency, and peak-to-peak ripple current in the inductor.
Equation 5 can be used to determine the minimum induc-
tance based on a given output ripple voltage:
Solving Equation 5 for a 10 mV
p-p
output ripple voltage
yields:
If the ripple voltage ends up less than that designed for, the
inductor can be made smaller until the ripple value is met.
This will allow optimal transient response and minimum
output decoupling.
The smallest possible inductor should be used to minimize
the number of output capacitors. Choosing a 650nH inductor
is a good choice for a starting point and gives a calculated
ripple current of 8.86A. The inductor should not saturate at
the peak current of 26.1A and should be able to handle the
sum of the power dissipation caused by the average current
of 21.7A in the winding and core loss.
Another important factor in the inductor design is the DC
Resistance (DCR), which is used for measuring the phase
currents. A large DCR will cause excessive power losses,
while too small a value will lead to increased measurement
error. A good rule of thumb is to have the DCR be about
1 to 1 1/2 times the droop resistance (R
O
). For our example,
we are using an inductor with a DCR of 1.6 m
Ω
.
Designing an Inductor
Once the inductance and Direct-Current resistance (DCR)
are known, the next step is either to design an inductor or
find a standard inductor that comes as close as possible to
meeting the overall design goals. It is also important to have
the inductance and DCR tolerance specified to keep the
accuracy of the system controlled. Using 15% for the induc-
tance and 8% for the DCR (at room temperature) are reason-
able tolerances that most manufacturers can meet.
The first decision in designing the inductor is to choose the
core material. There are several possibilities for providing
low core loss at high frequencies. Two examples are the
powder cores (e.g., Kool-Mm
from Magnetics, Inc. or
Micrometals) and the gapped soft ferrite cores (e.g., 3F3
or 3F4 from Philips). Low-frequency powdered iron cores
should be avoided due to their high core loss, especially
when the inductor value is relatively low and the ripple
current is high.
The best choice for a core geometry is a closed-loop type,
such as pot cores, PQ, U, and E cores, or toroids. A good
compromise between price and performance are cores with a
toroidal shape.
There are many useful references for quickly designing a
power inductor, such as:
Magnetics Design References
1.
Magnetic Designer Software: Intusoft
(www.intusoft.com)
2.
Designing Magnetic Components for High-Frequency
DC-DC Converters,
by William T. McLyman,
Kg Magnetics, Inc. ISBN 1883107008
VID
SS
DLY
VID
R
DLY
V
t
V
×
A
C
×
=
2
20
μ
(2)
DLY
DELAY
DLY
C
t
R
×
=
96
.
(3)
(
)
L
f
D
V
I
SW
O
R
×
×
=
1
(4)
(
)
(
V
)
RIPPLE
SW
O
VID
f
D
n
R
V
L
×
×
×
×
≥
1
(5)
(
)
nH
mV
kHz
m
V
L
534
10
228
375
.
1
3
×
5
=
×
×
Ω
≥
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相關代理商/技術參數 |
參數描述 |
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