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参数资料
| 型号： | FAN5019MTCX_NL |
| 厂商： | FAIRCHILD SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 4000 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封装： | LEAD FREE, TSSOP-28 |
| 文件页数： | 11/30页 |
| 文件大小： | 781K |
| 代理商： | FAN5019MTCX_NL |
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PRODUCT SPECIFICATION
FAN5019
REV. 1.0.7 1/5/04
19
Where tSS is the desired soft-start time. Assuming an RDLY
of 301k
and a desired a soft-start time of 3ms, CDLY is
35nF. A close standard value for CDLY is 47nF. Once CDLY
has been chosen, RDLY can be calculated for the current limit
latch-off time using:
If the result for RDLY is less than 200k
, then a smaller soft-
start time should be considered by recalculating the equation
for CDLY or a longer latch-off time should be used. In no
case should RDLY be less than 200k
. In this example, a
delay time of 8ms gives RDLY = 334k
. A close
standard 1% value is 301k
.
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 using
smaller-size inductors and, for a specied peak-to-peak
transient deviation, less total output capacitance. Conversely,
a higher inductance means lower ripple current and reduced
conduction losses, but requires larger-size 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 mVp-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 (RO). For our example,
we are using an inductor with a DCR of 1.6 m
.
Designing an Inductor
Once the inductance and DCR are known, the next step is
either to design an inductor or nd 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 specied to keep the accuracy of the system con-
trolled. Using 15% for the inductance and 8% for the DCR
(at room temperature) are reasonable tolerances that most
manufacturers can meet.
The rst 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 types,
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
DLY
V
t
R
V
A
C
×
×
=
2
20
(2)
DLY
DELAY
DLY
C
t
R
×
=
96
.
1
(3)
()
L
f
D
V
I
SW
O
R
×
×
=
1
(4)
()
RIPPLE
SW
O
VID
V
f
D
n
R
V
L
×
×
≥
1
(5)
()
nH
mV
kHz
m
V
L
534
10
228
375
.
0
1
3
.
1
5
.
1
=
×
×
×
≥
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