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參數(shù)資料
| 型號(hào)： | ADP3171 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Charger front end protection IC with 30V max Vin and 4.5V LDO output 8-WSON 0 to 125 |
| 中文描述： | 同步降壓控制器，配有雙線性穩(wěn)壓器控制器 |
| 文件頁(yè)數(shù)： | 7/12頁(yè) |
| 文件大?。?/td> | 310K |
| 代理商： | ADP3171 |
REV. 0
ADP3171
–7–
Inductance Selection
The choice of inductance determines the ripple current in the
inductor. Less inductance leads to more ripple current, which
increases the output ripple voltage and the conduction losses in
the MOSFETs but allows using smaller size inductors and, for a
specified peak-to-peak transient deviation, output capacitors
with less total 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. The following equa-
tion shows the relationship between the inductance, oscillator
frequency, peak-to-peak ripple current in an inductor, and input
and output voltages:
L
V
t
I
OUT
OFF
L RIPPLE
(
=
×
)
(4)
For 2.5 A peak-to-peak ripple current, which corresponds to
approximately 50% of the 5 A full-load dc current in an inductor,
Equation 4 yields an inductance of:
L
V
s
A
H
=
×
.
=
1 5
.
3 5
.
2 5
2 1
.
μ
μ
A 1.7
μ
H inductor can be used, which gives a calculated ripple
current of 3 A at no load. The inductor should not saturate at
the peak current of 8 A and should be able to handle the sum of
the power dissipation caused by the average current of 5 A in
the winding and the core loss.
Designing an Inductor
Once the inductance is 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. The first decision in
designing the inductor is to choose the core material. There are
several possibilities for providing low core loss at high frequen-
cies. Two examples are the powder cores (e.g., Kool Mu
from
Magnetics, Inc.) 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.
Two main core types can be used in this application. Open
magnetic loop types such as beads, beads on leads, and rods and
slugs, provide lower cost but do not have a focused magnetic
field in the core. The radiated EMI from the distributed mag-
netic field may create problems with noise interference in the
circuitry surrounding the inductor. Closed-loop types such as
pot cores, PQ, U, and E cores, or toroids, cost more but have
much better EMI/RFI performance. A good compromise between
price and performance are cores with a toroidal shape.
There are many useful references for quickly designing a power
inductor. Table I gives some examples.
Table I. Magnetics Design References
Magnetic Designer Software
Intusoft (www.intusoft.com)
Designing Magnetic Components for High-Frequency DC-
DC Converters; by William T. McLyman, Kg Magnetics
ISBN 1-883107-00-08
Selecting a Standard Inductor
The companies listed in Table II can provide design consultation
and deliver power inductors optimized for high power applications
upon request.
Table II. Power Inductor Manufacturers
Coilcraft
(847) 639-6400
www.coilcraft.com
Coiltronics
(561) 752-5000
www.coiltronics.com
Sumida Electric Company
(510) 668-0660
www.sumida.com
Vishay-Dale
(203) 452-5664
www.vishay.com
R
SENSE
The value of R
SENSE
is based on the required maximum output
current. The current comparator of the ADP3171 has a mini-
mum threshold of 69 mV. Note that this minimum value cannot
be used for the maximum specified nominal current, as head-
room is needed for ripple current and transients.
The current comparator threshold sets the peak of the inductor
current yielding a maximum output current, I
O(MAX)
, which
equals the peak value less half of the peak-to-peak ripple cur-
rent. Solving for R
SENSE
allowing a 20% margin for overhead
and using the minimum current sense threshold of 69 mV yields:
R
V
I
I
mV
3
A
A
2
m
SENSE
CS TH
(
MIN
O MAX
(
RIPPLE
2
=
+
=
+
=
)(
)
)
.
69
5
10 6
(5)
In this case, 7.5 m was chosen to provide ample headroom.
Once R
SENSE
has been chosen, the maximum output current at
the point where current limit is reached, I
OUT(CL)
, can be calcu-
lated using the maximum current sense threshold of 87 mV:
I
V
R
mV
m
I
I
A
2
A
OUT CL
CS TH
(
MAX
SENSE
L RIPPLE
(
2
OUT CL
(
)
)(
)
)
(
)
–
.
–
.
=
=
=
87
7 5
3
10 1
(6)
At output voltages below 450 mV, the current sense threshold is
reduced to 54 mV, and the ripple current is negligible. Therefore,
the worst-case dead short output current is reduced to:
I
V
R
mV
m
A
OUT SC
CS SC
(
SENSE
(
)
)
.
.
=
=
=
54
7 5
7 2
(7)
To safely carry the current under maximum load conditions, the
sense resistor must have a power rating of at least:
P
I
R
A
m
mW
R
O
SENSE
SENSE
=
×
=
×
=
2
2
5
7 5
.
188
(8)
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