ADP3163 Analog Devices, ADP3163 Datasheet - Page 8

• Manufacturer Part Number: ADP3163
• Description: 5-Bit Programmable 2-/3-Phase Synchronous Buck Controller
• Manufacturer: Analog Devices

ADP3163
tolerance, e.g., an MLC capacitor with NPO dielectric and with
5% or less tolerance.
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. In a three-phase
converter, a practical value for the peak-to-peak inductor ripple
current is under 50% of the dc current in the same inductor. A
choice of 50% for this particular design example yields a total
peak-to-peak output ripple current of 12% of the total dc output
current. The following equation shows the relationship between
the inductance, oscillator frequency, peak-to-peak ripple current
in an inductor and input and output voltages.
For an 11 A peak-to-peak ripple current, which corresponds to
50% of the 22 A full-load dc current in an inductor, Equation 1
yields an inductance of:
A 600 nH inductor can be used, which gives a calculated ripple
current of 10.9 A at no load. The inductor should not saturate
at the peak current of 27 A, and should be able to handle the
sum of the power dissipation caused by the average current of
22 A in the winding and the core loss.
The output ripple current is smaller than the inductor ripple
current due to the three phases partially canceling. This can be
calculated as follows:
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-M
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 mag-
netic field in the core. The radiated EMI from the distributed
magnetic field may create problems with noise interference in
L
L
I
O
V
12
(
V
(
I
IN
12
3 1 5
O
IN
V
12
V
–
f
V
V
SW
.
– .
600
n V
OUT
1 5
V
600
I
3
kHz
)
L RIPPLE
V
OUT
(
(
12
V
)
nH
V
IN
OUT
V
1 5
.
11
(
–
V
)
L
600
V
3 1 5
IN
A
–
f
kHz
OSC
.
n V
596
V
)
OUT
nH
7 81
)
.
A
®
from
(1)
(2)
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 III gives some examples.
Magnetic Designer Software
Intusoft (http://www.intusoft.com)
Designing Magnetic Components for High-Frequency DC-DC
Converters
McLyman, Kg Magnetics
ISBN 1-883107-00-08
Selecting a Standard Inductor
The companies listed in Table IV can provide design consulta-
tion and deliver power inductors optimized for high power
applications upon request.
Coilcraft
[phone]
http://www.coilcraft.com
Coiltronics
[phone]
http://www.coiltronics.com
Sumida Electric Company
[phone]
http://www.sumida.com
R
The value of R
current. The current comparator of the ADP3163 has a mini-
mum current limit threshold of 143 mV. Note that the 143 mV
value cannot be used for the maximum specified nominal cur-
rent, as headroom is needed for ripple current and tolerances.
The current comparator threshold sets the peak of the inductor
current yielding a maximum output current, I
the peak inductor current value less half of the peak-to-peak induc-
tor ripple current. From this, the maximum value of R
calculated as:
In this case, 5 m was chosen as the closest standard value.
Once R
where current limit is reached, I
the maximum current sense threshold of 173 mV:
R
SENSE
SENSE
I
I
OUT CL
OUT CL
(
(
SENSE
Table IV. Power Inductor Manufacturers
)
)
I
Table III. Magnetics Design References
n
O
V
has been chosen, the output current at the point
n
3
CSCL MIN
SENSE
I
L RIPPLE
173
V
(
(
5
CSCL MAX
R
2
is based on the maximum required output
m
SENSE
mV
)
(
)
)
3 10 9
65
3
OUT(CL)
n I
143
A
2
.
mV
L RIPPLE
10 9
A
(
2
, can be calculated using
.
2
87 5
A
)
.
O
, which equals
5 3
A
.
m
SENSE
is
(4)
(3)