ADP3164 Analog Devices, ADP3164 Datasheet - Page 9

• Manufacturer Part Number: ADP3164
• Description: 5-Bit Programmable 4-Phase Synchronous Buck Controller
• Manufacturer: Analog Devices

APPLICATION INFORMATION
The design parameters for a typical VRM 9.1-compliant CPU
application are as follows:
CT Selection—Choosing the Clock Frequency
The ADP3164 uses a fixed-frequency control architecture. The
frequency is set by an external timing capacitor, CT. The clock
frequency determines the switching frequency, which relates
directly to switching losses and the sizes of the inductors and
input and output capacitors. A clock frequency of 800 kHz sets
the switching frequency of each phase, f
represents a practical trade-off between the switching losses and
the sizes of the output filter components. To achieve an 800 kHz
oscillator frequency, the required timing capacitor value is 100 pF.
For good frequency stability and initial accuracy, it is recom-
mended to use a capacitor with low temperature coefficient
and tight tolerance, e.g., an MLC capacitor with NPO dielec-
tric 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 4-phase con-
verter, 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 8% 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 10 A peak-to-peak ripple current, which is 50% of the
20 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 10.8 A at no load. The inductor should not saturate
at the peak current of 26 A, and should be able to handle the
Input voltage (V
VID setting voltage (V
Nominal output voltage at no load (V
Nominal output voltage at 80 A load (V
Static output voltage drop based on a 0.95 mΩ load line
(R
1.4605 V – 1.3845 V = 76 mV
Maximum Output Current (I
Number of Phases (n) = 4
OUT
L
L
=
=
) from no load to full load (V
(
V
(
12
V
12
IN
IN
V
×
V
–
f
– .
SW
V
×
1 475
OUT
IN
800
×
) = 12 V
I
)
L RIPPLE
4
kHz
(
×
V
VID
V
)
OUT
×
) = 1.475 V
1 475
×
)
.
10
O
) = 81 A
A
V
∆
=
) = V
646
ONL
SW
OFL
, to 200 kHz, which
) = 1.4605 V
ONL
nH
) = 1.3845 V
– V
OFL
=
(1)
sum of the power dissipation caused by the average current of
20 A in the winding and the core loss.
The output ripple current is smaller than the inductor ripple
current due to the four 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 frequencies.
Two examples are the powder cores (e.g., Kool-Mµ
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 induc-
tor 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
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 II 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 III 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
I
O
∆
=
Table III. Power Inductor Manufacturers
4 1 475
Table II. Magnetics Design References
×
I
O
12
∆
.
=
V
n V
×
V
×
600
×
OUT
(
12
V
nH
IN
V
×
×
×
(
–
V
L
800
4 1 475
IN
×
×
–
f
kHz
OSC
n V
.
×
V
OUT
)
ADP3164
=
)
6 25
.
A
®
from
(2)