ADP3160 Analog Devices, ADP3160 Datasheet - Page 8

• Manufacturer Part Number: ADP3160
• Description: 5-Bit Programmable 2-Phase Synchronous Buck Controller
• Manufacturer: Analog Devices

ADP3160/ADP3167
3.3nF
C
The ADP3160 and ADP3167 use a fixed-frequency control archi-
tecture. The frequency is set by an external timing capacitor, C
The value of C
the graph in Figure 2.
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 400 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. From Figure 2, for 400 kHz
the required timing capacitor value is 150 pF. For good frequency
stability and initial accuracy, it is recommended to use a capacitor
with a low temperature coefficient and tight 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 2-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 46% for this particular design example yields a total
peak-to-peak output ripple current of 23% 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.
NC = NO CONNECT
C
26.1k
T
OC
Selection—Choosing the Clock Frequency
R
A
V
IN
100pF
11.0k
R
RTN
12V
B
V
C2
IN
FROM
CPU
T
for a given clock frequency can be selected using
R1
1k
C11
C1
150pF
1
2
3
4
5
6
7
8
VID4
VID3
VID2
VID1
VID0
COMP
FB
CT
ADP3160
C12
U1
OS-CON 16V
PWRGD
270 F
PWM2
PWM1
GND
VCC
REF
CS+
CS–
C13
Figure 6. 53.4 A Intel CPU Supply Circuit, VRM 9.0 FMB Design
16
15
14
13
12
11
10
4
9
4.7 F
C25 1nF
C14
C4
10
SW
1 F
R6
C6
, to 200 kHz, which
MBR052LTI
15nF
C23
D2
4.7 F
1
2
3
4
C26
ZMM5236BCT
NC
BST
IN
VCC
ADP3414
T
Z1
U3
.
DRVH
PGND
DRVL
SW
R5
2.4k
–8–
C10
1 F
20
R7
8
7
6
5
For 12.5 A peak-to-peak ripple current, which corresponds to
just under 50% of the 26.7 A full-load dc current in an induc-
tor, Equation 1 yields an inductance of:
A 600 nH inductor can be used, which gives a calculated ripple
current of 12.2 A at no load. The inductor should not saturate
at the peak current of 32.8 A and should be able to handle the
sum of the power dissipation caused by the average current of
26.7 A in the winding and the core loss.
The output ripple current is smaller than the inductor ripple
current due to the two 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-Mm
Magnetics) 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.
Q5
2N3904
1 F
C5
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2
3
4
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600nH
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–
BST
IN
NC
VCC
f
L2
V
V
– .
SW
600
ADP3414
V
4m
400
R4
1 635
AVG
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C15 C16 C17 C18 C19
IN
DRVH
PGND
nH
DRVL
kHz
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600nH
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L1
nH
A
C23
1.1V – 1.85V
53.4A
V
V
CC(CORE)
CC(CORE)
REV. B
®
from
(1)
(2)
RTN