ADP3178JR AD [Analog Devices], ADP3178JR Datasheet - Page 7

• Manufacturer Part Number: ADP3178JR
• Description: 4-Bit Programmable Synchronous Buck Controllers
• Manufacturer: AD [Analog Devices]

CT Selection for Operating Frequency
The ADP3158 and ADP3178 use a constant off-time architecture
with t
time the high-side N-channel MOSFET switch turns on, the volt-
age across CT is reset to 0 V. During the off-time, CT is charged
by a constant current of 150 A. Once CT reaches 3.0 V, a new
on-time cycle is initiated. The value of the off-time is calculated
using the continuous-mode operating frequency. Assuming a
nominal operating frequency (f
age of 1.7 V, the corresponding off-time is:
The timing capacitor can be calculated from the equation:
The converter only operates at the nominal operating frequency
at the above-specified V
of V
due to the parasitic voltage drops across the power devices. The
actual minimum frequency at V
195 kHz (see Equation 3), where:
f
MIN
R
(estimated value: 14 m )
R
(estimated value: 6 m )
R
(estimated value: 4 m )
R
(estimated value: 3 m )
DS(ON)HSF
DS(ON)LSF
SENSE
L
OUT
t
t
C
OFF
is the resistance of the inductor
OFF
OFF
T
t
, or under heavy load, the operating frequency decreases
OFF
1
determined by an external timing capacitor CT. Each
is the resistance of the sense resistor
t
OFF
V
1
1
is the resistance of the low-side MOSFET
V
is the resistance of the high-side MOSFET
T TH
–
IN
(
V
1 7
V
V
I
–
5
.
OUT
IN
)
CT
IN
V
I
O MAX
V
–
(
I
O MAX
OUT
(
. 3 3
)
200
f
NOM
(
1
)
and at light load. At higher values
R
NOM
s
DS ON HSF
1
3
(
kHz
R
OUT
V
(
150
DS ON HSF
) of 200 kHz at an output volt-
(
)
= 1.7 V is calculated to be
)
3 3
A
.
R
SENSE
150
R
s
SENSE
pF
R
L
R
–
L
R
) –
DS ON LSF
V
(
OUT
)
(1)
(2)
(3)
)
)
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 equation shows
the relationship between the inductance, oscillator frequency,
peak-to-peak ripple current in an inductor and input and
output voltages.
For 4 A peak-to-peak ripple current, which corresponds to
approximately 25% of the 15 A full-load dc current in an inductor,
Equation 4 yields an inductance of
A 1.5 H inductor can be used, which gives a calculated ripple
current of 3.8 A at no load. The inductor should not saturate at
the peak current of 17 A and should be able to handle the sum
of the power dissipation caused by the average current of 15 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-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
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.
L
L
V
1 7
.
OUT
I
L RIPPLE
V
(
4
A
t
3 3
OFF
.
)
s
1 4
.
ADP3158/ADP3178
H
®
from
(4)