ADP3208 ON Semiconductor, ADP3208 Datasheet - Page 28

• Manufacturer Part Number: ADP3208
• Description: 7-bit, Programmable, Dual-phase, Mobile, Cpu, Synchronous Buck Controller
• Manufacturer: ON Semiconductor

ADP3208
PWRGD DELAY TIMER
The PWRGD delay, t
period between the CLKEN assertion and the PWRGD
assertion. It is programmed by a capacitor connected to the
PGDELAY pin and calculated as follows:
IMVP-6+ specifies that the PWRGD delay is between 3 ms and
20 ms. Assuming a 5 ms PWRGD delay, C
INDUCTOR SELECTION
The choice of inductance determines the ripple current of the
inductor. Less inductance results in more ripple current, which
increases the output ripple voltage and the conduction losses in the
MOSFETs. However, this allows the use of smaller-size inductors,
and for a specified peak-to-peak transient deviation, it allows
less total output capacitance. Conversely, a higher inductance
results in lower ripple current and reduced conduction losses,
but it requires larger-size inductors and more output capacitance
for the same peak-to-peak transient deviation. For a multiphase
converter, the practical value for peak-to-peak inductor ripple
current is less than 50% of the maximum dc current of that
inductor. Equation 6 shows the relationship between the
inductance, oscillator frequency, and peak-to-peak ripple
current. Equation 7 can be used to determine the minimum
inductance based on a given output ripple voltage.
Solving Equation 7 for a 16 mV peak-to-peak output ripple
voltage yields
If the resultant ripple voltage is less than the initially selected
value, the inductor can be changed to a smaller value until the
ripple value is met. This iteration allows optimal transient
response and minimum output decoupling.
The smallest possible inductor should be used to minimize the
number of output capacitors. Choosing a 490 nH inductor is a
good choice for a starting point, and it provides a calculated
ripple current of 9.0 A. The inductor should not saturate at the
peak current of 24.5 A, and it should be able to handle the sum
of the power dissipation caused by the winding’s average current
(20 A) plus the ac core loss. In this example, 330 nH is used.
C
I
L
L
R
PGDELAY
≥
≥
=
V
1.
V
VID
3 4
VID
75
×
=
×
f
R
V
SW
1 (
2
O
×
μA
−
×
×
2
CPU_PWRGD
D
1 (
1 .
L
3
×
f
MIN
−
mΩ
0 0
SW
2
t
(
9 .
CPU
n
kHz
)
×
×
V
×
V
D
_ PWRGD
(
, is defined in IMVP-6+ as the
RIPPLE
1
MIN
×
−
16
2
))
×
mV
. 0
×
1 (
076
−
)
D
PGDELAY
1 (
MIN
−
. 0
)
076
is 4.7 nF.
)
Rev. 1 | Page 28 of 38 | www.onsemi.com
=
493
nH
(5)
(6)
(7)
Another important factor in the inductor design is the DCR,
which is used for measuring the phase currents. Too large of a
DCR causes excessive power losses, whereas too small of a value
leads to increased measurement error. For this example, an
inductor with a DCR of 0.8 mΩ is used.
Selecting a Standard Inductor
After the inductance and DCR are known, select a standard
inductor that best meets the overall design goals. It is also
important to specify the inductance and DCR tolerance to
maintain the accuracy of the system. Using 20% tolerance for
the inductance and 15% for the DCR at room temperature are
reasonable values that most manufacturers can meet.
Power Inductor Manufacturers
The following companies provide surface-mount power inductors
optimized for high power applications upon request:
•
•
•
•
Output Droop Resistance
The design requires that the regulator output voltage measured
at the CPU pins decreases when the output current increases. The
specified voltage drop corresponds to the droop resistance (R
The output current is measured by summing the currents of the
resistors monitoring the voltage across each inductor and by
passing the signal through a low-pass filter. The summing is
implemented by the CS amplifier that is configured with resistor
R
resistance of the regulator is set by the following equations:
where R
Either R
the current drive ability of the CSCOMP pin, the R
should be greater than 100 kΩ. For example, initially select R
to be equal to 200 kΩ, and then use Equation 9 to solve for C
PH(x)
Vishay Dale Electronics, Inc.
[phone]
Panasonic
[phone]
Sumida Electric Company
[phone]
NEC Tokin Corporation
[phone]
C
C
R
(summer) and resistors R
CS
O
CS
SENSE
CS
=
=
=
or R
R
0
R
R
8 .
PH
is the DCR of the output inductors.
SENSE
CS
PH(x)
(
mΩ
x
330
)
L
×
×
can be chosen for added flexibility. Due to
×
R
R
nH
200
SENSE
CS
kΩ
=
2
CS
1 .
and C
nF
CS
(filters). The output
CS
resistance
CS
CS
O
(8)
(9)
).
: