ADP3208C ON Semiconductor, ADP3208C Datasheet - Page 32

• Manufacturer Part Number: ADP3208C
• Description: 7-bit,programmable,dual- Phase,mobile,cpu,synchronous Buck Controller
• Manufacturer: ON Semiconductor

ADP3208C
current. Equation 4 can be used to determine the minimum
inductance based on a given output ripple voltage.
Solving Equation 4 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.
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:
•
•
•
•
Vishay Dale Electronics, Inc.
[phone]
Panasonic
[phone]
Sumida Electric Company
[phone]
NEC Tokin Corporation
[phone]
I
L
L
R
≥
≥
=
1.
V
V
3 4
VID
VID
75
×
f
×
SW
V
1 (
R
3
−
×
f
×
0 0
O
SW
D
L
2
×
1 .
kHz
MIN
1 (
×
mΩ
V
)
−
×
RIPPLE
(
×
16
n
(
1
×
mV
−
D
2
MIN
×
. 0
))
076
)
=
533
Rev. 1 | Page 32 of 41 | www.onsemi.com
nH
(3)
(4)
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 6 to solve for C
If C
example, if the optimal C
280 kΩ. For best accuracy, C
In this example, a 220 kΩ is used for R
Next, solve for R
The standard 1% resistor for R
Inductor DCR Temperature Correction
If the DCR of the inductor is used as a sense element and
copper wire is the source of the DCR, the temperature changes
associated with the inductor’s winding must be compensated
for. Fortunately, copper has a well-known temperature
coefficient (TC) of 0.39%/°C.
If R
change in resistance, it cancels the temperature variation of the
inductor’s DCR. Due to the nonlinear nature of NTC thermistors,
series resistors R
the NTC and produce the desired temperature coefficient tracking.
PH(x)
CS
CS
C
C
R
R
(summer) and resistors R
is not a standard capacitance, R
is designed to have an opposite but equal percentage of
CS
O
CS
PH
SENSE
CS
=
(
=
=
x
or R
)
R
0
R
≥
R
8 .
PH
is the DCR of the output inductors.
SENSE
CS
0
2
PH(x)
mΩ
(
8 .
1 .
x
CS1
330
)
PH(x)
L
mΩ
mΩ
×
×
and R
can be chosen for added flexibility. Due to
×
R
R
nH
by rearranging Equation 5 as follows:
200
SENSE
CS
×
220
CS2
CS
kΩ
capacitance is 1.5 nF, adjust R
(see Figure 36) are needed to linearize
kΩ
CS
=
PH(x)
should be a 5% NPO capacitor.
CS
2
=
1 .
and C
83
nF
is 86.6 kΩ.
8 .
CS
CS
kΩ
to achieve optimal results.
CS
can be tuned. For
(filters). The output
CS
resistance
CS
to
CS
CS
O
(5)
(6)
).
: