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ADP3207 Datasheet(PDF) 20 Page - ON Semiconductor

Part No. ADP3207
Description  CPU Synchronous Buck Controller
Download  29 Pages
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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ADP3207 Datasheet(HTML) 20 Page - ON Semiconductor

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ADP3207
Rev. 1 | Page 20 of 29 | www.onsemi.com
Solving Equation 6 for a 20 mV peak-to-peak output ripple
voltage yields
((
)
) ()
nH
356
mV
20
kHz
280
061
.
0
1
061
.
0
2
1
m
1
.
2
V
150
.
1
=
×
×
×
×
Ω
×
L
If the ripple voltage ends up being less than the initially selected
value, then 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. For this example, choosing a
360 nH inductor is a good starting point, and gives a calculated
ripple current of 10.7 A. The inductor should not saturate at the
peak current of 27.4 A, and should be able to handle the sum of
the power dissipation caused by the average current of 16 A in
the winding and core loss.
Another important factor in the inductor design is the DCR,
which is used to measure phase currents. A large DCR causes
excessive power losses, though too small a value leads to
increased measurement error. This example uses an inductor
with a DCR of 0.89 m
Ω.
Selecting a Standard Inductor
Once the inductance and DCR are known, the next step is to
either design an inductor or select a standard inductor that
comes as close as possible to meeting the overall design goals. It
is also important to have the inductance and DCR tolerance
specified to keep the accuracy of the system controlled; 20%
inductance and 15% DCR (at room temperature) are reasonable
assumptions 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.
http://www.vishay.com
Panasonic
http://www.panasonic.com
Sumida Corporation
http://www.sumida.com
NEC Tokin Corporation
http://www.nec-tokin.com
Output Droop Resistance
The inductor design requires that the regulator output voltage
measured at the CPU pins drops when the output current
increases. The specified voltage drop corresponds to a dc output
resistance (RO).
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. This summer-filter
is implemented by the CS amplifier that is configured with
resistors RPH(X) (summer), and RCS and CCS (filter). The output
resistance of the regulator is set by the following equations,
where
RL is the DCR of the output inductors:
L
X
PH
CS
O
R
R
R
R
×
=
)
(
(7)
CS
L
CS
R
R
L
C
×
=
(8)
Users have the flexibility of choosing either
RCS or RPH(X). Due to
the current drive ability of the CSCOMP pin, the
RCS resistance
should be larger than 100 k
Ω. For example, users should
initially select
RCS to be equal to 220 kΩ, then use Equation 8 to
solve for
CCS
nF
84
.
1
k
220
m
89
.
0
nH
360
=
Ω
×
Ω
=
CS
C
Because
CCS is not the standard capacitance, it is implemented
with two standard capacitors in parallel: 1.8 nF and 47 pF. For
the best accuracy,
CCS should be a 5% NPO capacitor.
Next, solve
RPH(X) by rearranging Equation 7.
Ω
=
Ω
×
Ω
Ω
k
2
.
93
k
220
m
1
.
2
m
89
.
0
)
(
X
PH
R
The standard 1% resistor for
RPH(X) is 93.1 kΩ.
Inductor DCR Temperature Correction
With the inductor DCR used as a sense element, and copper
wire being the source of the DCR, users need to compensate for
temperature changes in the inductor’s winding. Fortunately,
copper has a well-known temperature coefficient (TC) of
0.39%/°C.
If RCS is designed to have an opposite sign but equal percentage
change in resistance, then it cancels the temperature variation of
the inductor DCR. Due to the nonlinear nature of NTC
thermistors, series resistors, RCS1 and RCS2 (see Figure 11) are
needed to linearize the NTC and produce the desired
temperature coefficient tracking.
PLACE AS CLOSE AS POSSIBLE
TO NEAREST INDUCTOR
OR LOW–SIDE MOSFET
KEEP THIS PATH
AS SHORT AS POSSIBLE
AND WELL AWAY FROM
SWITCH NODE LINES
TO
SWITCH
NODES
TO
VOUT
SENSE
CSREF
CSSUM
CSCOMP
ADP3207
18
17
16
RPH1
RTH
RCS1
CCS
RCS2
RPH2
RPH3
Figure 11. Temperature Compensation Circuit Values


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