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ADP3207 Datasheet(PDF) 21 Page - ON Semiconductor

Part No. ADP3207
Description  CPU Synchronous Buck Controller
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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ADP3207 Datasheet(HTML) 21 Page - ON Semiconductor

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ADP3207
Rev. 1 | Page 21 of 29 | www.onsemi.com
The following procedure and equations yield values for RCS1, RCS2,
and RTH (the thermistor value at 25°C) for a given RCS value:
1.
Select an NTC to be used based on type and value. Because
there is no value yet, start with a thermistor with a value
close to RCS. The NTC should also have an initial tolerance
of better than 5%.
2.
Based on the type of NTC, find its relative resistance value
at two temperatures. Temperatures that work well are 50°C
and 90°C. These are called Resistance Value A (A is
RTH(50°C)/RTH(25°C)) and Resistance Value B (B is
RTH(90°C)/RTH(25°C)). Note that the relative value of NTC
is always 1 at 25°C.
3.
Next, find the relative value of RCS that is required for each
of these temperatures. This is based on the percentage of
change needed, which is initially 0.39%/°C. These are
called r1 and r2.
()
()
25
1
1
25
1
1
2
2
1
1
×
+
=
×
+
=
T
TC
r
T
TC
r
(9)
where:
TC = 0.0039
T1 = 50°C
T2 = 90°C.
4.
Compute the relative values for
rCS1, rCS2, and rTH using
()
()
()
()
()
(
)
()
1
2
2
1
2
1
2
1
1
2
2
1
2
1
1
1
1
1
1
1
1
1
1
1
CS
CS
TH
CS
CS
CS
CS
r
r
r
r
r
A
r
A
r
B
A
r
A
B
r
B
A
r
A
B
r
B
A
r
r
B
A
r
=
=
×
×
×
×
×
×
+
×
×
×
×
=
(10)
5.
Calculate
RTH = RTH × RCS, then select the closest value of
thermistor that is available. Also, compute a scaling factor
k based on the ratio of the actual thermistor value relative
to the computed one
)
(
)
(
CALCULATED
TH
ACTUAL
TH
R
R
k =
(11)
6.
Finally, calculate values for
RCS1 and RCS2 using
()
()
()
2
2
1
1
1
CS
CS
CS
CS
CS
CS
r
k
k
R
R
r
k
R
R
×
+
×
=
×
×
=
(12)
This example starts with a thermistor value of 100 k
Ω and uses
a Vishay NTHS0603N04 NTC thermistor (a 0603 size thermistor)
with A = 0.3359 and B = 0.0771. From this data, rCS1 = 0.359,
rCS2 = 0.729 and rTH = 1.094. Solving for RTH yields 240 kΩ, so
220 kΩ is chosen, making k = 0.914. Finally, RCS1 and RCS2 are
72.3 kΩ and 166 kΩ. Choosing the closest 1% resistor values
yields a choice of 71.5 kΩ and 165 kΩ.
COUT SELECTION
The required output decoupling for processors and platforms is
typically recommended by Intel. The following guidelines can
also be used if both bulk and ceramic capacitors in the system:
Select the total amount of ceramic capacitance. This is based
on the number and type of capacitors to be used. The best
location for ceramics is inside the socket; 20 pieces of
Size 0805 being the physical limit. Additional capacitors
can be placed along the outer edge of the socket.
Select the number of ceramics and find the total ceramic
capacitance (CZ). Combined ceramic values of 200 μF to
300 μF are recommended and are usually made up of
multiple 10 μF or 22 μF capacitors.
Note that there is an upper limit imposed on the total
amount of bulk capacitance (CX) when considering the
VID on-the-fly output voltage stepping (voltage step VV in
time tV with error of VERR), and also a lower limit based on
meeting the critical capacitance for load release at a given
maximum load step ΔIO. For a step-off load current, the
current version of the IMVP-6 specification allows a
maximum VCORE overshoot (VOSMAX) of 10 mV, plus 1.5% of
the VID voltage. For example, if the VID is 1.150 V, then
the largest overshoot allowed is 27 mV.
()
×
⎟⎟
⎜⎜
Δ
+
×
Δ
×
z
VID
O
OSMAX
O
O
MIN
x
C
V
I
V
R
n
I
L
C
(13)
z
O
V
VID
v
VID
V
2
O
2
MAX
X
C
L
nKR
V
V
t
V
V
R
nK
L
C
⎟⎟
⎜⎜
×
+
×
×
1
1
2
)
(
(14)
where:
=
V
ERR
V
V
n
K
1
(15)
To meet the conditions of these equations and transient
response, the ESR of the bulk capacitor bank (RX) should be less
than two times the droop resistance, RO. If the CX(MIN) is larger
than CX(MAX), the system does not meet the VID on-the-fly
and/or deeper sleep exit specification and can require a smaller
inductor or more phases (the switching frequency can also have
to be increased to keep the output ripple the same).


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