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ADP3208D Datasheet(PDF) 28 Page - ON Semiconductor

Part No. ADP3208D
Description  7-Bit, Programmable, Dual-Phase, Mobile, CPU, Synchronous Buck Controller
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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ADP3208D Datasheet(HTML) 28 Page - ON Semiconductor

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ADP3208D
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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 m
W 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.
Panasonic
Sumida Corporation
NEC Tokin Corporation
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 (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. The
summing is implemented by the CS amplifier that is
configured with resistor RPH(x) (summer) and resistors RCS
and CCS (filters). The output resistance of the regulator is set
by the following equations:
RO +
RCS
RPH(x)
RSENSE
(eq. 8)
CCS +
L
RSENSE RCS
(eq. 9)
where RSENSE is the DCR of the output inductors.
Either RCS or RPH(x) can be chosen for added flexibility.
Due to the current drive ability of the CSCOMP pin, the RCS
resistance should be greater than 100 k
W. For example,
initially select RCS to be equal to 200 kW, and then use
Equation 9 to solve for CCS:
CCS +
330 nH
0.8 mW
200 kW
+ 2.1 nF
(eq. 10)
If CCS is not a standard capacitance, RCS can be tuned. For
example, if the optimal CCS capacitance is 1.5 nF, adjust RCS
to 280 k
W. For best accuracy, CCS should be a 5% NPO
capacitor. In this example, a 220 k
W is used for RCS to
achieve optimal results.
Next, solve for RPH(x) by rearranging Equation 8 as
follows:
(eq. 11)
RPH(X) w
0.8 mW
2.1 mW
@ 220 kW + 83.8 kW
The standard 1% resistor for RPH(x) is 86.6 kW.
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 RCS is designed to have an opposite but equal
percentage of change in resistance, it cancels the
temperature variation of the inductor’s DCR. Due to the
nonlinear nature of NTC thermistors, series resistors RCS1
and RCS2 (see Figure 42) are needed to linearize the NTC and
produce the desired temperature coefficient tracking.
Figure 42. Temperature−Compensation Circuit
Values
ADP3208D
17
19
18
CSCOMP
CSSUM
CSREF
+
-
CCS2
R
R
R
TH
Place as close as possible
to nearest inductor
RR
R
To Switch Nodes
To V
Sense
OUT
Keep This Path As Short
As Possible And Well Away
From Switch Node Lines
CCS1
CS2
CS1
PH2
PH1
PH3
The following procedure and expressions 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 its type and
value. Because the value needed is not yet
determined, start with a thermistor with a value
close to RCS and an NTC with an initial tolerance
of better than 5%.
2. Find the relative resistance value of the NTC at
two temperatures. The appropriate temperatures
will depend on the type of NTC, but 50
°C and
90
°C have been shown to work well for most types
of NTCs. The resistance values are called A (A is
RTH(50°C)/RTH(25°C)) and B (B is
RTH(90°C)/RTH(25°C)). Note that the relative
value of the NTC is always 1 at 25
°C.
3. Find the relative value of RCS required for each of
the two temperatures. The relative value of RCS is
based on the percentage of change needed, which
is initially assumed to be 0.39%/
°C in this
example.
The relative values are called r1 (r1 is 1/(1+ TC ×
(T1 − 25))) and r2 (r2 is 1/(1 + TC × (T2 − 25))),
where TC is 0.0039,
T1 is 50°C, and T2 is 90°C.


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