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ADP3207C Datasheet(PDF) 23 Page - ON Semiconductor

Part No. ADP3207C
Description  7-Bit Programmable, Multi-Phase Mobile, CPU Synchronous Buck Controller
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

ADP3207C Datasheet(HTML) 23 Page - ON Semiconductor

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ADP3207C
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Application Information
The design parameters for a typical Intel IMVP6−compliant
CPU core VR application are as follows:
Maximum input voltage (VINMAX) = 19 V
Minimum input voltage (VINMIN) = 7.0 V
Output voltage by VID setting (VVID) = 1.150 V
Maximum output current (IO) = 44 A
Load line slope (RO) = 2.1 mW
Maximum output current step (DIO) = 34.5 A
Maximum output thermal current (IOTDC) = 32 A
Number of phases (n) = 2
Switching frequency per phase (fSW) = 280 kHz
Duty cycle at maximum input voltage (DMIN) = 0.061
Duty cycle at minimum input voltage (DMAX) = 0.164
Setting the Clock Frequency for PWM Mode
In PWM mode operation, the ADP3207C uses a
fixed−frequency control architecture. The frequency is set
by an external timing resistor (RT). The clock frequency and
the number of phases determine the switching frequency per
phase, which directly relates to switching losses and the
sizes of the inductors and input and output capacitors. In a
2−phase design, a clock frequency of 560 kHz sets the
switching frequency to 280 kHz per phase. This selection
represents a trade−off between the switching losses and the
minimum sizes of the output filter components. To achieve
a 560 kHz oscillator frequency at VID voltage 1.150 V, RT
has to be 237 k
W. Alternatively, the value for RT can be
calculated using:
RT +
VVID ) 1.0 V
n
fSW 16 pF
* 5kW
(eq. 1)
where 16 pF and 5 k
W are internal IC component values. For
good initial accuracy and frequency stability, it is
recommended to use a 1% resistor.
Current Monitor Output
The IMON pin output a current proportional to the inductor
current.
Inductor Selection
The choice of inductance determines the ripple current in
the inductor. Less inductance leads to more ripple current,
which increases the output ripple voltage and 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 means lower ripple current
and reduced conduction losses but requires larger size
inductors and more output capacitance for the same
peak−to−peak transient deviation. In a multi−phase converter,
the practical peak−to−peak inductor ripple current is less than
50% of the maximum dc current in the same inductor.
Equation 2 shows the relationship between the inductance,
oscillator frequency, and peak−to−peak ripple current.
Equation 3 can be used to determine the minimum inductance
based on a given output ripple voltage.
IR +
VVID
1 * DMIN
fSW L
(eq. 2)
L w
VVID RO (1 * (n DMIN)) (1 * DMIN)
fSW VRIPPLE
(eq. 3)
Solving Equation 3 for a 20 mV peak−to−peak output ripple
voltage yields:
L w
1.150 V
2.1 mW
(1 * (2
0.061))
(1 * 0.061)
280 kHz
20 mV
+ 356 nH
(eq. 4)
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
W.
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 expectations 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 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


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