ADP3210 Datasheet, PDF (25/38 Pages) ON Semiconductor – 7-Bit Programmable Multiphase Mobile CPU Synchronous

ADP3210
APPLICATION INFORMATION
The design parameters for a typical Intel IMVP6.5-compliant
CPU Core VR application are as follows:
• Maximum input voltage (VINMAX) = 19 V
• Minimum input voltage (VINMIN) = 7 V
• Output voltage by VID setting (VVID) = 1.150 V
• Maximum output current (IO) = 55 A
• Load line slope (RO) = 2.1 mΩ
• Maximum output current step (ΔIO) = 34.5 A
• Maximum output thermal current (IOTDC) = 32 A
• Number of phases (n) = 3
• 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 ADP3210 uses a fixed-frequency
control architecture. The frequency is set by an external timing
resistor (R_T). 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, R_T has to be 196 kΩ. Alternatively, the value for
R_T can be calculated using
R_T
=
n
V_VID +1.0 V _{−16 kΩ}
× 2 × f_SW × 9 pF
(1)
where 9 pF and 16 kΩ are internal IC component values. For
good initial accuracy and frequency stability, it is recommended
to use a 1% resistor.
(3)
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 multiphase
converter, the practical peak-to-peak inductor ripple current is
less than 50% of the maximum dc current in the same inductor.
Equation 5 shows the relationship between the inductance,
oscillator frequency, and peak-to-peak ripple current. Equation
6 can be used to determine the minimum inductance based on
a given output ripple voltage.
) _IR
=
V_VID
× (1 − D
f_SW × L
MIN
(5)
L ≥ ^{VVID × RO × (1 − (n × D MIN ))× (1 − D MIN )}
(6)
f _SW ×V_RIPPLE
Solving Equation 6 for a 20 mV peak-to-peak output ripple
voltage yields
1.150 V × 2.1mΩ × (1 − (2 × 0.061))× (1 − 0.061)
L≥
= 356 nH
280 kHz × 20 mV
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 mΩ.
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%
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