ADP3208C ON Semiconductor, ADP3208C Datasheet - Page 31

• Manufacturer Part Number: ADP3208C
• Description: 7-bit,programmable,dual- Phase,mobile,cpu,synchronous Buck Controller
• Manufacturer: ON Semiconductor

APPLICATION INFORMATION
The design parameters for a typical IMVP-6+-compliant CPU
core VR application are as follows:
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SETTING THE CLOCK FREQUENCY FOR PWM
In PWM operation, the ADP3208C 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 relates directly to the
switching losses and the sizes of the inductors and input and
output capacitors. For a dual-phase design, a clock frequency
of 600 kHz sets the switching frequency to 300 kHz per phase.
This selection represents the trade-off between the switching
losses and the minimum sizes of the output filter components.
To achieve a 600 kHz oscillator frequency at a VID voltage of
1.2 V, RT must be 187 kΩ. Alternatively, the value for RT can
be calculated by using the following equation:
where:
9 pF and 16 kΩ are internal IC component values.
V
n is the number of phases.
f
For good initial accuracy and frequency stability, it is
recommended to use a 1% resistor.
When VARFREQ pin is connected to ground, the switching
frequency does not change with VID. The value for RT can be
calculated by using the following equation.
SW
VID
is the switching frequency in hertz for each phase.
Maximum input voltage (V
Minimum input voltage (V
Output voltage by VID setting (V
Maximum output current (I
Droop resistance (R
Nominal output voltage at 40 A load (V
Static output voltage drop from no load to full load
(ΔV) = V
Maximum output current step (ΔI
Number of phases (n) = 2
Switching frequency per phase (f
Duty cycle at maximum input voltage (D
Duty cycle at minimum input voltage (D
is the VID voltage in volts.
R
R
T
T
=
=
2
n
ONL
×
×
n
f
V
− V
SW
×
VID
1
V
f
OFL
×
SW
+
9
O
= 1.4375 V − 1.3535 V = 84 mV
1
pF
×
) = 2.1 mΩ
V
9
pF
−
INMIN
INMAX
16
O
) = 40 A
−
k
) = 8 V
16
Ω
) = 19 V
SW
VID
O
) = 300 kHz
k
) = 27.9 A
) = 1.4375 V
Ω
OFL
MIN
MAX
) = 1.3535 V
) = 0.076 V
) = 0.18 V
Rev. 1 | Page 31 of 41 | www.onsemi.com
(1)
(2)
For good initial accuracy and frequency stability, it is
recommended to use a 1% resistor.
SETTING THE SWITCHING FREQUENCY FOR
RPM OPERATION OF PHASE 1
During the RPM mode operation of Phase 1, the ADP3208C
runs in pseudo constant frequency, given that the load current
is high enough for continuous current mode. While in
discontinuous current mode, the switching frequency is
reduced with the load current in a linear manner. When
considering power conversion efficiency in light load, lower
switching frequency is usually preferred for RPM mode.
However, the V
limitation for lowest switching frequency. Therefore, depending
on the inductor and output capacitors, the switching frequency
in RPM mode can be equal, larger, or smaller than its
counterpart in PWM mode.
A resistor from RPM to GND sets the pseudo constant
frequency as following:
where:
A
C
R
internal ramp magnitude.
Because R
switching frequency in RPM operation.
INDUCTOR SELECTION
The choice of inductance determines the ripple current of the
inductor. Less inductance results in more ripple current, which
increases the output ripple voltage and the 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
results in lower ripple current and reduced conduction losses,
but it requires larger-size inductors and more output capacitance
for the same peak-to-peak transient deviation. For a multiphase
converter, the practical value for peak-to-peak inductor ripple
current is less than 50% of the maximum dc current of that
inductor. Equation 3 shows the relationship between the
inductance, oscillator frequency, and peak-to-peak ripple
R
R
R
is an external resistor on the RAMPADJ pin to set the
is the internal ramp capacitor value.
is the internal ramp amplifier gain.
R
R
RPM
RPM
R
=
= 280 kΩ, the following resistance sets up 300 kHz
=
. 1
150
V
2
CORE
VID
×
2
V
280
×
+
+
ripple specification in the IMVP-6 sets the
R
1
k
1
Ω
0 .
0 .
T
V
V
×
×
462
0
A
5 .
R
R
×
k
×
Ω
1 (
R
1 (
×
−
×
. 0
5
C
−
pF
061
R
D
×
×
)
)
ADP3208C
300
×
×
f
. 1
SW
V
150
VID
kHz
−
−
0
500
5 .
k
Ω
Ω
=
202
(24)
k
Ω