ADP3208CJCPZ-RL View Datasheet(PDF) - ON Semiconductor

Part Name

Description

Manufacturer

ADP3208CJCPZ-RL

7-Bit, Programmable, Dual-Phase, Mobile, CPU, Synchronous Buck Controller

ON Semiconductor 

ADP3208C

APPLICATION INFORMATION

The design parameters for a typical IMVP-6+-compliant CPU

core VR application are as follows:

• Maximum input voltage (VINMAX) = 19 V

• Minimum input voltage (VINMIN) = 8 V

• Output voltage by VID setting (VVID) = 1.4375 V

• Maximum output current (IO) = 40 A

• Droop resistance (RO) = 2.1 mΩ

• Nominal output voltage at 40 A load (VOFL) = 1.3535 V

• Static output voltage drop from no load to full load

(ΔV) = VONL − VOFL = 1.4375 V − 1.3535 V = 84 mV

• Maximum output current step (ΔIO) = 27.9 A

• Number of phases (n) = 2

• Switching frequency per phase (fSW) = 300 kHz

• Duty cycle at maximum input voltage (DMAX) = 0.18 V

• Duty cycle at minimum input voltage (DMIN) = 0.076 V

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:

RT

=

VVID + 1V

2 × n × fSW × 9 pF

− 16kΩ

(1)

where:

9 pF and 16 kΩ are internal IC component values.

VVID is the VID voltage in volts.

n is the number of phases.

fSW is the switching frequency in hertz for each phase.

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.

RT

=

n×

1V

fSW × 9 pF

−16kΩ

(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 VCORE ripple specification in the IMVP-6 sets the

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:

R RPM

= 2 × RT ×

VVID + 1.0 V

AR × (1 − D) ×VVID

RR × CR × f SW

− 0.5kΩ

(24)

where:

AR is the internal ramp amplifier gain.

CR is the internal ramp capacitor value.

RR is an external resistor on the RAMPADJ pin to set the

internal ramp magnitude.

Because RR = 280 kΩ, the following resistance sets up 300 kHz

switching frequency in RPM operation.

R RPM

= 2 × 280 kΩ × 0.5 × (1 − 0.061) ×1.150

1.150 V + 1.0 V 462 k Ω × 5 pF × 300 kHz

− 500 Ω = 202 kΩ

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

Rev. 1 | Page 31 of 41 | www.onsemi.com

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