REV. 0

ADP3204

–13–

Power Switching Circuitry

ADP3415, MOSFETs, and Input Capacitors

3.

Locate the ADP3415 near the MOSFETs so that the loop

inductance in the path of the top gate drive returned to the

SW pin is small, and similarly for the bottom gate drive

whose return path is the ground plane. The GND pin

should have at least one very close via into the ground plane.

4.

Locate the input bypass MLC capacitors close to the MOSFETs

so that the physical area of the loop enclosed in the electrical

path through the bypass capacitor and around through the

top and bottom MOSFETs (drain-source) is small and wide.

This is the switching power path loop.

5.

Make provisions for thermal management of all the MOSFETs.

Heavy copper and wide traces to ground and power planes

will help to pull the heat out. Heat sinking by a metal tap

soldered in the power plane near the MOSFETs will help.

Even just a small airflow can help tremendously. Paralleled

MOSFETs to achieve a given resistance will help spread the heat.

6.

An external Schottky diode (across the bottom MOSFET)

may increase efficiency by a small amount (< ~1%), depend-

ing on its forward voltage drop compared to the MOSFET’s

body diode at a given current; a MOSFET with a built-in

Schottky is more effective. For an external Schottky, it should

be placed next to the bottom MOSFET or it may not be

effective at all.

7.

The VCC bypass capacitor should be close to the VCC pin

and connected on either a very short trace to the GND pin

or to the GND plane.

Output Filter

Output Inductor and Capacitors, Current Sense Resistor

8.

Locate the current sense resistors very near to the output

voltage plane.

9.

The load-side heads of sense resistors should join as closely

as possible for accurate current signal measurement of

each phase.

10.

PCB trace resistances from the current sense resistors to

the regulation point should be minimized, known (calcu-

lated or measured), and compensated for as part of the

design if it is significant. (Remote sensing is not sufficient

for relieving this requirement.) A square section of 1-ounce

copper trace has a resistance of ~0.5 m

Ω, which adds to

the specified DC output resistance of the power converter.

The output capacitors should similarly be close to the

regulation point and well tied into power planes as imped-

ance here will add to the “AC output resistance” (i.e., the

ESR) that is implicitly specified as well.

11.

Whenever high currents must be routed between PCB

layers, vias should be used liberally to create parallel current

paths so that the resistance and inductance are minimized

and the via current rating is not exceeded.

Control Circuitry

ADP3204, Control Components

12.

If the ADP3204 cannot be placed as previously recom-

mended, care should be taken to keep the device and

surrounding components away from radiation sources

(e.g., from power inductors) and capacitive coupling from

noisy power nodes.

13.

Noise immunity can be improved by the use of a devoted

signal ground plane for the power controller and its

surrounding components. Space for a ground plane might

readily be available on a signal plane of the PCB since it is

often unused in the vicinity of the power controller.

14.

If critical signal lines (i.e., signals from the current sense

resistor leading back to the ADP3204) must cross through

power circuitry, it is best if a signal ground plane can be

interposed between those signal lines and the traces of the

power circuitry. This serves as a shield to minimize noise

injection into the signals.

15.

Absolutely avoid crossing any signal lines over the switching

power path loop, described previously.

16.

Accurate voltage positioning depends on accurate current

sensing, so the control signals that monitor the voltage

differentially across the current sense resistor should be

Kelvin-connected. Please refer to ADI Evaluation Board of

the ADP3204 and its documentation for control signal

connection with sense resistors.

17.

The RC filter used for the current sense signal should be

located near the control components as this serves the

dual purpose of filtering out the effect of the current sense

resistors’ parasitic inductance and the noise picked up along

the routing of the signal. The former purpose is achieved by

having the time constant of the RC filters approximately

matched to that of the sense resistors, and is important

for maintaining the accuracy of the current signal.

APPLICATION INFORMATION

Theoretical Background

This application section presents the theoretical background for

multiphase dc-to-dc converters using the ADP320x family of

controllers for mobile CPUs. Members of that family control

multiphase ripple regulators (also called hysteretic regulators) in

a configuration that allows employing ADOPT, Analog Devices’

optimal voltage positioning technique to implement the desired

output voltage and load line, both statically and dynamically, as

required by Intel’s IMVP-II and IMVP-III specifications.

Single-Phase Hysteretic Regulator with ADOPT

Figure 2 shows the conventional single-phase hysteretic

regulator and the characteristic waveforms. The operation is as

follows. During the time the upper transistor, Q1, is turned on,

the inductor current, I

increase. When V

hysteretic comparator, Q1 is turned off, Q2 is turned on, and

the inductor current and the output voltage decrease. The cycle

repeats after V

comparator.