HIP6301

FN4765 Rev 6.00

Page 15 of 17

December 27, 2004

connections with vias to this layer. Dedicate another solid layer as

a power plane and break this plane into smaller islands of

common voltage levels. Keep the metal runs from the PHASE

the input power and output power nodes. Use copper filled

polygons on the top and bottom circuit layers for the phase nodes.

Use the remaining printed circuit layers for small signal wiring.

The wiring traces from the driver IC to the MOSFET gate and

source should be sized to carry at least one ampere of current.

Component Selection Guidelines

Output Capacitor Selection

The output capacitor is selected to meet both the dynamic

load requirements and the voltage ripple requirements. The

load transient for the microprocessor CORE is characterized

by high slew rate (di/dt) current demands. In general,

multiple high quality capacitors of different size and dielectric

are paralleled to meet the design constraints.

Modern microprocessors produce severe transient load rates.

High frequency capacitors supply the initially transient current

and slow the load rate-of-change seen by the bulk capacitors.

The bulk filter capacitor values are generally determined by

the ESR (effective series resistance) and voltage rating

requirements rather than actual capacitance requirements.

High frequency decoupling capacitors should be placed as

close to the power pins of the load as physically possible. Be

careful not to add inductance in the circuit board wiring that

could cancel the usefulness of these low inductance

components. Consult with the manufacturer of the load on

specific decoupling requirements.

Use only specialized low-ESR capacitors intended for

switching-regulator applications for the bulk capacitors. The

bulk capacitor’s ESR determines the output ripple voltage

and the initial voltage drop following a high slew-rate

transient’s edge. In most cases, multiple capacitors of small

case size perform better than a single large case capacitor.

Bulk capacitor choices include aluminum electrolytic, OS-

Con, Tantalum and even ceramic dielectrics. An aluminum

electrolytic capacitor’s ESR value is related to the case size

with lower ESR available in larger case sizes. However, the

equivalent series inductance (ESL) of these capacitors

increases with case size and can reduce the usefulness of

the capacitor to high slew-rate transient loading.

Unfortunately, ESL is not a specified parameter. Consult the

capacitor manufacturer and measure the capacitor’s

impedance with frequency to select a suitable component.

Output Inductor Selection

One of the parameters limiting the converter’s response to a

load transient is the time required to change the inductor

current. Small inductors in a multi-phase converter reduces

the response time without significant increases in total ripple

current.

The output inductor of each power channel controls the

ripple current. The control IC is stable for channel ripple

current (peak-to-peak) up to twice the average current. A

single channel’s ripple current is approximately:

The current from multiple channels tend to cancel each other

and reduce the total ripple current. Figure 12 gives the total

ripple current as a function of duty cycle, normalized to the

parameter

at zero duty cycle. To determine

the total ripple current from the number of channels and the

duty cycle, multiply the y-axis value by

.

I

–



--------------------------------

----------------



=

+12V

VIA CONNECTION TO GROUND PLANE

ISLAND ON POWER PLANE LAYER

ISLAND ON CIRCUIT PLANE LAYER

KEY

PHASE

VCC

USE INDIVIDUAL METAL RUNS

COMP

HIP6301

PWM

FB

VSEN

ISEN

HIP6601B

FS/DIS

PVCC

LOCATE NEXT TO IC PIN

LOCATE NEXT

TO FB PIN

LOCATE NEXT TO IC PIN(S)

ISOLATE OUTPUT STAGES

FOR EACH CHANNEL TO HELP

LOCATE NEAR TRANSISTOR

FIGURE 11. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS

Vo





Vo



