9

FN9038.4

March 9, 2006

by placing them nearby. Position both the ceramic and bulk

input capacitors as close to the upper MOSFET drain as

possible. Position the output inductor and output capacitors

between the upper MOSFET and lower diode and the load.

The critical small signal components include any bypass

capacitors, feedback components, and compensation

the VCC pin with a via directly to the ground plane. Place the

PWM converter compensation components close to the FB

and COMP pins. The feedback resistors for both regulators

should also be located as close as possible to the relevant

FB pin with vias tied straight to the ground plane as required.

Component Selection Guidelines

Output Capacitor Selection

Output capacitors are required to filter the output and supply

the load transient current. The filtering requirements are a

function of switching frequency and output current ripple.

The load transient requirements are a function of the

transient load current slew rate (di/dt) and magnitude. These

requirements are generally met with a mix of capacitors and

careful layout.

PWM REGULATOR OUTPUT CAPACITORS

Modern digital ICs can produce high transient load slew

rates. High frequency capacitors initially supply the transient

current and slow the load rate-of-change seen by the bulk

capacitors. The bulk filter capacitor selection is generally

determined by the effective series resistance (ESR) 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.

Specialized low-ESR capacitors intended for switching-

regulator applications are recommended 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 edge. Aluminum electrolytic, tantalum,

and special polymer capacitor ESR values are 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.

Work with your capacitor supplier and measure the

capacitor’s impedance with frequency to select a suitable

component. In most cases, multiple electrolytic capacitors of

small case size perform better than a single large case

capacitor.

LINEAR REGULATOR OUTPUT CAPACITORS

The output capacitors for the linear regulator provide

dynamic load current. The linear controller uses dominant

pole compensation integrated into the error amplifier and is

relatively insensitive to output capacitor selection. Output

capacitors should be selected for transient load regulation.

PWM Output Inductor Selection

The PWM converter requires an output inductor. The output

inductor is selected to meet the output voltage ripple

requirements and sets the converter response time to a load

transient. The inductor value determines the converter’s

ripple current and the ripple voltage is also a function of the

ripple current. The ripple voltage and current are

approximated by the following equations:

Increasing the value of inductance reduces the output ripple

current and voltage ripple. However, increasing the

+5 VCC

ISLAND ON POWER PLANE LAYER

ISLAND ON CIRCUIT PLANE LAYER

KEY

COMP

ISL6528

UGATE

R4

FB

DRIVE2

GND

VCC

D1

FIGURE 7. PRINTED CIRCUIT BOARD POWER PLANES AND

ISLANDS

R1

Q2

BOOT

FB2

VIA CONNECTION TO GROUND PLANE

Q1

R5

R6

PHASE

D2

∆I

–

×

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

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

×

=

∆

I

∆

ESR

×

=

(EQ. 11)

(EQ. 12)

ISL6528