ISL6537A

FN9143 Rev 5.00

Page 14 of 16

Jul 18, 2007

Component Selection Guidelines

Output Capacitor Selection - PWM Buck Converter

An output capacitor is required to filter the inductor current

and supply the load transient current. The filtering

requirements are a function of the switching frequency and

the ripple current. The load transient requirements are a

function of the slew rate (di/dt) and the magnitude of the

transient load current. These requirements are generally met

with a mix of capacitors and careful layout.

DDR memory systems are capable of producing transient

load rates above 1A/ns. High frequency capacitors initially

supply the transient and slow the current load rate 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 will determine the output ripple voltage

and the initial voltage drop after a high slew-rate transient. 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. 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.

Output Capacitor Selection - LDO Regulators

The output capacitors used in LDO regulators are used to

provide dynamic load current. The amount of capacitance

and type of capacitor should be chosen with this criteria in

mind.

Output Inductor Selection

The output inductor is selected to meet the output voltage

ripple requirements and minimize the converter’s response

time to the load transient. The inductor value determines the

converter’s ripple current and the ripple voltage is a function

of the ripple current. The ripple voltage and current are

approximated by the following equations:

Increasing the value of inductance reduces the ripple current

and voltage. However, the large inductance values reduce

the converter’s response time to a load transient.

One of the parameters limiting the converter’s response to

a load transient is the time required to change the inductor

current. Given a sufficiently fast control loop design, the

ISL6537A will provide either 0% or 100% duty cycle in

response to a load transient. The response time is the time

required to slew the inductor current from an initial current

value to the transient current level. During this interval the

difference between the inductor current and the transient

current level must be supplied by the output capacitor.

Minimizing the response time can minimize the output

capacitance required.

The response time to a transient is different for the

application of load and the removal of load. The following

equations give the approximate response time interval for

application and removal of a transient load:

response time to the removal of load. The worst case

response time can be either at the application or removal of

load. Be sure to check both of these equations at the

minimum and maximum output levels for the worst case

response time.

Input Capacitor Selection - PWM Buck Converter

Use a mix of input bypass capacitors to control the voltage

overshoot across the MOSFETs. Use small ceramic

capacitors for high frequency decoupling and bulk capacitors

to supply the current needed each time the upper MOSFET

turns on. Place the small ceramic capacitors physically close

to the MOSFETs and between the drain of upper MOSFET

and the source of lower MOSFET.

The important parameters for the bulk input capacitance are

the voltage rating and the RMS current rating. For reliable

operation, select bulk capacitors with voltage and current

ratings above the maximum input voltage and largest RMS

current required by the circuit. Their voltage rating should be

at least 1.25 times greater than the maximum input voltage,

while a voltage rating of 1.5 times is a conservative

guideline. For most cases, the RMS current rating

requirement for the input capacitor of a buck regulator is

approximately 1/2 the DC load current.

The maximum RMS current required by the regulator may be

closely approximated through the following equation:

I =

Fs x L

x

(EQ. 7)

(EQ. 8)

MAX

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

MAX

2

1

12

------

–



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

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







2



+







=

(EQ. 9)