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FN9030.7

March 4, 2005

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. For example, Intel

recommends that the high frequency decoupling for the

Pentium-Pro be composed of at least forty (40) 1.0

µF

ceramic capacitors in the 1206 surface-mount package.

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 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

ISL6522 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. With a +5V input

source, the worst case response time can be either at the

application or removal of load and dependent upon the

output voltage setting. Be sure to check both of these

equations at the minimum and maximum output levels for

the worst case response time.

Input Capacitor Selection

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 Q1 turns on. Place the

small ceramic capacitors physically close to the MOSFETs

and between the drain of Q1 and the source of Q2.

The important parameters for the bulk input capacitor are the

voltage rating and the RMS current rating. For reliable

operation, select the bulk capacitor with voltage and current

ratings above the maximum input voltage and largest RMS

current required by the circuit. The capacitor voltage rating

should be at least 1.25 times greater than the maximum

input voltage and a voltage rating of 1.5 times is a

conservative guideline. The RMS current rating requirement

for the input capacitor of a buck regulator is approximately

1/2 the DC load current.

For a through-hole design, several electrolytic capacitors

(Panasonic HFQ series or Nichicon PL series or Sanyo MV-GX

or equivalent) may be needed. For surface mount designs,

solid tantalum capacitors can be used, but caution must be

exercised with regard to the capacitor surge current rating.

These capacitors must be capable of handling the surge-

current at power-up. The TPS series available from AVX, and

the 593D series from Sprague are both surge current tested.

MOSFET Selection/Considerations

The ISL6522 requires two N-Channel power MOSFETs.

requirements, and thermal management requirements.

In high-current applications, the MOSFET power dissipation,

package selection and heatsink are the dominant design

factors. The power dissipation includes two loss

components; conduction loss and switching loss. The

conduction losses are the largest component of power

dissipation for both the upper and the lower MOSFETs.

These losses are distributed between the two MOSFETs

according to duty factor. The switching losses seen when

sourcing current will be different from the switching losses seen

when sinking current. When sourcing current, the upper

MOSFET realizes most of the switching losses. The lower

∆I =

Fs x L

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ISL6522