11

FN9257.0

February 13, 2006

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 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

ISL8104 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 a 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, a voltage rating of 1.5 times greater 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 ISL8104 requires at least 2 N-Channel power

gate supply 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. At a

300kHz switching frequency, 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 (see the

following equations). Only the upper MOSFET exhibits

switching losses, since the schottky rectifier clamps the

switching node before the synchronous rectifier turns on.

These equations assume linear voltage-current transitions

and do not adequately model power loss due the reverse-

recovery of the lower MOSFETs body diode. The

gate-charge losses are dissipated by the ISL8104 and don't

heat the MOSFETs. However, large gate-charge increases

∆V

∆I =

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V

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L

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T

RISE

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V

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ISL8104