FAC1591 datasheet(10/12 Pages) FS

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Inductor

The inductor is required to supply constant

current to the output load while being driven by

the switched input voltage. A larger value

inductor results in less ripple current that in

turn results in lower output ripple voltage.

However, the larger value inductor has a larger

physical size, higher series resistance, and/or

lower saturation current. Choose an inductor

that does not saturate under the worst-case

load conditions. A good rule for determining

the inductance is to allow the peak-to-peak

ripple current in the i nductor to be approximately

30% of the maximum load

current. Also, make sure that the peak

inductor current (the load current plus half the

peak-to-peak inductor ripple current) is below

the TBDA minimum current limit. The

inductance value can be calculated by the

equation:

L = (VOUT) * (VIN-VOUT) / VIN * f * ΔI

Where VOUT is the output voltage, VIN is the

input voltage, f is the switching frequency, and

ΔI is the peak-to-peak inductor ripple current.

Input Capacitor

The input current to the step-down converter is

discontinuous, and so a capacitor is required

to supply the AC current to the step-down

converter while maintaining the DC input

voltage. A low ESR capacitor is required to

keep the noise at the IC to a minimum.

Ceramic capacitors are preferred, but tantalum

or low-ESR electrolytic capacitors may also

suffice.

The input capacitor value should be greater

than 10μF. The capacitor can be electrolytic,

tantalum or ceramic. However since it absorbs

the input switching current it requires an

adequate ripple current rating. Its RMS current

rating should be greater than approximately

1/2 of the DC load current.

For insuring stable operation should be

placed as close to the IC as possible.

Alternately a smaller high quality ceramic

0.1μF capacitor may be placed closer to the IC

and a larger capacitor placed further away. If

using this technique, it is recommended that

the larger capacitor be a tantalum or

electrolytic type. All ceramic capacitors should

be places close to the FAC1591.

Output Capacitor

The output capacitor is required to maintain

the DC output voltage. Low ESR capacitors

are preferred to keep t he output voltage ripple

low. The characteristics of the output

capacitor also affect the stability of the

regulation control system. Ceramic, tantalum,

or low ESR electrolytic capacitors are

recommended. In the case of ceramic

capacitors, the impedance at the switching

frequency is dominated by the capacitance,

and so the output voltage ripple is mostly

independent of the ESR

. The output voltage

ripple is estimated to be:

VRIPPLE ~= 1.4 * VIN * (fLC/fSW)^2

Where VRIPPLE is the output ripple voltage, VIN

is the input voltage, fLC is the resonant

frequency of the LC filter, fSW is the switching

frequency. In the case of tanatalum or low-

ESR electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency, and so the output ripple is

calculated as:

VRIPPLE ~= ΔI * RESR

Where VRIPPLE is the output voltage ripple, ΔI is

the inductor ripple current, and RESR is the

equivalent series resistance of the output

capacitors.

Output Rectifier Diode

The output rectifier diode supplies the current

to the inductor when the high-side switch is off.

To reduce losses due to the diode forward

voltage and recovery times, use a Schottky

rectifier.

Table 1 provides the Schottky rectifier part

numbers based on the maximum input voltage

and current rating.

Choose a rectifier who’s maximum reverse

voltage rating is greater than the maximum

input voltage, and who’s current rating is

greater than the maximum load current.

2008.01. 28

10/12

Revision No : 0

FAC1591

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