Function Description(Cont.)

output voltage down by the ratio:

VFB = VOUT * R2 / (R1 + R2)

Thus the output voltage is:

VOUT = 0.8 * (R1 + R2) / R2

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 inductor 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 *

∆I

Where VOUT is the output voltage, VIN is the input

voltage, f is the switching frequency, and

∆I 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 alsosuffice.

The input capacitor value should be greater than 10

μF.

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 0.1

μF

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 FAC1513.

Output Capacitor

The output capacitor is required to maintain the DC

output voltage. Low ESR capacitors are preferred to

keep the 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 iscalculated as:

VRIPPLE ~=

∆I * RESR

Where VRIPPLE is the output voltage ripple,

∆I is

FAC1513

2011.06. 17

12/15

Revision No : 1