SC2440AEVB datasheet(16/28 Pages) SEMTECH

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Applications Information (Continued)

SC2440A

Output Capacitor

The output ripple voltage DV

OUT of a buck converter can

be expressed as

(7)

where C

OUT is the output capacitance.

Inductor ripple current DI

L increases as D decreases [Equa-

tion (3)]. The output ripple voltage is therefore the high-

est when V

IN is at its maximum. The first term in (7) results

from the ESR of the output capacitor while the second

term is due to the charging and discharging of C

OUT by

the inductor ripple current. Substituting DI

L = 0.65A, f =

550kHz and C

OUT = 22mF ceramic with ESR = 3mW in (7),

Depending upon operating frequency and the type of

capacitor, ripple voltage resulting from the charging and

discharging of C

OUT may be higher than that due to the

ESR of the output capacitor. A 0mF to 47mF X5R ceramic

capacitor is found adequate for output filtering in most

applications. Ripple current in the output capacitor is not

a concern because the inductor current of a buck convert-

er directly feeds C

OUT, resulting in very low ripple current.

Avoid using Z5U and Y5V ceramic capacitors for output

filtering because these types of capacitors have high tem-

perature and voltage coefficients.

Freewheeling Diode

Use of Schottky barrier diodes as freewheeling rectifiers

reduces diode reverse recovery current spikes, easing

high-side current sensing in the SC2440A. These diodes

should have an average current rating between A and

2A. The reverse blocking voltage of the Schottky diode

should be derated by 0%-20% for reliability. The Schott-

ky diode used in a 2V input step-down converter should

have a reverse voltage rating of at least 6V (20% derat-

ing). For switching regulators operating at low duty cy-

cles (i.e. low output voltage to input voltage conversion

ratios), it is beneficial to use freewheeling diodes with

somewhat higher average current ratings (thus lower for-

ward voltages). This is because the diode conduction in-

Figure 6. Input Voltage Ripple Generated by the DC-

DC Converter in Figure 1.

(a) Channel 1 is delivering 1.8A with Channel

2 shut off. The input ripple voltage ≈105mV.

(b) Channel 2 is delivering 1.8A with Channel

1 shut off. The input ripple voltage ≈ 130mV.

loads. Interleaved switching reduces the

input ripple voltage to about 60mV.

(c)

400ns/div

200mV/div

AC Coupled

5V/div

(b)

SW2

5V/div

200mV/div

AC Coupled

400ns/div

(a)

400ns/div

SW1

5V/div

200mV/div

AC Coupled

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