SC4530 datasheet(15/23 Pages) SEMTECH

15 / 23 page

SC4530

when large feedback resistors are used. The regulator

becomes insensitive to switching disturbances after it

enters continuous-conduction mode.

The operating mode transition can be significantly

smoothed by filtering the FB node. A capacitor between

FB pin and ground (Capacitor C

5, as shown in Figures

3(a), page 8) serves this purpose. It should be chosen

so that it improves mode transition without significantly

slowing down load transients. Switching waveforms of a

5V output regulator (Figure 3(a), page 8) immediately

before it enters continuous-conduction mode are shown

in Figure 7. The inductor current waveform appears to be

more jagged without filtering. Moreover, transition to CCM

occurs at an output current of 76mA, instead of 26mA

with FB filtering. Figure 8 compares the corresponding

switching waveforms of an output 3.3V (Figure 4(a),

page 9) regulator.

Applications Information (Continued)

If the converter output voltage is .8V or less, or if R

2 is

reduced to below 2kW, then C

5 will not be necessary. C5 is

also optional in Figures 3(a) and 4(a) if the maximum V

never exceeds 2V. Bench testing shows that removing C

fromtheseconvertersstillresultsinacceptabletransitional

behavior, provided that V

IN < 2V.

5 can be estimated using the following empirical

equation:

()

FB filtering has no significant impact on the output ripple

voltage. However, it improves the converter efficiency by

0.25% to 0.5% around the mode transition point (Figure

9). Regulator efficiencies are slightly lower (< 0.25%) at

light loads when filtering the FB voltage. Positive values

in Figure 9 imply that FB filtering improves efficiency

compared to no filtering.

Effect of FB Filtering on

Efficiencies vs Load Current

-1.0

-0.5

0.0

0.5

1.0

100

1000

Load Current (mA)

VIN = 28V

VOUT = 5V

VOUT = 3.3V

Effect of FB Filtering on Converter Efficiency (V

IN = 28V)

Plotted Efficiency = the Efficiency of a FB-Filtered Converter -

the Efficiency of the Same Converter without FB-Filtering

Figure 9.

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