SC415_08 datasheet(20/30 Pages) SEMTECH

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SC415

Applications Information (continued)

Figure 14 shows example of this.

FB trip point

False trigger on FB

Idealized ESL ripple

Actual ripple with CBP

Figure 14

There are two ways to deal with this issue. One is to

use a larger ceramic capacitor, typically 2.2–10μF, which

significantly smooths the ripple waveform as shown in

Figure 15.

FB trip point

Actual ripple with

CB = (~2.2 – 10 uF)

Ripple from ESL

Figure 15

A second solution is to add a small RC filter in series

with the FB resistor path, shown by RF/CF. This RC filter

is intended to remove the high-frequency noise but still

allow the ripple to reach the FB pin as shown in Figure 16.

Recommended values are 10 ohms and 10nF.

To FB

VOUT

VOUT ripple

FB ripple

Figure 16

Dropout Performance

The VOUT adjust range for continuous-conduction

operation is limited by the fixed 330nsec (typical)

Minimum Off-time One-shot. When working with low

input voltages, the duty-factor limit must be calculated

using worst-case values for on and off times.

The IC duty-factor limitation is given by:

DUTY = TONMIN/(TONMIN + TOFFMAX)

Be sure to include inductor resistance and MOSFET on-

state voltage drops when performing worst-case dropout

duty-factor calculations.

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