SC416 datasheet(24/34 Pages) SEMTECH

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SC416

Figure 18 shows examples of this.

FB trip point

False trigger on FB

Idealized ESL ripple

Actual ripple with CBP

Idealized ESL ripple

Figure 18

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

a larger ceramic capacitor, typically 2.2–10μF, which sig-

nificantly smooths the ripple waveform as shown in Fig-

ure 19.

Large CB (~2.2 – 10 uF)

FB trip point

Actual ripple with CBP

Ripple due to ESL

Figure 19

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

the FB resistors, shown by RF/CF in Figure 20. This RC

filter is intended to remove the high-frequency noise but

still allow the ripple to reach the FB pin. Recommended

values are 10 ohms and 10nF.

To FB

VOUT

VOUT ripple

FB ripple

Figure 20

Dropout Performance

The VOUT adjust range for continuous-conduction op-

eration is limited by the fixed 330nsec (typical) Minimum

Off-time One-shot. When working with low input voltag-

es, 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 = TON

MIN

/ (TON

MIN

+ TOFF

MAX

)

Be sure to include inductor resistance and MOSFET on-

state voltage drops when performing worst-case drop-

out duty-factor calculations.

SC416 System DC Accuracy (VOUT Controller)

Three factors affect VOUT accuracy: the trip point of the

FB error comparator, the switching frequency variation

with line and load, and the external resistor tolerance.

The error comparator is trimmed to trip when the FB pin

is 750mV, +/-1% over the range of 0 to 85°C.

Applications Information (continued)

SC416 Datasheet(PDF) 24 Page - Semtech Corporation

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