SC415_08 datasheet(18/30 Pages) SEMTECH

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SC415

Applications Information (continued)

This virtual ESR ramp is created by integrating the voltage

across the inductor, and coupling the signal into the FB

pin as shown in Figure 9.

To FB

COUT

Figure 9

Double-Pulsing

Double-pulsing occurs because the ripple waveform seen

at the FB pin is either too small, or because the FB and

VOUT ripple waveform is very noisy and prone to cause

premature triggering of the FB comparator. Both are

discussed below.

Increasing FB Ripple

If the ripple waveform at FB is too small, the FB waveform

will be susceptible to switching noise. Note that under

normal conditions the FB voltage is within 10-20mV of the

750mV trip point. Noise can couple into the FB point from

either side1 or side1, or even from an external circuit. This

causes the FB comparator to trigger too quickly after the

330nsec minimum off-time has expired. Double-pulsing

will result in higher ripple voltage at the output but in

most cases is harmless.

A way to remedy this is to couple more ripple into FB from

VOUT. Note that the feedback resistor divider attenuates

the FB ripple. This can be compensated by placing a small

capacitor in parallel with the top resistor, which effectively

increases the ripple that appears at FB.

The schematic with added capacitor C as shown in Figure

10.

Figure 10

This capacitor should be left out until confirmation that

double-pulsing exists. It is best to leave a spot on the PCB

in case it is needed.

FB/VOUT Ripple Waveform

Because the constant on-time control method triggers a

DH pulse whenever the FB waveform reaches the 750mV

trip point, it is important that the VOUT and FB ripple

waveforms are well shaped. This waveform will depend

on the output capacitors.

The idealized circuit has an inductor, a capacitor COUT

with series ESR. The charging and discharging of COUT is

generally much smaller than the ripple voltage due to ESR,

so the resulting ripple waveform is generally determined

by ESR only.

To FB

COUT

VOUT

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