SC415_08 datasheet(19/30 Pages) SEMTECH

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SC415

Applications Information (continued)

The result is a well-defined sawtooth waveform, as shown

in Figure 11.

To FB

COUT

VOUT

LOAD

ESR

Ripple Current

VOUT ripple from ESR

FB ripple

750 mV

(to MOSFETs)

Figure 11

In real applications, the output capacitor also has some

series inductance (ESL), and this can have a large effect

on ripple. The ripple current creates voltage across the

ESL; this is a square wave similar to the LX waveform. The

result is shown below. Note the fast rising and falling

edges created by the ESL as show in Figure 12.

To FB

COUT

VOUT

LOAD

(to MOSFETs)

ESR

Ripple Current

VOUT ripple from ESR

FB ripple

750 mV

ESL

VOUT ripple from ESL

Figure 12

In addition to the ESL, most applications also have a small

capacitor in parallel with COUT; this is typically a small

ceramic capacitor intended to absorb high frequency

noise not filtered by the output capacitor, as shown by CB

in Figure 13.

To FB

COUT

VOUT

LOAD

ESR

Ripple Current

VOUT ripple from ESR

FB ripple

750 mV

ESL

VOUT ripple from ESL

(to MOSFETs)

Figure 13

This capacitor CB can have a large effect on the ripple

waveform. The switch transitions are fast, typically

10-30nsec. At this high speed, the output capacitor

impedance is dominated by ESL. In parallel with this is

CBP. The effective circuit is a parallel L-C filter, with ESL

in parallel with CPB. Thus the choice of CB can have

significant effect on the ripple waveform. The parallel LC

circuit can ring at high levels, and this can cause the VOUT

and therefore FB ripple to go below the trip point too

early. Since the FB waveform goes below the threshold

soon after the DH pulse is finished, there is the potential

for double-pulsing.

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