SC416 datasheet(23/34 Pages) SEMTECH

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SC416

To FB

COUT

VOUT

LOAD

ESR

Ripple Current

VOUT ripple from ESR

FB ripple

750 mV

ESL

VOUT ripple from ESL

Total VOUT ripple

Figure 16

Note also that the effect of ESL becomes increasingly

more noticeable with lower VOUT and higher VIN. The

square wave due to ESL is basically the same square

wave seen at the LX node, scaled down by the ratio of the

output inductor and the ESL. As VOUT gets lower, the

output inductor gets smaller, and this directly increases

the ESL square wave. Moreover, lower output voltages

have tighter ripple requirements, so there is generally

less room available for pk-pk ripple.

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 17.

To FB

COUT

VOUT

LOAD

ESR

ESL

Figure 17

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 (COUT)

impedance is dominated by ESL, which is in parallel with

CB. The effective circuit is a parallel L-C filter. The choice

of CB can have significant effect on the ripple waveform.

This parallel LC circuit can lead to ringing at VOUT. Since

the FB waveform goes below the 750mV threshold soon

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

double-pulsing; the ringing can cause the FB waveform

to go below the trip point too early.

Applications Information (continued)

SC416 Datasheet(PDF) 23 Page - Semtech Corporation

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