SC416 datasheet(22/34 Pages) SEMTECH

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SC416

The schematic with added capacitor C is shown below.

To FB

COUT

VOUT

Figure 14

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

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 case for the output filter has an inductor

and a capacitor COUT with series ESR. The voltage ripple

due to charging and discharging COUT is typically much

smaller than the ripple voltage due to ESR, so the over-

all ripple waveform is generally determined by ESR only.

The result is a well-defined sawtooth, see Figure 15.

Applications Information (continued)

Figure15

In many 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.

To FB

COUT

VOUT

LOAD

ESR

Inductor Ripple

Current

VOUT ripple

from ESR

FB ripple

750 mV

SC416 데이터시트(PDF) 22 Page - Semtech Corporation