SC461 datasheet(25/31 Pages) SEMTECH

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SC461

Applications Information (continued)

DCR

D x VIN

Figure 18 — FB Voltage by CL Voltage

The magnitude of the FB ripple contribution due to C

L is

shown by the following equation.

VFBc

Figure 19 shows the equivalent circuit for calculating the

magnitude of the ripple contribution due to the output

voltage ripple.

COUT

DCR

VOUT

COUT

Figure 19 — FB Voltage by Output Voltage

The magnitude of the FB ripple contribution due to output

voltage ripple is shown by the following equation.

The purpose of this network is to couple the inductor

current ripple information into the feedback voltage such

that the feedback voltage has 90 degrees phase lag to the

switching node similar to the case of using standard high

ESR capacitors. This is illustrated in Figure 20.

VOUT

FB contribution

by CL

FB contribution by

output voltage ripple

Combined FB

Figure 20 — FB voltage in Phaser Diagram

The magnitude of the feedback ripple voltage, which is

dominated by the contribution from C

L , is controlled by

the value of R

1, R2 and CC . If the corner frequency of (R1//

2) x CC is too high, the ripple magnitude at the FB pin will

be smaller, which can lead to double-pulsing. Conversely,

if the corner frequency of (R

1// R2) x CC is too low, the ripple

magnitude at FB pin will be higher. Since the SC461 regu-

lates to the valley of the ripple voltage at the FB pin, a high

ripple magnitude is undesirable as it significantly impacts

the output voltage regulation. As a result, it is desirable to

select a corner frequency for (R

1// R2) x CC to achieve

enough, but not excessive, ripple magnitude and phase

margin. The component values for R

1, R2, and CC should

be calculated using the following procedure.

Select C

L (typical 10nF) and RL to match with L and DCR

time constant using the following equation.

Select C

C by using the following equation.

The resistor values (R

1 and R2) in the voltage divider circuit

set the V

OUT for the switcher. The typical value for CC is

from 10pF to 1nF.

Dropout Performance

The output voltage adjust range for continuous-conduc-

tion operation is limited by the fixed 250ns (typical)

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