Design Guide (Continued)
20189029
This circuit uses an additional resistor in series with the
inductor to add ripple at the output. It is placed in this location
and used in combination with the feed-forward capacitor
(CFF) to provide ripple to the feedback pin, without adding
ripple or a DC offset to the output. The benefit of using a
ceramic capacitor is still obtained with this technique. Be-
cause the addition of the resistor results in power loss, this
circuit implementation is only recommended for low currents
(2A and below). The power loss and rating of the resistor
should be taken into account when selecting this compo-
nent.
This circuit implementation utilizing the feed-forward capaci-
tor begins to experience limitations when the output voltage
is small. Previously the circuit relied on the CFF for all the
ripple at the feedback node by assuming that the resistor
divider was negligible. As VOUT decreases this can not be
assumed. The resistor divider contributes a larger amount of
ripple which is problematic as it is also out of phase. There-
fore the resistor location should be changed to be in series
with the output capacitor. This can be viewed as adding an
effective ESR to the output capacitor.
20189030
FEED-FORWARD CAPACITOR
time, but large enough to prevent attenuation of the ripple
voltage. In general a small ceramic capacitor in the range of
1nF to 10nF is sufficient.
The feed-forward capacitor is used across the top feedback
resistor to provide a lower impedance path for the high
frequency ripple without degrading the DC accuracy. Typi-
cally the value for this capacitor should be small enough to
prevent load transient errors because of the discharging
If CFF is used then it can be assumed that the ripple voltage
seen at the feedback pin is the same as the ripple voltage at
the output. The attenuation factor H no longer needs to be
used. However, in these conditions, it is recommended to
have a minimum of 20mV ripple at the feedback pin. The use
11
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