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

(C

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 C

ripple at the feedback node by assuming that the resistor

divider was negligible. As V

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

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

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.

If C

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

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