Datasheet

LM2599

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SNVS123C –APRIL 1998–REVISED APRIL 2013

— A Feedforward Capacitor C

, shown across R2 in Figure 24 is used when the output voltage is greater

than 10V or when C

OUT

has a very low ESR. This capacitor adds lead compensation to the feedback loop and

increases the phase margin for better loop stability. For C

selection, see the Design Procedure section.

If the output ripple is large (> 5% of the nominal output voltage), this ripple can be coupled to the feedback pin

through the feedforward capacitor and cause the error comparator to trigger the error flag. In this situation,

adding a resistor, R

, in series with the feedforward capacitor, approximately 3 times R1, will attenuate the

ripple voltage at the feedback pin.

INPUT CAPACITOR

— A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin. It

must be located near the regulator using short leads. This capacitor prevents large voltage transients from

appearing at the input, and provides the instantaneous current needed each time the switch turns on.

The important parameters for the Input capacitor are the voltage rating and the RMS current rating. Because of

the relatively high RMS currents flowing in a buck regulator's input capacitor, this capacitor should be chosen for

its RMS current rating rather than its capacitance or voltage ratings, although the capacitance value and voltage

rating are directly related to the RMS current rating.

The RMS current rating of a capacitor could be viewed as a capacitor's power rating. The RMS current flowing

through the capacitors internal ESR produces power which causes the internal temperature of the capacitor to

rise. The RMS current rating of a capacitor is determined by the amount of current required to raise the internal

temperature approximately 10°C above an ambient temperature of 105°C. The ability of the capacitor to dissipate

this heat to the surrounding air will determine the amount of current the capacitor can safely sustain. Capacitors

that are physically large and have a large surface area will typically have higher RMS current ratings. For a given

capacitor value, a higher voltage electrolytic capacitor will be physically larger than a lower voltage capacitor, and

thus be able to dissipate more heat to the surrounding air, and therefore will have a higher RMS current rating.

Figure 32. RMS Current Ratings for Low

ESR Electrolytic Capacitors (Typical)

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