LM2599 datasheet(20/31 Pages) NSC

20 / 31 page

Application Information (Continued)

DELAY CAPACITOR

DELAY — Provides delay for the error flag output. See the

upper curve in

Figure 13, and also refer to timing diagrams in

Figure 14. A capacitor on this pin provides a time delay be-

tween the time the regulated output voltage (when it is in-

creasing in value) reaches 95% of the nominal output volt-

age, and the time the error flag output goes high. A 3 µA

constant current from the delay pin charges the delay ca-

pacitor resulting in a voltage ramp. When this voltage

reaches a threshold of approximately 1.3V, the open collec-

tor error flag output (or power OK) goes high. This signal can

be used to indicate that the regulated output has reached the

correct voltage and has stabilized.

If, for any reason, the regulated output voltage drops by 5%

or more, the error output flag (Pin 3) immediately goes low

(internal transistor turns on). The delay capacitor provides

very little delay if the regulated output is dropping out of

regulation. The delay time for an output that is decreasing is

approximately a 1000 times less than the delay for the rising

output. For a 0.1 µF delay capacitor, the delay time would be

approximately 50 ms when the output is rising and passes

through the 95% threshold, but the delay for the output drop-

ping would only be approximately 50 µs.

Pull Up — The error flag output, (or power OK) is the collec-

tor of a NPN transistor, with the emitter internally grounded.

To use the error flag, a pullup resistor to a positive voltage is

needed. The error flag transistor is rated up to a maximum of

45V and can sink approximately 3 mA. If the error flag is not

used, it can be left open.

FEEDFORWARD CAPACITOR

(Adjustable Output Voltage Version)

FF - A Feedforward Capacitor CFF, shown across R2 in Fig-

ure 1 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

FF selection, see the de-

sign procedure section.

If the output ripple is large (> 5% of the nominal output volt-

age), 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

FF,

in series with the feedforward capacitor, approximately 3

times R1, will attenuate the ripple voltage at the feedback

pin.

INPUT CAPACITOR

IN — A low ESR aluminum or tantalum bypass capacitor is

needed between the input pin and ground pin. It must be lo-

cated near the regulator using short leads. This capacitor

prevents large voltage transients from appearing at the in-

put, 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 in-

put 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 di-

rectly 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 approxi-

mately 10˚C above an ambient temperature of 105˚C. The

ability of the capacitor to dissipate this heat to the surround-

ing 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 electro-

lytic capacitor will be physically larger than a lower voltage

capacitor, and thus be able to dissipate more heat to the sur-

rounding air, and therefore will have a higher RMS current

rating.

The consequences of operating an electrolytic capacitor

above the RMS current rating is a shortened operating life.

The higher temperature speeds up the evaporation of the ca-

pacitor’s electrolyte, resulting in eventual failure.

Selecting an input capacitor requires consulting the manu-

facturers data sheet for maximum allowable RMS ripple cur-

rent. For a maximum ambient temperature of 40˚C, a gen-

eral guideline would be to select a capacitor with a ripple

current rating of approximately 50% of the DC load current.

For ambient temperatures up to 70˚C, a current rating of

75% of the DC load current would be a good choice for a

conservative design. The capacitor voltage rating must be at

least 1.25 times greater than the maximum input voltage,

and often a much higher voltage capacitor is needed to sat-

isfy the RMS current requirements.

DS012582-33

FIGURE 15. RMS Current Ratings for Low

ESR Electrolytic Capacitors (Typical)

DS012582-34

FIGURE 16. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low ESR Electrolytic Capacitor)

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