LM2598 datasheet(21/33 Pages) NSC

21 / 33 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

between the time the regulated output voltage (when it is

increasing in value) reaches 95% of the nominal output

voltage, 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

dropping would only be approximately 50 µs.

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

lector 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

Figure 1 is used when the output voltage is greater than 10V

or then 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

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 com-

parator to trigger the error flag. In this situation, adding a

resistor, R

FF, in series with the feedforward capacitor, ap-

proximately 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

located 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

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 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 elec-

trolytic 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 cur-

rent rating.

DS012593-65

FIGURE 15. External 3.7V Soft-Start Clamp

LM2598 Datasheet(PDF) 21 Page - National Semiconductor (TI)

LM2598 Datasheet(HTML) 21 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Datasheet Download

Link URL