LM2597 datasheet(22/34 Pages) NSC

22 / 34 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 1) 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.

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

DS012440-34

FIGURE 14. Timing Diagram for 5V Output

DS012440-75

FIGURE 15. External 3.7V Soft-Start Clamp

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