LM2597 datasheet(28/34 Pages) NSC

28 / 34 page

Application Information (Continued)

To use the bias supply feature with output voltages between

4V and 15V, wire the bias pin to the regulated output. Since

the V

BS pin requires a minimum of 4V to operate, the 3.3V

part cannot be used this way. When the V

BS pin is left open,

the intemal regulator circuitry is powered from the input

voltage.

SHUTDOWN /SOFT-START

The circuit shown in

Figure 28 is a standard buck regulator

with 24V in, 12V out, 100 mA load, and using a 0.068 µF

Soft-start capacitor. The photo in

Figure 26 and Figure 27

show the effects of Soft-start on the output voltage, the input

current, with, and without a Soft-start capacitor.

Figure 26

also shows the error flag output going high when the output

voltage reaches 95% of the nominal output voltage. The

reduced input current required at startup is very evident

when comparing the two photos. The Soft-start feature re-

duces the startup current from 700 mA down to 160 mA, and

delays and slows down the output voltage rise time.

This reduction in start up current is useful in situations where

the input power source is limited in the amount of current it

can deliver. In some applications Soft-start can be used to

replace undervoltage lockout or delayed startup functions.

If a very slow output voltage ramp is desired, the Soft-start

capacitor can be made much larger. Many seconds or even

minutes are possible.

If only the shutdown feature is needed, the Soft-start capaci-

tor can be eliminated.

DS012440-43

FIGURE 24. Effects of Bias Supply Feature on 5V

Regulator Efficiency

DS012440-45

FIGURE 25. Effects of Bias Supply Feature on 12V

Regulator Efficiency

DS012440-44

FIGURE 26. Output Voltage, Input Current, Error Flag

Signal, at Start-Up, WITH Soft-start

DS012440-46

FIGURE 27. Output Voltage, Input Current, at Start-Up,

WITHOUT Soft-start

LM2597 Datasheet(PDF) 28 Page - National Semiconductor (TI)

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