LM2575EP datasheet(20/26 Pages) NSC

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Additional Applications (Continued)

UNDERVOLTAGE LOCKOUT

In some applications it is desirable to keep the regulator off

until the input voltage reaches a certain threshold. An und-

ervoltage lockout circuit which accomplishes this task is

shown in Figure 12, while Figure 13 shows the same circuit

applied to a buck-boost configuration. These circuits keep

the regulator off until the input voltage reaches a predeter-

mined level.

≈ V

Z1 +2VBE (Q1)

DELAYED STARTUP

The ON /OFF pin can be used to provide a delayed startup

feature as shown in Figure 14. With an input voltage of 20V

and for the part values shown, the circuit provides approxi-

mately 10 ms of delay time before the circuit begins switch-

ing. Increasing the RC time constant can provide longer

delay times. But excessively large RC time constants can

cause problems with input voltages that are high in 60 Hz or

120 Hz ripple, by coupling the ripple into the ON /OFF pin.

ADJUSTABLE OUTPUT, LOW-RIPPLE

POWER SUPPLY

A 1A power supply that features an adjustable output voltage

is shown in Figure 15. An additional L-C filter that reduces

the output ripple by a factor of 10 or more is included in this

circuit.

20113216

Typical Load Current

200 mA for VIN = −5.2V

500 mA for VIN = −7V

Note: Pin numbers are for TO-220 package.

FIGURE 11. Negative Boost

20113217

Note: Complete circuit not shown.

Note: Pin numbers are for the TO-220 package.

FIGURE 12. Undervoltage Lockout for Buck Circuit

20113218

Note: Complete circuit not shown (see Figure 10).

Note: Pin numbers are for the TO-220 package.

FIGURE 13. Undervoltage Lockout

for Buck-Boost Circuit

20113219

Note: Complete circuit not shown.

Note: Pin numbers are for the TO-220 package.

FIGURE 14. Delayed Startup

LM2575EP Datasheet(PDF) 20 Page - National Semiconductor (TI)

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