LM2574 datasheet(20/24 Pages) NSC

20 / 24 page

Additional Applications (Continued)

Because of the boosting function of this type of regulator, the

switch current is relatively high, especially at low input volt-

ages. Output load current limitations are a result of the maxi-

mum current rating of the switch. Also, boost regulators can

not provide current limiting load protection in the event of a

shorted load, so some other means (such as a fuse) may be

necessary.

UNDERVOLTAGE LOCKOUT

In some applications it is desirable to keep the regulator off

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

voltage lockout circuit which accomplishes this task is shown

Figure 13 while Figure 14 shows the same circuit applied

to a buck-boost configuration. These circuits keep the regu-

lator off until the input voltage reaches a predetermined

level.

TH ≈ VZ1 +2VBE (Q1)

DELAYED STARTUP

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

feature as shown in

Figure 15. 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 de-

lay 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 500 mA power supply that features an adjustable output

voltage is shown in

Figure 16. An additional L-C filter that re-

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

in this circuit.

DS011394-20

Note: Pin numbers are for 8-pin DIP package.

FIGURE 12. Negative Boost

DS011394-21

Note: Complete circuit not shown.

Note: Pin numbers are for 8-pin DIP package.

FIGURE 13. Undervoltage Lockout for Buck Circuit

DS011394-22

Note: Complete circuit not shown (see

Figure 11 ).

Note: Pin numbers are for 8-pin DIP package.

FIGURE 14. Undervoltage Lockout

for Buck-Boost Circuit

DS011394-23

Note: Complete circuit not shown.

Note: Pin numbers are for 8-pin DIP package.

FIGURE 15. Delayed Startup

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