LM2576 datasheet(19/24 Pages) NSC

19 / 24 page

Additional Applications (Continued)

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 3A 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.

Definition of Terms

BUCK REGULATOR

A switching regulator topology in which a higher voltage is

converted to a lower voltage. Also known as a step-down

switching regulator.

BUCK-BOOST REGULATOR

A switching regulator topology in which a positive voltage is

converted to a negative voltage without a transformer.

DUTY CYCLE (D)

Ratio of the output switch’s on-time to the oscillator period.

CATCH DIODE OR CURRENT STEERING DIODE

The diode which provides a return path for the load current

when the LM2576 switch is OFF.

EFFICIENCY (

η)

The proportion of input power actually delivered to the load.

01147617

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

FIGURE 13. Undervoltage Lockout

for Buck-Boost Circuit

01147618

Note: Complete circuit not shown.

FIGURE 14. Delayed Startup

01147619

FIGURE 15. 1.2V to 55V Adjustable 3A Power Supply with Low Output Ripple

LM2576 Datasheet(PDF) 19 Page - National Semiconductor (TI)