LM2576 datasheet(19/23 Pages) ARTSCHIP

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LM2576/LM2576HV

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Additional Applications (Continued)

Note: Complete circuit not shown (see Figure 10)

FIGURE 13. Undervoltage Lockout for Buck-Boost Circuit

DELAYED STARTUP

The

/OFF pin can be used to provide a delayed start up

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

for the part values shown, the circuit provides approximately

10ms of delay time before the circuit beigns switching.

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 60Hz

or 120Hz ripple, by coupling the ripple into the

/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.

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

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.

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