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LM2575EP, LM2575HVEP

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SNVS301B – DECEMBER 2004 – REVISED APRIL 2013

UNDERVOLTAGE LOCKOUT

In some applications it is desirable to keep the regulator off until the input voltage reaches a certain threshold. An

undervoltage lockout circuit which accomplishes this task is shown in Figure 33, while Figure 34 shows the same

circuit applied to a buck-boost configuration. These circuits keep the regulator off until the input voltage reaches

a predetermined level.

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DELAYED STARTUP

The ON /OFF pin can be used to provide a delayed startup feature as shown in Figure 35. With an input voltage

of 20V and for the part values shown, the circuit provides approximately 10 ms of delay time before the circuit

begins 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 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 36. An additional L-C filter that

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

Note: Complete circuit not shown.

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

Figure 33. Undervoltage Lockout for Buck Circuit

Note: Complete circuit not shown (see Figure 31).

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

Figure 34. Undervoltage Lockout

for Buck-Boost Circuit

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