-5V output, but other output voltages are possible by selecting other

This example uses the LM2575/LM2575HV-5.0 to generate a

Application Information (Continued)

DELAYED STARTUP

The circuit in

Figure 21 uses the the ON /OFF pin to provide

a time delay between the time the input voltage is applied

and the time the output voltage comes up (only the circuitry

pertaining to the delayed start up is shown). As the input

voltage rises, the charging of capacitor C1 pulls the ON /OFF

pin high, keeping the regulator off. Once the input voltage

reaches its final value and the capacitor stops charging, and

resistor R

2 pulls the ON /OFF pin low, thus allowing the

circuit to start switching. Resistor R

1 is included to limit the

maximum voltage applied to the ON /OFF pin (maximum of

25V), reduces power supply noise sensitivity, and also limits

the capacitor, C1, discharge current. When high input ripple

voltage exists, avoid long delay time, because this ripple can

be coupled into the ON /OFF pin and cause problems.

This delayed startup feature is useful in situations where the

input power source is limited in the amount of current it can

deliver. It allows the input voltage to rise to a higher voltage

before the regulator starts operating. Buck regulators require

less input current at higher input voltages.

UNDERVOLTAGE LOCKOUT

Some applications require the regulator to remain off until

the input voltage reaches a predetermined voltage. An und-

ervoltage lockout feature applied to a buck regulator is

shown in

Figure 22, while Figure 23 and 24 applies the same

feature to an inverting circuit. The circuit in

Figure 23 fea-

tures a constant threshold voltage for turn on and turn off

(zener voltage plus approximately one volt). If hysteresis is

needed, the circuit in

Figure 24 has a turn ON voltage which

is different than the turn OFF voltage. The amount of hyster-

esis is approximately equal to the value of the output volt-

age. If zener voltages greater than 25V are used, an addi-

tional 47 k

Ω resistor is needed from the ON /OFF pin to the

ground pin to stay within the 25V maximum limit of the ON

/OFF pin.

INVERTING REGULATOR

The circuit in

Figure 25 converts a positive input voltage to a

negative output voltage with a common ground. The circuit

operates by bootstrapping the regulator’s ground pin to the

negative output voltage, then grounding the feedback pin,

output voltage versions, including the adjustable version.

Since this regulator topology can produce an output voltage

that is either greater than or less than the input voltage, the

maximum output current greatly depends on both the input

and output voltage. The curve shown in

Figure 26 provides a

guide as to the amount of output load current possible for the

different input and output voltage conditions.

01258336

FIGURE 21. Delayed Startup

01258337

FIGURE 22. Undervoltage Lockout

for Buck Regulator

01258338

This circuit has an ON/OFF threshold of approximately 13V.

FIGURE 23. Undervoltage Lockout

for Inverting Regulator

12

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LM2575

/LM2575HV

5.0

LM2575

/LM2575HV

5.0

LM2575

/LM2575HV

5.0

2A Step-Down Voltage Regulator

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R

UMW LM2575/LM2575HV

UMW LM2575/LM2575HV