LM2574_06 datasheet(20/26 Pages) ONSEMI

Part #	LM2574

Description	0.5 A, Adjustable Output Voltage, Step?묭own Switching Regulator

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LM2574, NCV2574

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The circuit in Figure 31 shows the negative boost

configuration. The input voltage in this application ranges

from −5.0 to −12 V and provides a regulated −12 V output.

If the input voltage is greater than −12 V, the output will rise

above −12 V accordingly, but will not damage the regulator.

1N5817

330

mH

Output

7

1

Feedback

Vout = −12 V

Load Current

60 mA for Vin = −5.2 V

120 mA for Vin = −7.0 V

Vin

L1

D1

Cout

1000

mF

Cin

22

mF

LM2574−12

5

3

4

ON/OFF

Pwr

Gnd

+Vin

2

Sig

Gnd

−5.0 to −12 V

Figure 31. Negative Boost Regulator

(5)

(12)

(3)

(14)

(4)

(6)

Design Recommendations:

The same design rules as for the previous inverting

buck−boost converter can be applied. The output capacitor

Cout must be chosen larger than what would be required for

a standard buck converter. Low input voltages or high output

currents require a large value output capacitor (in the range

of thousands of

mF). The recommended range of inductor

values for the negative boost regulator is the same as for

inverting converter design.

Another important point is that these negative boost

converters cannot provide any current limiting load

protection in the event of a short in the output so some other

means, such as a fuse, may be necessary to provide the load

protection.

Delayed Startup

There are some applications, like the inverting regulator

already mentioned above, which require a higher amount of

startup current. In such cases, if the input power source is

limited, this delayed startup feature becomes very useful.

To provide a time delay between the time when the input

voltage is applied and the time when the output voltage

comes up, the circuit in Figure 32 can be used. As the input

voltage is applied, the capacitor C1 charges up, and the

voltage across the resistor R2 falls down. When the voltage

on the ON/OFF pin falls below the threshold value 1.3 V, the

regulator starts up. Resistor R1 is included to limit the

maximum voltage applied to the ON/OFF pin. It reduces the

power supply noise sensitivity, and also limits the capacitor

C1 discharge current, but its use is not mandatory.

When a high 50 Hz or 60 Hz (100 Hz or 120 Hz

respectively) ripple voltage exists, a long delay time can

cause some problems by coupling the ripple into the

ON/OFF pin, the regulator could be switched periodically

on and off with the line (or double) frequency.

R1

47 k

LM2574−XX

5

2

and

4

3

Gnds

Pins

ON/OFF

R2

47 k

+Vin

C1

0.1

mF

Cin

22

mF

NOTE: This picture does not show the complete circuit.

Figure 32. Delayed Startup Circuitry

(5)

(12)

(4)

and

(6)

Undervoltage Lockout

Some applications require the regulator to remain off until

the input voltage reaches a certain threshold level. Figure 33

shows an undervoltage lockout circuit applied to a buck

regulator. A version of this circuit for buck−boost converter

is shown in Figure 34. Resistor R3 pulls the ON/OFF pin

high and keeps the regulator off until the input voltage

reaches a predetermined threshold level, which is

determined by the following expression:

V

th [

V

Z1 )

1.0

) R2

R1

V

BE

(Q1)

R1

10 k

Z1

1N5242B

R2

10 k

Q1

2N3904

R3

47 k

Cin

22

mF

LM2574−XX

5

2

and

4

3

Gnds

Pins

ON/OFF

+Vin

NOTE: This picture does not show the complete circuit.

Figure 33. Undervoltage Lockout Circuit for

Buck Converter

(5)

(12)

(4)

and

(6)

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