LM2575 datasheet(19/28 Pages) MOTOROLA

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LM2575

MOTOROLA ANALOG IC DEVICE DATA

Figure 26. Inverting Buck–Boost

Regulator with Delayed Startup

1N5819

100

µH

Output

Feedback

Unregulated

DC Input

12 V to 25 V

Cin

100

µF

/50 V

5ON/OFF

Gnd

+Vin

Regulated

Output

–12 V @ 0.35 A

Cout

1800

µF

/16 V

LM2575–12

0.1

µF

47 k

It has been already mentioned above, that in some

situations, the delayed startup or the undervoltage lockout

features could be very useful. A delayed startup circuit

applied to a buck–boost converter is shown in Figure 26.

Figure 32 in the “Undervoltage Lockout” section describes an

undervoltage lockout feature for the same converter

topology.

Figure 27. Inverting Buck–Boost Regulator Shut Down

Circuit Using an Optocoupler

LM2575–XX

Gnd

ON/OFF

+Vin

47 k

Cin

100

µF

NOTE:

This picture does not show the complete circuit.

47 k

470

Shutdown

Input

MOC8101

–Vout

Off

5.0 V

+Vin

With the inverting configuration, the use of the ON/OFF pin

requires some level shifting techniques. This is caused by the

fact, that the ground pin of the converter IC is no longer at

ground. Now, the ON/OFF pin threshold voltage (1.4 V

approximately) has to be related to the negative output

voltage level. There are many different possible shut down

methods, two of them are shown in Figures 27 and 28.

Figure 28. Inverting Buck–Boost Regulator Shut Down

Circuit Using a PNP Transistor

NOTE:

This picture does not show the complete circuit.

5.6 k

2N3906

LM2575–XX

Gnd

ON/OFF

12 k

–Vout

+Vin

Shutdown

Input

Off

+Vin

Cin

100

µF

Negative Boost Regulator

This example is a variation of the buck–boost topology and

is called a negative boost regulator. This regulator

experiences relatively high switch current, especially at low

input voltages. The internal switch current limiting results in

lower output load current capability.

The circuit in Figure 29 shows the negative boost

configuration. The input voltage in this application ranges

from –5.0 V 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.

Figure 29. Negative Boost Regulator

1N5817

150

µH

Output

Feedback

Regulated

Output

Vout = –12 V

Load Current from

200 mA for Vin = –5.2 V

to 500 mA for Vin = –7.0 V

Unregulated

DC Input

–Vin = –5.0 V to –12 V

Cout

1000

µF

/16 V

Cin

100

µF

/50 V

LM2575–12

ON/OFF

Gnd

+Vin

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