LM2575 datasheet(19/26 Pages) ONSEMI

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LM2575

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Using a delayed startup arrangement, the input capacitor

can charge up to a higher voltage before the switch−mode

regulator begins to operate.

The high input current needed for startup is now partially

supplied by the input capacitor Cin.

Design Recommendations:

The inverting regulator operates in a different manner

than the buck converter and so a different design procedure

has to be used to select the inductor L1 or the output

capacitor Cout.

The output capacitor values must be larger than is

normally required for buck converter designs. 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

inverting converter design is between 68

mH and 220 mH. To

select an inductor with an appropriate current rating, the

inductor peak current has to be calculated.

The following formula is used to obtain the peak inductor

current:

where ton +

in )

fosc

, and fosc + 52 kHz.

peak [

Load

in )

)

xton

Under normal continuous inductor current operating

conditions, the worst case occurs when Vin is minimal.

Note that the voltage appearing across the regulator is the

absolute sum of the input and output voltage, and must not

exceed 40 V.

Figure 26. Inverting Buck−Boost

Regulator with Delayed Startup

1N5819

100

Output

Feedback

Unregulated

DC Input

12 V to 25 V

Cin

100

/50 V

5ON/OFF

GND

+Vin

Regulated

Output

−12 V @ 0.35 A

Cout

1800

/16 V

LM2575−12

0.1

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

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

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.

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