LM2576 datasheet(19/26 Pages) ONSEMI

19 / 26 page

LM2576

http://onsemi.com

This circuit configuration is able to deliver approximately

0.7 A to the output when the input voltage is 12 V or higher.

At lighter loads the minimum input voltage required drops

to approximately 4.7 V, because the buck−boost regulator

topology can produce an output voltage that, in its absolute

value, is either greater or less than the input voltage.

Since the switch currents in this buck−boost configuration

are higher than in the standard buck converter topology, the

available output current is lower.

This type of buck−boost inverting regulator can also

require a larger amount of start−up input current, even for

light loads. This may overload an input power source with

a current limit less than 5.0 A.

Such an amount of input start−up current is needed for at

least 2.0 ms or more. The actual time depends on the output

voltage and size of the output capacitor.

Because of the relatively high start−up currents required

by this inverting regulator topology, the use of a delayed

start−up or an undervoltage lockout circuit is recommended.

Using a delayed start−up 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 start−up is now partially

supplied by the input capacitor Cin.

It has been already mentioned above, that in some

situations, the delayed start−up or the undervoltage lockout

features could be very useful. A delayed start−up circuit

applied to a buck−boost converter is shown in Figure 27,

Figure 33 in the “Undervoltage Lockout” section describes

an undervoltage lockout feature for the same converter

topology.

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 what 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 )

1.0

fosc

, and fosc + 52 kHz.

peak [

Load

in )

)

xton

Under normal continuous inductor current operating

conditions, the worst case occurs when Vin is minimal.

Figure 27. Inverting Buck−Boost Regulator

with Delayed start−up

1N5822

Output

Feedback

12 V to 25 V

Unregulated

DC Input

Cin

100

/50 V

5ON/OFF

+Vin

−12 V @ 700 m A

Regulated

Output

Cout

2200

/16 V

LM2576−12

0.1

47 k

Figure 28. Inverting Buck−Boost Regulator Shutdown

Circuit Using an Optocoupler

LM2576−XX

5GN

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

(1.3 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 28 and 29.

LM2576 Datasheet(PDF) 19 Page - ON Semiconductor

LM2576 Datasheet(HTML) 19 Page - ON Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Datasheet Download

Link URL