LM2576D2T-3.3G 데이터 시트보기 (PDF) - ON Semiconductor

부품명

상세내역

제조사

LM2576D2T-3.3G

3.0 A, 15 V, Step−Down Switching Regulator

ON Semiconductor 

LM2576

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 startup 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 startup 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 startup currents required by

this inverting regulator topology, the use of a delayed startup

or an undervoltage lockout circuit is recommended.

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.

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 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:

Ipeak

[

ILoad (Vin )

Vin

|VO|)

)

Vin x ton

2L1

where ton

+

|VO|

Vin ) |VO|

x

1.0

fosc

,

and

fosc

+

52 kHz.

Under normal continuous inductor current operating

conditions, the worst case occurs when Vin is minimal.

12 V to 25 V

Unregulated

Feedback

DC Input

+Vin

Cin

100 mF

1

C1

/50 V 0.1 mF

LM2576−12

4

Output

L1

68 mH

2

5 ON/OFF 3 GN

R1

47 k R2

D

D1

1N5822

47 k

Cout

2200 mF

/16 V

−12 V @ 700 m A

Regulated

Output

Figure 27. Inverting Buck−Boost Regulator

with Delayed startup

+Vin

Shutdown

5.0 V

Input

Off

0 On

R3

470

+Vin

LM2576−XX

1

Cin

R1

100 mF 47 k

5 ON/OFF 3 GN

D

MOC8101

R2

47 k

−Vout

NOTE: This picture does not show the complete circuit.

Figure 28. Inverting Buck−Boost Regulator Shutdown

Circuit Using an Optocoupler

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.

http://onsemi.com

19

Share Link: