LM2596 Просмотр технического описания (PDF) - ON Semiconductor

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LM2596

3.0 A, Step-Down Switching Regulator

ON Semiconductor 

LM2596

ADDITIONAL APPLICATIONS

Inverting Regulator

An inverting buck−boost regulator using the

LM2596−ADJ is shown in Figure 22. This circuit converts

a positive input voltage to a negative output voltage with a

common ground by bootstrapping the regulators ground to

the negative output voltage. By grounding the feedback pin,

the regulator senses the inverted output voltage and

regulates it.

In this example the LM2596−12 is used to generate a

−12 V output. The maximum input voltage in this case

cannot exceed +28 V because the maximum voltage

appearing across the regulator is the absolute sum of the

input and output voltages and this must be limited to a

maximum of 40 V.

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 29 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 to 40 V

Feedback

R4

Unregulated

DC Input

+Vin

LM2596−ADJ

L1

33 mH

Cin

100 mF/50 V

C1

0.1 mF

ON/OFF GND

R2

47k

D1

Cout

1N5822 220 mF

R3

−12 V @ 0.7 A

Regulated

Output

Figure 23. Inverting Buck−Boost Develops −12 V

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