LM2576T 查看數據表（PDF） - Motorola => Freescale

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LM2576T

Easy Switcher 3.0 A Step–Down Voltage Regulator

Motorola => Freescale 

LM2576

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 µF).

The recommended range of inductor values for the

inverting converter design is between 68 µH and 220 µH. 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 |VO|)

Vin

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.

Figure 27. Inverting Buck–Boost Regulator

with Delayed start–up

12 V to 25 V

Unregulated

DC Input

+Vin

Cin

100 µF

1

C1

/50 V 0.1 µF

LM2576–12

Feedback

4

Output

L1

68 µH

2

5 ON/OFF 3 Gnd

R1

47 k R2

47 k

D1

1N5822

Cout

2200 µF

/16 V

–12 V @ 700 m A

Regulated

Output

Figure 28. Inverting Buck–Boost Regulator Shutdown

Circuit Using an Optocoupler

+Vin

Shutdown

5.0 V

Input

Off

0 On

R3

470

+Vin

LM2576–XX

1

Cin R1

100 µF 47 k

5 ON/OFF 3 Gnd

MOC8101

R2

47 k

–Vout

NOTE: This picture does not show the complete circuit.

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.

MOTOROLA ANALOG IC DEVICE DATA

19

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