AN557 datasheet(48/52 Pages) STMICROELECTRONICS

48 / 52 page

AN557 APPLICATION NOTE

48/52

In the buck converter, the average input current is:

DESIGN EXAMPLE FOR L4974A

Figure 67. Design Example for L4974A

Up Down Converter

In some applications it is required to stabilize a voltage starting from an input voltage which can be lower

or higher then the output regulated voltage.

In this case a well known buck-boost topology is suggested.

The fig. 68 which shows the electrical diagram of the up-down converter, makes use of the L4974A to gen-

erate an output voltage of 12V at 3A. For output current lower or higher than 3A other devices of this family

can be used. For input voltage less than 20V the zener diode can be avoided.

Such circuit can also be used as a simple step-up. In this case there is a structure of the "asymmetrical

two transistor converter" type, that in the case of a short circuit is automatically protected since the internal

transistor turn-off, disconnecting the power supply.

This doesn’t happen in the classical step-up converter topology, in which, during the short circuit only the

power transistor is protected, but the current in the coil and the freewheeling diode is not limited.

Negative Output Voltage

Often it becomes necessary, in the multioutput power supplies, to generate negative voltages with current

(a)

(b)

Vo = 5.1V Io = 3.5A

Po = 17.85W

Vo = 12V Io = 3.5A

Po = 42W

Ii = 0.388A

Ii = 0.933A

With an operating input voltage of 60V the pre-

regulator will dissipate:

Pd = 5.82W

Pd =13.4W

The overall efficiency will be:

n = 68%

n = 70%

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⋅

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⋅

L4974A

818

15K

SB560

220nF

150

µH

3 x

100

µF

EKR

2200

µF

Vi=50 to 70V

D93IN005

BDW23C

µF

390pF

Vo=5V

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