A5974AD datasheet(36/49 Pages) STMICROELECTRONICS

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Application information

A5974AD

36/49

Doc ID 018762 Rev 1

8.6

Positive buck-boost regulator

The device can be used to implement a step-up/down converter with a positive output

voltage.

The output voltage is given by:

Equation 33

where the ideal duty cycle D for the buck-boost converter is:

Equation 34

However, due to power losses in the passive elements, the real duty cycle is always higher

than this. The real value (that can be measured in the application) should be used in the

following formulas.

The peak current flowing in the embedded switch is:

Equation 35

while its average current is equal to:

Equation 36

This is due to the fact that the current flowing through the internal power switch is delivered

to the output only during the OFF phase.

The switch peak current must be lower than the minimum current limit of the overcurrent

protection (see Table 4 for details) while the average current must be lower than the rated

DC current of the device.

As a consequence, the maximum output current is:

Equation 37

where ISW MAX represents the rated current of the device.

The current capability is reduced by the term (1-D) and so, for example, with a duty cycle of

0.5, and considering an average current through the switch of 2 A, the maximum output

current deliverable to the load is 1 A.

Figure 25 below shows the circuit schematic of this topology for a 12 V output voltage and

5 V input.

OUT

–

-------------

⋅

OUT

------------------------------

LOAD

–

---------------

RIPPLE

--------------------

LOAD

–

---------------

⋅

-----------

---------

⋅

LOAD

–

---------------

OUT MAX

SW MAX

–

()

⋅

≅

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