A5975AD datasheet(37/50 Pages) STMICROELECTRONICS

37 / 50 page

A5975AD

Application information

Doc ID 018761 Rev 1

37/50

Figure 25.

PCB layout (front side)

8.7

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.

OUT

–

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

⋅

OUT

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

LOAD

–

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

RIPPLE

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

LOAD

–

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

⋅

-----------

---------

⋅

LOAD

–

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