Техническая Спецификация для STMicroelectronics Evaluation board for A5970AD EVALA5970AD EVALA5970AD

DocID14661 Rev 5

31/42

42

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

The output voltage is given by:

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

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:

while its average current is equal to: 

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 

 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:

where I

 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 1 A, the maximum output 
current deliverable to the load is 0.5 A.

V

OUT

V

IN

D

1 D

–

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



=

D

V

OUT

V

IN

V

OUT

+

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

=

I

SW

I

LOAD

1 D

–

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

I

RIPPLE

2

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

+

I

LOAD

1 D

–

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

V

IN

2 L



-----------

D

f

SW

---------



+

=

=

I

SW

I

LOAD

1 D

–

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

=

I

OUT MAX

I

SW MAX

1 D

–







