R5973D datasheet(30/39 Pages) STMICROELECTRONICS

30 / 39 page

Application information

R5973D

30/39

DocID026876 Rev 1

Figure 18 shows the schematic circuit of this topology for a 12 V output voltage and 5 V

input.

Figure 18. Positive buck-boost regulator

8.6

Negative buck-boost regulator

In Figure 19 the schematic circuit for a standard buck-boost topology is shown. The output

voltage is:

Equation 37

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

Equation 38

The considerations given in Section 8.5: Positive buck-boost regulator for the real duty cycle

are still valid here.

Also the Equation 34 till Equation 36 can be used to calculate the maximum output current.

So, as an example, considering the conversion VIN = 12 V to VOUT = -5 V, ILOAD = 0.5 A:

Equation 39

Equation 40

An important thing to take into account is that the ground pin of the device is connected to

the negative output voltage. Therefore, the device is subjected to a voltage equal to

VIN - VO, which must be lower than 36 V (the maximum operating input voltage).

VOUT

VIN

–

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



– OUT

VIN VOUT

–

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

512

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

0.706

ISW

ILOAD

–

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

0.5

10.706

–

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

1.7A

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