A5975AD datasheet(38/50 Pages) STMICROELECTRONICS

38 / 50 page

Application information

A5975AD

38/50

Doc ID 018761 Rev 1

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 3 A, the maximum output

current deliverable to the load is 1.5 A.

Figure 26 below shows the circuit schematic of this topology for a 12 V output voltage and

5 V input.

Figure 26.

Positive buck-boost regulator

8.8

Negative buck-boost regulator

In Figure 27, the schematic circuit for a standard buck-boost topology is shown. The output

voltage is:

Equation 38

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

Equation 39

The considerations given in Section 8.8 for the real duty cycle are still valid here. Also

Equation 35 to 37 can be used to calculate the maximum output current. So, for example,

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

OUT MAX

SW MAX

–

()

⋅

≅

AM09669v1

10V

A5975AD

4.7

6.8

470

ESR>35m

OUT

–

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

⋅

–

OUT

–

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

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