AN1518 datasheet(25/30 Pages) STMICROELECTRONICS

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AN1518

Application ideas

25/30

Application ideas

10.1

Positive buck-boost regulator

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

voltage. The figure below shows the schematic circuit of this topology for a 12 V output

voltage.

The input voltage can range from 5 V and 35 V. The output voltage is given by VO=VIN · D/

(1-D), where D is the duty cycle. The maximum output current is given by IOUT=1× (1-D).

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

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

current deliverable to the load is 1 A. 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.

Figure 24.

Positive buck-boost regulator

10.2

Buck-boost regulator

In Figure 25, the schematic circuit for a standard buck-boost topology is shown. The output

voltage is given by VO=-VIN · D/(1-D).

The maximum output current is equal to IOUT=1 · (1-D), for the same reason as that of the

up/down converter. 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).

AN1518 Datasheet(PDF) 25 Page - STMicroelectronics