AN1517 datasheet(25/30 Pages) STMICROELECTRONICS

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AN1517

Application ideas

Positive buck-boost regulator

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

voltage. Figure 24 shows the schematic diagram of this topology for an output voltage of

12 V.

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, the maximum output current deliverable to the load is 0.75 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

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 in 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 has to be

lower than 36 V (the maximum operating input voltage).

Figure 25. Buck-boost regulator

VIN=5V

10uF

10V

Ceramic

STPS2L25U

Vcc

COMP

GND

OUT

L5972D

4.7k

33uH

24k

2.7k

22nF

100uF

16V

VOUT=12V/0.45A

220pF

STPS2L25U

STN4NE03L

AM00136v1

L5972D

VIN=5V

10uF

10V

Ceramic

STPS2L25U

Vcc

COMP

GND

OUT

L5972D

4.7k

33uH

2.7k

24k

22nF

100uF

16V

VOUT=-12V/0.45A

220pF

10uF

25V

Ceramic

AM00135v1

L5972D

AN1517 Datasheet(PDF) 25 Page - STMicroelectronics

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