L7981 datasheet(35/44 Pages) STMICROELECTRONICS

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L7981

Application ideas

Doc ID 15182 Rev 3

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Application ideas

7.1

Positive buck-boost

The L7981 can implement the step up/down converter with a positive output voltage.

Figure 33. shows the schematic: one power MOSFET and one Schottky diode are added to

the standard buck topology to provide 12 V output voltage with input voltage from 4.5 V to 28

Figure 33.

Positive buck-boost regulator

The relationship between input and output voltage is:

Equation 33

So the duty cycle is:

Equation 34

The output voltage isn’t limited by the maximum operating voltage of the device (28 V),

because the output voltage is sense only through the resistor divider. The external power

MOSFET maximum drain to source voltage, must be higher than output voltage; the

maximum gate to source voltage must be higher than the input voltage (in Figure 33., if VIN

is higher than 16V, the gate must be protected through zener diode and resistor)

The current flowing through the internal power MOSFET is transferred to the load only

during the OFF time, so according to the maximum DC switch current (3.0 A), the maximum

output current for the buck boost topology can be calculated from the following equation.

OUT

–

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

⋅

OUT

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