R7986ATR datasheet(30/37 Pages) STMICROELECTRONICS

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Application ideas

R7986A

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DocID026997 Rev 1

Application ideas

7.1

Positive buck-boost

The R7986A device can implement the step-up/down converter with a positive output

voltage.

Figure 18 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

38 V.

Figure 18. Positive buck-boost regulator

The relationship between input and output voltage is:

Equation 38

so the duty cycle is:

Equation 39

The output voltage isn’t limited by the maximum operating voltage of the device (38 V),

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

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

maximum gate to source voltage must be higher than the input voltage (in Figure 18, if VIN is

higher than 16 V, the gate must be protected through a 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 Equation 40.

VOUT

VIN

–

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

VOUT

VOUT VIN

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