L7985 datasheet(33/42 Pages) STMICROELECTRONICS

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L7985

Application ideas

7.1

Positive buck-boost

The L7985 can implement the step-up/down converter with a positive output voltage.

Figure 34 shows the schematic: one power MOSFET and one Schottky diode are added to

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

38 V.

Figure 34. Positive buck-boost regulator

The relationship between input and output voltage is:

Equation 37

so the duty cycle is:

Equation 38

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 output voltage; the

maximum gate to source voltage must be higher than the input voltage (in Figure 34, if VIN is

higher than 16 V, 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 (2.0 A), the maximum

output current for the buck-boost topology can be calculated from Equation 39.

VOUT

VIN

–

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



VOUT

VOUT VIN

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L7985 Datasheet(PDF) 33 Page - STMicroelectronics