R7986A datasheet(32/37 Pages) STMICROELECTRONICS

32 / 37 page

Application ideas

R7986A

32/37

DocID026997 Rev 1

Equation 42

where VD is the voltage drop across the diodes, VSW and VSWE across the internal and

external power MOSFET.

7.2

Inverting buck-boost

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

voltage.

Figure 18 shows the schematic to regulate -5 V: no further external components are added

to the standard buck topology.

The relationship between input and output voltage is:

Equation 43

so the duty cycle is:

Equation 44

As in the positive one, in the inverting buck-boost the current flowing through the 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 can be calculated from Equation 40,

where the duty cycle is given by Equation 44.

Figure 20. Inverting buck-boost regulator

The GND pin of the device is connected to the output voltage so, given the output voltage,

the input voltage range is limited by the maximum voltage the device can withstand across

VCC and GND (38 V). Therefore, if the output is -5 V, the input voltage can range from 4.5 V

to 33 V.

VOUT 2VD



VIN VSW VSWE VOUT 2V



–

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

VOUT

VIN

–

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



VOUT

VOUT VIN

–

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

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