L7986TR datasheet(36/43 Pages) STMICROELECTRONICS

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Application ideas

L7986

36/43

DocID022450 Rev 6

Equation 41

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 L7986 can implement the step-up/down converter with a negative output voltage.

Figure 34. 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 42

so the duty cycle is:

Equation 43

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 39,

where the duty cycle is given by Equation 43.

Figure 36. 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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