L7985 datasheet(35/42 Pages) STMICROELECTRONICS

35 / 42 page

DocID022446 Rev 7

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L7985

Application ideas

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 L7985 device 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 (2.0 A), the maximum output current can be calculated from Equation 38,

where the duty cycle is given by Equation 42.

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

VOUT 2VD



VIN VSW VSWE VOUT 2V



–

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

VOUT

VIN

–

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



VOUT

VOUT VIN

–

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

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