L7986TA datasheet(37/43 Pages) STMICROELECTRONICS

37 / 43 page

L7986TA

Application ideas

Doc ID 022098 Rev 3

37/43

Equation 43

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 L7986TA can implement the step-up/down converter with a negative output voltage.

Figure 33. 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 44

so the duty cycle is:

Equation 45

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 41,

where the duty cycle is given by Equation 45 Equation 45.

Figure 35.

Inverting buck-boost regulator

OUT

⋅

SWE

OUT

⋅

–

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

OUT

–

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

⋅

OUT

–

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

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