A7985ATR datasheet(38/44 Pages) STMICROELECTRONICS

38 / 44 page

Application ideas

A7985A

38/44

DocID023128 Rev 5

Figure 35. Maximum output current according to max. DC switch current (2.0 A):

VO = 12 V

Equation 44

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 A7985A 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 45

so the duty cycle is:

Equation 46

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 42,

where the duty cycle is given by Equation 46.

VOUT 2VD



VIN VSW VSWE VOUT 2V



–

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

VOUT

VIN

–

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



VOUT

VOUT VIN

–

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

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