L7981 datasheet(37/44 Pages) STMICROELECTRONICS

Part #	L7981

Description	3 A step-down switching regulator

PDF	44 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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37 / 44 page

L7981

Application ideas

Doc ID 15182 Rev 3

37/44

Figure 34.

Maximum output current according to max DC switch current (3.0 A):

VO=12 V

Equation 37

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

power MOSFET.

7.2

Inverting buck-boost

The L7981 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 38

So the duty cycle is:

Equation 39

OUT

⋅

SWE

OUT

⋅

–

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

OUT

–

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

⋅

OUT

–

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L7981 Datasheet(PDF) 37 Page - STMicroelectronics