Datasheet

L7981 Application ideas
Doc ID 15182 Rev 3 37/44
Figure 34. Maximum output current according to max DC switch current (3.0 A):
V
O
=12 V
Equation 37
where V
D
is the voltage drop across diodes, V
SW
and V
SWE
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
D
V
OUT
2V
D
⋅+
V
IN
V
SW
V
SWE
V
OUT
2V⋅
D
++––
------------------------------------------------------------------------------------------- -=
V
OUT
V
IN
–
D
1D–
-------------
⋅=
D
V
OUT
V
OUT
V
IN
–
----------------------------- -=