Datasheet

L7986TA Application ideas
Doc ID 022098 Rev 3 37/43
Equation 43
where V
D
is the voltage drop across the diodes, V
SW
and V
SWE
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
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
–
------------------------------- -=