Datasheet

Application ideas L7985
36/43 Doc ID 022446 Rev 3
Equation 40
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 L7985 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 41
so the duty cycle is:
Equation 42
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 38,
where the duty cycle is given by Equation 42.
Figure 36. Inverting buck-boost regulator
The GND pin of the device is connected to the output voltage so, given the output voltage,
the input voltage range is limited by the maximum voltage the device can withstand across
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
–
------------------------------- -=