Datasheet

Application ideas L7981
38/44 Doc ID 15182 Rev 3
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 the Equation
35, where the duty cycle is given by Equation 39.
Figure 35. Inverting buck-boost regulator
The GND pin of the device is connected to the output voltage so, given the output voltage,
input voltage range is limited by the maximum voltage the device can withstand across VCC
and GND (28 V). Thus if the output is -5 V the input voltage can range from 4.5 V to 23 V.
As in the positive buck-boost, the maximum output current according to application
conditions is shown in Figure 36. The dashed line considers a more accurate estimation of
the duty cycles given by Equation 40, where power losses across diodes and internal power
MOSFET are taken into account.
Equation 40
Figure 36. Maximum output current according to switch max peak current (3.0 A):
V
O
=-5 V
D
V
OUT
V
D
–
V–
IN
V
SW
V
OUT
V
D
–+–
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