L7981 datasheet(38/44 Pages) STMICROELECTRONICS

38 / 44 page

Application ideas

L7981

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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):

VO=-5 V

OUT

–

OUT

–

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L7981 Datasheet(PDF) 38 Page - STMicroelectronics