L7986 datasheet(35/43 Pages) STMICROELECTRONICS

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L7986

Application ideas

Equation 39

where ISW is the average current in the embedded power MOSFET in the on-time.

To chose the right value of the inductor and to manage transient output current, that for

a short time can exceed the maximum output current calculated by Equation 39, also the

peak current in the power MOSFET must be calculated. The peak current, shown in

Equation 40, must be lower than the minimum current limit (3.7 A).

Equation 40

where r is defined as the ratio between the inductor current ripple and the inductor DC

current:

So, in the buck-boost topology the maximum output current depends on the application

conditions (firstly input and output voltage, secondly switching frequency and inductor

value).

In Figure 35 the maximum output current for the above configuration is depicted varying the

input voltage from 4.5 V to 38 V.

The dashed line considers a more accurate estimation of the duty cycles given by Equation

41, where power losses across diodes, external power MOSFET, and internal power

MOSFET are taken into account.

Figure 35. Maximum output current according to max DC switch current (3.0 A):

VO = 12 V

ISW

IOUT

–

------------- 3 A



ISW,PK

IOUT

–

-------------

---

3.7A





L7986 Datasheet(PDF) 35 Page - STMicroelectronics