R7986A datasheet(31/37 Pages) STMICROELECTRONICS

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R7986A

Application ideas

Equation 40

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, which for

a short time can exceed the maximum output current calculated by Equation 40, also the

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

Equation 41, must be lower than the minimum current limit (3.7 A).

Equation 41

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 19, 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

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

MOSFET are taken into account.

Figure 19. Maximum output current according to max. DC switch current (3.0 A):

VO = 12 V

ISW

IOUT

–

------------- 2 A



ISW,PK

IOUT

–

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

---

3.7A





VOUT

IOUT LFSW



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

–





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