L7986TA datasheet(36/43 Pages) STMICROELECTRONICS

36 / 43 page

Application ideas

L7986TA

36/43

Doc ID 022098 Rev 3

Equation 41

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

To choose 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 41, also the peak

current in the power MOSFET must be calculated. The peak current, shown in Equation 42,

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

Equation 42

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 34 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

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

MOSFET are taken into account.

Figure 34.

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

VO=12 V

OUT

–

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

3 A

SW,PK

OUT

–

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

---

3.7 A

⋅

OUT

⋅⋅

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

–

()

⋅

L7986TA Datasheet(PDF) 36 Page - STMicroelectronics