L7980ATR datasheet(36/44 Pages) STMICROELECTRONICS

36 / 44 page

Application ideas

L7980

36/44

Doc ID 15181 Rev 4

Equation 35

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 short

time can exceed the maximum output current calculated by Equation 35, also the peak

current in the power MOSFET has to be calculated. The peak current, showed in Equation

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

Equation 36

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 28 V.

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

37, where power losses across diodes, external power MOSFET, internal power MOSFET

are taken into account.

OUT

–

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

2 A

SW,PK

OUT

–

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

---

2.5A

⋅

OUT

⋅⋅

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

–

()

⋅

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