A7985ATR datasheet(37/44 Pages) STMICROELECTRONICS

37 / 44 page

DocID023128 Rev 5

37/44

A7985A

Application ideas

The current flowing through the internal Power MOSFET is transferred to the load only

during the OFF time, so according to the maximum DC switch current (2.0 A), the maximum

output current for the buck boost topology can be calculated from Equation 42.

Equation 42

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 42, also the

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

Equation 43, must be lower than the minimum current limit (2.5 A).

Equation 43

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

current.

Therefore, 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 Equation 44,

where power losses across diodes, the external Power MOSFET, and the internal Power

MOSFET are taken into account.

ISW

IOUT

–

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



ISW,PK

IOUT

–

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

---

3.7A





VOUT

IOUT LFSW



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

–





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