Datasheet

Application ideas L7981
36/44 Doc ID 15182 Rev 3
Equation 35
where I
SW
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.
I
SW
I
OUT
1D–
-------------
3 A<=
I
SW,PK
I
OUT
1D–
-------------
1
r
2
---+ 3.7A<⋅=
r
V
OUT
I
OUT
LF
SW
⋅⋅
------------------------------------
1D–()
2
⋅=