Datasheet
L7985 Application ideas
Doc ID 022446 Rev 3 35/43
Equation 38
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, which can
exceed the maximum output current calculated by Equation 38 for a short time, also the
peak current in the power MOSFET must be calculated. The peak current, shown in
Equation 39, must be lower than the minimum current limit (2.5 A).
Equation 39
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 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 by Equation
40, where power losses across diodes, external power MOSFET, and internal power
MOSFET are taken into account.
Figure 35. Maximum output current according to max. DC switch current (2.0 A):
V
O
=12 V
I
SW
I
OUT
1D–
-------------
2 A<=
I
SW,PK
I
OUT
1D–
-------------
1
r
2
---+ 2.5A<⋅=
r
V
OUT
I
OUT
LF
SW
⋅⋅
--------------------------------------------
1D–()
2
⋅=