Datasheet
DocID14117 Rev 4 29/44
B5973D Application information
44
Equation 29
where V
D
is the voltage drop across the diode, DCR
L
is the series resistance of the inductor.
In short-circuit conditions V
OUT
is negligible so during T
OFF
the voltage across the inductor
is very small as equal to the voltage drop across parasitic components (typically the DCR of
the inductor and the V
FW
of the free wheeling diode) while during T
ON
the voltage applied
the inductor is instead maximized as approximately equal to V
IN
.
So Equation 28 and Equation 29 in overcurrent conditions can be simplified to:
Equation 30
considering T
ON
that has been already reduced to its minimum.
Equation 31
considering that f
SW
has been already reduced to one third of the nominal.
In case a short-circuit at the output is applied and V
IN
= 12 V the inductor current is
controlled in most of the applications (see Figure 15). When the application must sustain the
short-circuit condition for an extended period, the external components (mainly the inductor
and diode) must be selected based on this value.
In case the V
IN
is very high, it could occur that the ripple current during T
OFF
(Equation 31)
does not compensate the current increase during T
ON
(Equation 30). Figure 17 shows an
example of a power up phase with V
IN
= V
IN MAX
= 36 V where
IL TON
>
IL TOFF
so the
current escalates and the balance between Equation 30 and Equation 31 occurs at
a current slightly higher than the current limit. This must be taken into account in particular
to avoid the risk of an abrupt inductor saturation.
I
L TOFF
V
D
V
out
DCR
L
I++–
L
--------------------------------------------------------------- T
OFF
=
I
L TON
V
IN
DCR
L
R
DSON
+I–
L
---------------------------------------------------------------- T
ON MIN
V
IN
L
--------- 250ns=
I
L TOFF
V
D
V
out
DCR
L
I++–
L
--------------------------------------------------------------- 3T
SW
V
D
V
out
DCR
L
I++–
L
--------------------------------------------------------------- 12s=