Datasheet

DocID023432 Rev 4 29/40
LED2000 Application information
40
In overcurrent condition, the duty cycle is strongly reduced and, in most applications, this is
enough to limit the switch current to the current threshold.
The inductor current ripple during ON and OFF phases can be written as:
 ON phase
Equation 40
 OFF phase
Equation 41
where DCR
L
is the series resistance of the inductor.
The pulse-by-pulse current limitation is effective to implement constant current protection
when:
Equation 42
From Equation 40 and Equation 41 it can be seen that the implementation of the constant
current protection becomes more critical the lower the V
OUT
and the higher the V
IN
.
In fact, in short-circuit condition the voltage applied to the inductor during the OFF-time
becomes equal to the voltage drop across parasitic components (typically the DCR of the
inductor and the R
DSON
of the low-side switch) since VOUT is negligible, while during T
ON
the voltage applied at the inductor is maximized and is approximately equal to V
IN
.
In general, the worst case scenario is heavy short-circuit at the output with maximum input
voltage. Equation 40 and Equation 41 in overcurrent conditions can be simplified to:
Equation 43
considering T
ON
which has already been reduced to its minimum.
Equation 44
where T
SW
= 1/F
SW
and considering the nominal F
SW
.
At higher input voltage
I
L TON
may be higher than I
L TOFF
and so the inductor current can
escalate. As a consequence, the system typically meets Equation 42 at a current level
I
L TON

V
IN
V
OUT
– DCR
L
R
DSON HS
+I–
L
----------------------------------------------------------------------------------------------- T
ON
=
I
L TON

V
OUT
DCR
L
R
DSON LS
+I+–
L
---------------------------------------------------------------------------------------- T
OFF
=
I
L TON
 I
L TOFF
=
I
L TON

V
IN
DCR
L
R
DSON HS
+I–
L
------------------------------------------------------------------------ T
ON MIN

V
IN
L
--------- 90ns=
I
L TOFF

DCR
L
R
DSON LS
+– I
L
------------------------------------------------------------- - T
SW
90ns–
DCR
L
R
DSON LS
+– I
L
------------------------------------------------------------- - 1,18s=