Datasheet

DocID024346 Rev 1 29/42
LED2001 Application information
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
Δ
I
L TON
may be higher than
Δ
I
L TOFF
and so the inductor current can
escalate. As a consequence, the system typically meets Equation 42 at a current level
higher than the nominal value thanks to the increased voltage drop across stray
components. In most of the application conditions the pulse-by-pulse current limitation is
effective to limit the inductor current. Whenever the current escalates, a second level current
protection called “Hiccup mode” is enabled. Hiccup protection offers an additional protection
against heavy short-circuit conditions at very high input voltage even considering the spread
I
L TON
Δ
V
IN
V
OUT
DCR
L
R
DSON HS
+()I
L
----------------------------------------------------------------------------------------------- T
ON
()=
I
L TON
Δ
V
OUT
DCR
L
R
DSON LS
+()I+()
L
---------------------------------------------------------------------------------------- T
OFF
()=
I
L TON
Δ I
L TOFF
Δ=
I
L TON
Δ
V
IN
DCR
L
R
DSON HS
+()I
L
------------------------------------------------------------------------ T
ON MIN
()
V
IN
L
--------- 90ns()=
I
L TOFF
Δ
DCR
L
R
DSON LS
+() I
L
-------------------------------------------------------------- T
SW
90ns()
DCR
L
R
DSON LS
+() I
L
-------------------------------------------------------------- 1.18μs()=