Datasheet

DocID18279 Rev 5 27/37
ST1CC40 Application information
37
The pulse-by-pulse current limitation is effective in implementing constant current protection
when:
Equation 38
From Equation 36 and Equation 37 we can gather that the implementation of the constant
current protection becomes more critical the lower the V
OUT
is and the higher V
IN
is.
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
DS(on)
of the low-side switch) since V
OUT
is negligible, while during T
ON
the voltage applied at the inductor is maximized and it is approximately equal to V
IN
.
In general, the worst case scenario is heavy short-circuit at the output with maximum input
voltage. Equation 36 and Equation 37 in overcurrent conditions can be simplified to:
Equation 39
considering T
ON
that has already been reduced to its minimum.
Equation 40
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
may escalate. As a consequence, the system typically meets Equation 38 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 condition at very high input voltage even considering the spread
of the minimum conduction time of the power element. If the hiccup current level (6.2 A typ.)
is triggered, the switching activity is prevented for 12 cycles.
Figure 15 shows the operation of the constant current protection when a short-circuit is
applied at the output at the maximum input voltage.
I
L TON
 I
L TOFF
=
DCR R I V
I
L TON

V
IN
DCR
L
R
DS(on) HS
+I–
L
------------------------------------------------------------------------- T
ON MIN

V
IN
L
--------- 90ns=
I
L TOFF

DCR
L
R
DS(on) LS
+– I
L
--------------------------------------------------------------- T
SW
90ns–
DCR
L
R
DS(on) LS
+– I
L
--------------------------------------------------------------- 1,18s=