Datasheet

Application information ST1S14
28/46 DocID17977 Rev 2
reduced and, in most applications, this is enough to limit the switch current to the
active current threshold, nominal or foldback depending on the FB voltage.
The inductor current ripple during ON and OFF phases can be written as:
• ON phase
Equation 29
• OFF phase
Equation 30
where V
D
is the voltage drop across the diode, DCR
L
is the series resistance of the inductor.
The pulse-by-pulse current limitation is effective in implementing constant current protection
when:
Equation 31
The overcurrent protection is operating over the entire output voltage, which goes from the
regulated output voltage (V
O_SET
) down to GND during heavy short circuit applied at the
output.
From Equation 29 and Equation 30 we can gather that the implementation of the constant
current protection becomes more critical the lower is the V
OUT
and the higher is V
IN
.
In fact, 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 V
FW
of the free
wheeling diode) when VOUT is negligible, while during T
ON
the voltage applied 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.
7.4.1 300 mV < V
FB
< 1.22 V
The nominal output voltage can be written as:
Equation 32
From Equation 32 the voltage can be expressed as:
Equation 33
I
L TON
Δ
V
IN
V
OUT
– DCR
L
R
DSON
+()I⋅–
L
------------------------------------------------------------------------------------------
T
ON
()=
I
L TON
Δ
V
D
V
OUT
DCR
L
I⋅++()–
L
--------------------------------------------------------------------
T
OFF
()=
I
L TON
Δ I
L TOFF
Δ=
V
O_SET
V
FB
1
R
1
R
2
------ -+
⎝⎠
⎛⎞
⋅ 1.22 1
R
1
R
2
------ -+
⎝⎠
⎛⎞
⋅==
1
R
1
R
2
------ -+
⎝⎠
⎛⎞
V
O_SET
1.22
-------------------=