Datasheet

DocID17977 Rev 2 29/46
ST1S14 Application information
so the output voltage is:
Equation 34
Equation 29 and 30, in overcurrent conditions, can be simplified to:
Equation 35
considering T
ON
which has already been reduced to its minimum.
Equation 36
where T
SW
=1/f
SW
and considering the nominal f
SW
.
The voltage divider introduces a gain factor K between the V
O_SET
and V
FB
that affect the
effectiveness of the current protection. The worst case scenario is the minimum K, that is
the minimum output voltage, over the input voltage (Chapter 5.2: Minimum output voltage
over V
IN
range).
As a consequence the minimum feedback voltage to keep the inductor current limited over
the input voltage range can be expressed making Equation 35 equal to Equation 36 and
expressing V
O_SET
as given in Equation 1:
Equation 37
Equation 37 expresses the worst case scenario as it considers the minimum K gain of the
voltage divider over the entire input voltage range. The Figure 16 plots the Equation 37
considering the minimum value of the peak current limit given in Table 6: Electrical
characteristics on page 8.
V
O
V
FB
1
R
1
R
2
------ -+
⎝⎠
⎛⎞
⋅ V
FB
V
O_SET
1.22
-------------------
⋅==
I
L TON
Δ
V
IN
DCR
L
R
DSON
+()I⋅–
L
-------------------------------------------------------------------
T
ON MIN
()
V
IN
L
---------
90ns()≅=
I
L TOFF
Δ
V
D
V
O_MIN
DCR
L
I⋅++()–
L
------------------------------------------------------------------------
T
SW
90ns–()
V
D
V
FB
V
O_SET
1.22
-------------------
⋅ DCR
L
I⋅++
⎝⎠
⎛⎞
–
L
------------------------------------------------------------------------------------------- -
1.18μs()≅=
V
FB
V
IN
()1.22
1.22 T
SW
⋅
V
IN
T
ON_MIN
⋅
--------------------------------------
V
D
I
L
DCR⋅()+()⋅
⎝⎠
⎛⎞
–=