Datasheet

DocID17977 Rev 2 31/46
ST1S14 Application information
prevented for 16 ms and then a new soft-start phase takes place (see Figure 17).
7.4.2 V
FB
< 300 mV
The device reduces the switching frequency by five times the nominal value when V
FB
<300
mV. The frequency foldback makes the pulse-by-pulse current protection effective to keep
the current limited when the output voltage is shorted and V
OUT
negligible.
Equation 29 and 30 in overcurrent conditions can be simplified to:
Equation 38
considering T
ON
which has already been reduced to its minimum.
Equation 39
taking into consideration the frequency foldback feature.
Figure 18. Current and frequency foldback triggered when V
FB
<300 mV (red trace)
The content given in Section 7.4.1 is valid and the equivalent expression of Equation 37 is:
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
5
----------- 90ns–
⎝⎠
⎛⎞
V
D
V
FB
V
O_SET
1.22
-------------------
⋅ DCR
L
I⋅++
⎝⎠
⎛⎞
–
L
------------------------------------------------------------------------------------------- -
5.9μs()≅=
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