Datasheet

DocID022450 Rev 7 13/45
L7986 Functional description
45
This kind of overcurrent protection is effective if the output current is limited. To prevent the
current from diverging, the current ripple in the inductor during the on-time must not be
higher than the current ripple during the off-time. That is:
Equation 3
If the output voltage is shorted, V
OUT
≅ 0, I
OUT
= I
LIM
, D / F
SW
= T
ON_MIN
,
(1 - D) / F
SW
≅ 1 / F
SW
. So, from Equation 3, the maximum switching frequency that
guarantees to limit the current results:
Equation 4
With R
DSON
= 300 mΩ, DCR = 0.08 Ω, the worst condition is with V
IN
= 38 V, I
LIM
= 3.7 A;
the maximum frequency to keep the output current limited during the short-circuit results
88 kHz.
The pulse-by-pulse mechanism, which reduces the switching frequency down to one eighth
the maximum F
SW
, adjusted by the FSW pin, that assures a full effective output current
limitation, is 88 kHz * 8 = 706 kHz.
If, with V
IN
= 38 V, the switching frequency is set higher than 706 kHz, during short-circuit
condition the system finds a different equilibrium with higher current. For example, with
F
SW
= 800 kHz and the output shorted to ground, the output current is limited around:
Equation 5
where F
SW
* is 800 kHz divided by eight.
V
IN
V
OUT
R
DSON
I
OUT
DCR I
OUT
⋅–⋅––
LF
SW
⋅
------------------------------------------------------------------------------------------------------------ D
V
OUT
V
F
R
DSON
I
OUT
DCR I
OUT
⋅+⋅++
LF
SW
⋅
----------------------------------------------------------------------------------------------------------- 1D–()⋅=⋅
F
SW
*
V
F
DCR I⋅+
LIM
()
V
IN
R
DSON
DCR+()I
LIM
⋅–()
-------------------------------------------------------------------------------
1
T
ON_MIN
----------------------⋅=
I
OUT
V
IN
F
SW
*
⋅ V
F
T
ON_MIN
⁄–
DCR T
ON_MIN
⁄()R
DSON
DCR+()F
SW
*
⋅+
---------------------------------------------------------------------------------------------------------------- 4.2A==