Datasheet

DocID022446 Rev 8 13/44
L7985 Functional description
44
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
= 2.5 A;
the maximum frequency to keep the output current limited during the short-circuit results
74 kHz.
The pulse-by-pulse mechanism, which reduces the switching frequency down to one eighth
of the maximum F
SW
, adjusted by the FSW pin, assures that a full effective output current
limitation is 74 kHz * 8 = 592 kHz.
If, with V
IN
= 38 V, the switching frequency is set higher than 592 kHz, during short-circuit
condition the system finds a different equilibrium with higher current. For example, with
F
SW
= 700 kHz and the output shorted to ground, the output current is limited around:
Equation 5
where F
SW
* is 700 kHz divided by eight.
Figure 8. Overcurrent protection
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
–
DRC T
ON_MIN
R
DSON
DCR+F
SW
*
+
--------------------------------------------------------------------------------------------------------------- - 3.68A==