Datasheet

DocID022801 Rev 5 13/42
A7986A Functional description
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 the above equation, the maximum switching frequency that guarantees to limit the
current results:
Equation 4
With R
DS(on)
= 300 mΩ, DRC = 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.
Based on the pulse-by-pulse mechanism, that reduces the switching frequency down to one
eighth, the maximum F
SW
, adjusted by the FSW pin, which assures that 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
⁄–
DRC T
ON_MIN
⁄()R
DSON
DCR+()F
SW
*
⋅+
---------------------------------------------------------------------------------------------------------------- 4.2A==