Datasheet

L7985 Functional description
Doc ID 022446 Rev 3 13/43
5.4 Overcurrent protection
The L7985 implements overcurrent protection by sensing current flowing through the power
MOSFET. Due to the noise created by the switching activity of the power MOSFET, the
current sensing is disabled during the initial phase of the conduction time. This avoids an
erroneous detection of a fault condition. This interval is generally known as “masking time”
or “blanking time”. The masking time is about 200 ns.
If the overcurrent limit is reached, the power MOSFET is turned off, implementing pulse-by-
pulse overcurrent protection. In the overcurrent condition, the device can skip turn-on pulses
in order to keep the output current constant and equal to the current limit. If, at the end of the
“masking time”, the current is higher than the overcurrent threshold, the power MOSFET is
turned off and one pulse is skipped. If, at the following switching on, when the “masking
time” ends, the current is still higher than the overcurrent threshold, the device skips two
pulses. This mechanism is repeated and the device can skip up to seven pulses. While, if at
the end of the “masking time”, the current is lower than the overcurrent threshold, the
number of skipped cycles is decreased by one unit (see Figure 8).
So, the overcurrent/short-circuit protection acts by switching off the power MOSFET and
reducing the switching frequency down to one eighth of the default switching frequency, in
order to keep constant the output current around the current limit.
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, DRC=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:
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
-------------------------
⋅=