L7986TA STMicroelectronics, L7986TA Datasheet - Page 13

• Manufacturer Part Number: L7986TA
• Description: 3A step-down switching regulator
• Manufacturer: STMicroelectronics

Specifications of L7986TA

• Low Dropout Operation: 100% duty cycle

L7986TA
5.4
Overcurrent protection
The L7986TA implements the overcurrent protection 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 the pulse
by pulse overcurrent protection. Under 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 will
skip 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 of one unit.(see
So the overcurrent/short-circuit protection acts 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 to diverge, the current ripple in the inductor during the on-time has not to be higher
than the current ripple during the off-time. That is:
Equation 3
If the output voltage is shorted, V
from the above equation the maximum switching frequency that guarantees to limit the
current results:
Equation 4
With R
maximum frequency to keep the output current limited during the short-circuit results 88 kHz
Accounting on the pulse by pulse mechanism, that reduces the switching frequency down to
one eighth, the maximum F
current limitation is 88 kHz*8 = 706 kHz.
If, with V
condition the system finds a different equilibrium with higher current. For example with
F
SW
V
------------------------------------------------------------------------------------------------------------ D
IN
=800 kHz and the output shorted to ground, the output current is limited around:
–
DSon
V
OUT
IN
=38 V, the switching frequency is set higher than 706 kHz, during short-circuit
=300 mΩ, DRC=0.08 Ω, the worst condition is with V
–
R
DSON
L F
⋅
SW
⋅
I
OUT
F
*
SW
–
SW
DCR I
=
Doc ID 022098 Rev 1
, adjusted by the FSW pin, that assures a full effective output
------------------------------------------------------------------------------ -
(
V
OUT
IN
⋅
OUT
–
(
≅0, I
(
V
R
F
⋅
DSON
+
OUT
DCR I ⋅
=
+
V
-----------------------------------------------------------------------------------------------------------
=I
OUT
DCR
LIM
LIM
, D/F
+
) I
)
V
⋅
F
LIM
SW
+
R
)
=T
DSON
⋅
Figure
----------------------
T
L F
ON_MIN
ON_MIN
⋅
1
⋅
IN
SW
I
OUT
=38 V, I
7)
Functional description
, (1-D)/F
+
DCR I
LIM
SW
=3.7 A; the
⋅
OUT
≅1/F
⋅
(
SW
1 D
–
. So
13/43
)