4.3.1
SD Pin
The SD is an input/output pin (open drain type if used as output). Taking into account the voltage reference on SD
(3.3 V), a pull up resistor of 10 kΩ (R28) is used to guarantee the right bias and consequently to keep the current
on the open drain DMOS (Iod) lower than 3 mA.
The filter on SD (R28 and C20) has to be sized to obtain the desired re-starting time after a fault event and placed
as close as possible to the SD pin.
A shutdown event can be managed by the MCU, in this case the SD functions as the input pin.
Conversely, the SD functions as an output pin when an overcurrent or undervoltage condition is detected.
4.3.2
Fault management
The SLLIMM 2nd series integrates a specific kind of fault management, useful when SD is functioning as output,
able to identify the type of fault event.
As previously described, as soon as a fault occurs, the open-drain (DMOS) is activated and LVGx outputs are
forced low.
Two types of fault can be signaled:
•
Overcurrent (OC) sensed by the internal comparator (CIN);
•
Undervoltage (UVLO) on supply voltage (VCC).
Each fault enables the SD open drain for a different time (see the table below).
The duration of a shutdown event therefore tells us the type of failure that has occurred.
Table 1.
Fault timing
Symbol
Parameter
Event time
SD open-drain enable time
result
OC
Overcurrent event
≤ 24 μs
24 μs
> 24 μs
OC time
UVLO
Undervoltage lockout event
≤ 70 μs
70 μs
> 70 μs until VCC_LS
exceeds the VCC_LS
UV turn on threshold
UVLO time
Note:
typical value (TJ = -40 °C to 125 °C)
Note:
without contribution of RC network on SD
Figure 10. SD failure due to overcurrent
shows a shutdown as the result of an overcurrent event. During the
overcurrent, the voltage on the comparator (CIN) exceeds the threshold (0.51 V typ.) and the shutdown is able to
stop the application. In this case, the SD event time is about 24 μs (for OC event less than 24 μs).
UM2702
Overcurrent protection
UM2702
-
Rev 1
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