參數(shù)資料
型號： KIT33298DWEVB
廠商： Freescale Semiconductor
文件頁數(shù)： 13/28頁
文件大小： 0K
描述： KIT EVAL FOR MC33298 SMART SW
標準包裝： 1
Analog Integrated Circuit Device Data
20
Freescale Semiconductor
33298
FUNCTIONAL DESCRIPTION
FAULT LOGIC OPERATION
ARCHIVE
INFORMATION
ARCHIVE
INFORMATION
Figure 19. Output OFF Open Load Fault
An Output OFF Open Load Fault is indicates when the
output voltage is less than the Output Threshold Voltage
(VTHRES) of 0.6 to 0.8 x VDD. Since the 33298 outputs function
as switches, during normal operation, each MOSFET output
should either be completely turned ON or OFF. By design,
the threshold voltage was selected to be between the ON and
OFF voltage of the MOSFET. During normal operation, the
ON state VDS voltage of the MOSFET is less than the
threshold voltage and the OFF state VDS voltage is greater
than the threshold voltage. This design approach provides
using the same threshold comparator for Output Open Load
Detect in the OFF state and Short-circuit Detect in the ON
state. See Figure 20 for an understanding of the Short-circuit
Detect circuit. With VDD = 5.0V, an OFF state output voltage
of less than 3.0V will be detected as an Output OFF Open
Load Fault while voltages greater than 4.0V will not be
detected as a fault.
The 33298 has an internal pull-down current source of
50A, illustrated in Figure 19 between the MOSFET drain
and ground. This current source prevents the output from
floating up to VPWR if there is an open load or internal wire
bond failure. The internal comparator compares the drain
voltage with a reference voltage, VTHRES (0.6 to 0.8 x VDD). If
the output voltage is less than this reference voltage, the
33298 will declare the condition to be an open load fault.
During steady-state operation, the minimum load
resistance (RL) required to prevent false fault reporting during
normal operation can be located using the following equation.
Therefore, the load resistance necessary to prevent false
open load fault reporting is (using Ohm’s Law) equal to 92k
Ω
or less
.
During output switching, especially with capacitive loads,
a false output OFF Open Load Fault may be triggered. To
prevent this false fault from being reported an internal fault
filter in the range of 25 to 100s is incorporated. The duration
in which a false fault may be reported is a function of the load
impedance (RL,CL,LL), RDS(ON), and COUT of the MOSFET as
well as the supply voltage (VPWR). The rising edge of CS
triggers a built-in fault delay timer which must time-out (25 or
100s) before the fault comparator is enabled to detect at
faulted threshold. The circuit automatically returns to normal
operation once the condition causing the Open Load Fault is
removed.
SHORTED LOAD FAULT
A short load, or over-current fault can be caused by any
output being shorted directly to supply, or an output
experiencing a current greater than the current limit.
There are three safety circuits progressively in operation
during load short conditions providing system protection.
They are:
1. The output current of the device is monitored in an
analog fashion using a SENSEFET
approach and
current limited.
2. The output current of the device is sensed by
monitoring the MOSFET drain voltage.
3. The output thermal limit of the device is sensed, and
when attained, causes only the specific faulted output
to be latched OFF, allowing all remaining outputs to
operate normally.
Each of the three protection mechanisms are incorporated
in their output providing robust independent output operation.
The analog current limit circuit is always active, monitoring
the output drain current. An over-current condition causes the
gate control circuitry to reduce the gate-to-source voltage
imposed on the output MOSFET, re-establishing the load
current in compliance with current limit (3.0 to 6.0A) range.
Time required for the current limit circuitry to act is less than
20s. Therefore, currents higher than 3.0 to 6.0A will never
be seen for more than 20s (a typical duration is 10s). If the
current of an output attempts to exceed the predetermined
limit of 3.0 to 6.0A (4.0A nominal), the VDS voltage will
exceed the VTHRES voltage and the over-current comparator
will be tripped, illustrated in Figure 20.
Figure 20. Short Circuit Detect and
Analog Current Limiting Circuit
The status of SFPD determines whether the 33298 will
shut down immediately, or continue to operate in an analog
current limited mode until either the short-circuit (over-
VTHRES
2.5 to 3.5V
+
–
Low = Fault
50A
Output
VPWR
RL
33298
MOSFET OFF
VTHRES
2.5 to 3.5V
+
–
High = Fault
Output
VPWR
RL
33298
+
–
MOSFET ON
Digital
Analog
VREF
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