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參數(shù)資料
| 型號(hào)： | MC33298 |
| 廠商： | 飛思卡爾半導(dǎo)體(中國)有限公司 |
| 英文描述： | Eight Output Switch with Serial Peripheral Interface I/O(帶SPI接口的八輸出開關(guān)) |
| 中文描述： | 8個(gè)串行外圍接口的I / O（帶的SPI接口的八輸出開關(guān)輸出開關(guān)） |
| 文件頁數(shù)： | 20/28頁 |
| 文件大?。?/td> | 642K |
| 代理商： | MC33298 |
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33298
20
Go to: www.freescale.com
prevent
false
faults from being reported when switching
capacitive loads.
If the SFPD pin is at 5.0 V, or V
DD
, an output will not be
disabled when an over current is detected. The specific output
will, within 5.0 to 10 μs of encountering the short circuit, go into
an analog current limited mode. This feature is especially useful
when switching incandescent lamp loads, where high in-rush
currents experienced during startup last for 10 to 20
milliseconds.
Each output of the 33298 has its own over current shutdown
circuitry. Over temperature, and the over voltage faults are not
affected by the SFPD pin’s state.
Both load current sensing and output voltage sensing are
incorporated for Short Fault detection with actual detection
occurring slightly
after
the onset of current limit. The current
limit circuitry incorporates a SENSEFET
approach to
measure the total drain current. This calls for the current
through a small number of cells in the power MOSFET to be
measured and the result multiplied by a constant, giving the
total current. Wherein output shutdown circuitry measures the
drain-to-source voltage, shutting down the output if its threshold
(V
Thres
) is exceeded.
Short fault detection is accomplished by sensing the output
voltage and comparing it to V
Thres
. The lowest V
Thres
requires
a voltage of 2.5 V to be sensed. For an enabled output, with
V
DD
= 5.0
±
0.5 V, an output voltage in excess of 3.5 V
will
be
detected as a
short
, or over current condition, while voltages
less than 2.5 V will
not
be detected as
shorts
.
Over Current Recovery
If the SFPD pin is in a high logic state, the circuit returns to
normal operation automatically after the short circuit is removed
(unless thermal shutdown has occurred).
If the SFPD pin is grounded and over current shutdown
occurs, removing the short circuit will result in the output
remaining OFF until the next write cycle. If the short circuit is
not
removed, the output will turn ON for the delay time (70 to 250
μs) and then turn OFF for every write cycle commanding a turn
ON.
SFPD Pin Voltage Selection
Since the voltage condition of the SFPD pin controls the
activation of the short fault protection (i.e., shutdown) mode
equally for all eight outputs, the load having the longest duration
of in-rush current determines what voltage (state) the SFPD pin
should be. Usually if at least one load is, an incandescent lamp
for example, the in-rush current on that input will be
milliseconds in duration. Therefore, setting SFPD at 5.0 V will
prevent shutdown of the output due to the in-rush current. The
system relies only on the over temperature shutdown to protect
the outputs and the loads. The 33298 was designed to switch
GE194 incandescent lamps, or equivalents, with the SFPD pin
in a grounded state. Considerably larger lamps can be switched
with the SFPD pin held in a high logic state.
Sometimes both a delay period greater than 70 to 250 μs
(current limiting of the output) followed by an immediate over
current shutdown is necessary. This can be accomplished by
programming the SFPD pin to 5.0 V for the extended delay
period, allowing the outputs to remain ON in a current limited
mode, then grounding it to accomplish the immediate shutdown
after a period of time. Additional external circuitry is required to
implement this type of function. An MCU parallel output port can
be devoted to controlling the SFPD voltage during and after the
delay period, is often a much better method. In either case, care
should be taken to execute the SFPD start-up routine every
time start-up or reset occurs.
Under Voltage Shutdown
An under voltage V
DD
condition will result in the global
shutdown of all outputs. The under voltage threshold is between
2.5 V and 3.5 V. When V
DD
goes below the threshold, all
outputs are turned OFF, thereby resetting the Serial Output
Data register to indicate the same.
An under voltage condition at the V
PWR
pin will
not
cause
output shutdown and reset. When V
PWR
is between 5.5 V and
9.0 V, the outputs will operate per the command word.
However, the status as reported by the SO pin may not be
accurate below 9.0 V V
PWR
. Proper operation at V
PWR
voltages
below 5.5 V are not be guaranteed.
Deciphering Fault Type
The 33298 SO pin can be used to determine what kind of
system fault has occurred. With eight outputs having open load,
over current, over temperature, and over voltage faults; a total
of 25 different faults are possible. The SO status word received
by the MCU will be compared with the word sent to the 33298
during the previous write cycle. For a specific output, if the SO
bit compares with the corresponding SI bit of the previous word;
the output is operating normal with no fault. Only when the SO
bit and previous word SI bit differ is there a fault indicated. If the
two words are
not
the same, the MCU should be programmed
to determine which output or outputs are faulted.
If, for a specific output, the initial SI command bit were logic
high, the output would be programmed to be OFF; if, upon the
next command word being entered, a logic low came back on
SO, for that specific output’s corresponding bit, an Output-OFF
Open-Load fault would be indicated. The resulting SO bit, for
that specific output, would be different from that entered during
the previous word for that SI bit, indicating the fault. The eight
output-off open-load faults are therefore most easily detected.
If for a specific output, the initial SI command bit were a logic
low, calling for the output to be programmed
on
; upon the next
word command being entered, the corresponding bit came
back with a logic high on SO, an output over current fault would
be indicated. An over current fault is always reported by the SO
output and is independent of the logic state existing on the
F
Freescale Semiconductor, Inc.
For More Information On This Product,
n
.
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