MC33298 Motorola, MC33298 Datasheet - Page 15

• Manufacturer Part Number: MC33298
• Description: OCTAL SERIAL SWITCH (SPI Input/Output)
• Manufacturer: Motorola

will shut down or continue to operate in an analog current
limited mode until either the short circuit is removed or
thermal shutdown is reached.
protection shutdown circuitry. Consider a load short (output
short to supply) occurring on an output before, during, and
after output turn “on.” When the CSB signal rises to the high
logic state, the corresponding output is turned “on” and a
delay timer activated. The duration of the delay timer is
25 to 100 s. If the short circuit takes place before the output
is turned “on,” the delay experienced is the entire
25 to 100
during the delay time, the shutdown still occurs after the
delay time has elapsed. If the short circuit occurs after the
delay time, shut– down is immediate (within 20
sensing). The purpose of the delay timer is to prevent false
faults from being reported when switching capacitive loads.
disabled when overcurrent is detected. The specific output
will, within 5.0 to 10 s 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 ms.
shutdown circuitry. Over temperature faults and the
overvoltage faults are not affected by the SFPD pin.
incorporated for Short Fault detection with actual detection
occurring slightly after the onset of current limit. The current
limit circuitry incorporates a SENSEFET
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 to give the
total current. Whereas output shutdown circuitry measures
the drain to source voltage and shuts down if a threshold
(V Thres ) is exceeded.
output voltage and comparing it to V Thres . The lowest V Thres
requires a voltage of 0.6 times 4.5 V (the minimum V DD
voltage) or 2.7 V to be sensed. For an enabled output, with
V DD = 5.0 0.5 V, an output voltage in excess of 4.4 V will be
detected as a “short” while voltages less than 2.7 V will not be
detected as “shorts.”
Over Current Recovery
normal operation automatically after the short circuit is
removed (unless thermal shutdown has occurred).
occurs; removal of 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
(25 to 100
commanding a turn “on.”
SFPD Pin Voltage Selection
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)
MOTOROLA ANALOG IC DEVICE DATA
The status of SFPD will determine whether the MC33298
Grounding the SFPD pin will enable the short fault
If the SFPD pin is at 5.0 V (or V DD) , an output will not be
Each output of the MC33298 has its own overcurrent
Both load current sensing and output voltage sensing are
Short Fault detection is accomplished by sensing the
If the SFPD pin is in a high logic state, the circuit returns to
If the SFPD pin is grounded and overcurrent shutdown
Since the voltage condition of the SFPD pin controls the
s followed by shutdown. If the short occurs
s) and then turn “off” for every write cycle
approach to
s after
MC33298
the SFPD pin should be at. Usually if at least one load is, say
an incandescent lamp, 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
MC33298 was designed to switch GE194 incandescent
lamps with the SFPD pin in a grounded state. Considerably
larger lamps can be switched with the SFPD pin held in a
high logic state.
(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 to afford the outputs to remain “on” in a current limited
mode and then grounding it to accomplish the immediate
shutdown after some 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.
Undervoltage Shutdown
shutdown of all outputs. The undervoltage threshold is
between 2.5 V and 4.5 V. When V DD goes below the
threshold, all outputs are turned “off” and the SO register is
reset to indicate the same.
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 serial output (SO) pin
may not be accurate. Proper operation at V PWR voltages
below 5.5 V cannot be guaranteed.
Deciphering Fault Type
kind of system fault has occurred. With eight outputs having
open load, over current and over temperature 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
MC33298 during the previous write cycle. If the two words
are not the same, then the MCU should be programmed to
determine which output or outputs are indicating faults. If the
command bit for any of the output switches indicating a fault
is high, the fault is an open load.
easily detected. Over current and over temperature faults are
often related. Turning the affected output switches “off” and
waiting for some time should make these faults go away.
Over current and over temperature faults can not be
differentiated in normal application usage.
multiple faults can be detected with only one pin (SO) being
used for fault status indication.
will immediately be turned “off” and remain latched “off.”
A new command word is required to turn the outputs back
“on” following an overvoltage condition.
Sometimes both a delay period greater than 25 to 100 s
An undervoltage V DD condition will result in the global
An undervoltage condition at the V PWR pin will not cause
The MC33298 SO pin can be used to understand what
The eight open load faults are therefore the ones most
One advantage of the synchronous serial output is that
If V PWR experiences an overvoltage condition, all outputs
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