mc33291l Freescale Semiconductor, Inc, mc33291l Datasheet - Page 19

• Manufacturer Part Number: mc33291l
• Description: Eight-output Switch With Serial Peripheral Interface I/o
• Manufacturer: Freescale Semiconductor, Inc

protection shutdown circuitry. Consider a load short (output
short to supply) occurring on an output before, during, and
after output turn ON. When the
logic state, the corresponding output is turned ON and a
delay timer is activated. The duration of the delay timer is
70 µs to 250 µs. If the short circuit takes place before the
output is turned ON, the delay experienced is the entire 70 µs
to 250 µs followed by shutdown. If the short occurs during the
delay time, the shutdown still occurs after the delay time has
elapsed. However, if the short circuit occurs after the delay
time, shutdown is immediate (within 20 µs after sensing). The
purpose of the delay timer is to prevent false faults from being
reported when switching capacitive loads.
disabled when an overcurrent is detected. The specific output
will, within 5.0 µs 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 ms to
20 ms.
shutdown circuitry. Overtemperature faults and overvoltage
faults are not affected by the SFPD pin’s state.
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. Whereas an output shutdown circuitry
measures the drain-to-source voltage, shutting down the
output if its threshold (V
output voltage and comparing it to V
requires a voltage of 2.5 V to be sensed. For an enabled
output, with V
3.5 V will be detected as a short (overcurrent condition), while
voltages less than 2.5 V will not be detected as shorts.
OVERCURRENT RECOVERY
normal operation automatically after the short circuit is
removed (unless thermal shutdown has occurred).
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 µs
to 250 µs) and then turn OFF for every write cycle
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)
Analog Integrated Circuit Device Data
Freescale Semiconductor
Grounding the SFPD pin will enable the short fault
If the SFPD pin is at 5.0 V (or V
Each output of the 33291L 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
DD
= 5.0 ± 0.5 V, an output voltage in excess of
THRES
) will be exceeded.
CS
DD
signal rises to the high
Thres
), an output will not be
. The lowest V
Thres
the SFPD pin should be. 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 overtemperature shutdown to
protect the outputs and the loads. The 33291L 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.
250 µs (current limiting of the output) followed by an
immediate overcurrent 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; this is often a much better method. In either case,
care should be taken to execute the SFPD start-up routine
every time startup or reset occurs.
UNDERVOLTAGE SHUTDOWN
shutdown of all outputs. The undervoltage threshold is
between 2.5 V and 3.5 V. When V
threshold, all outputs are turned OFF and the Serial Output
data register is reset to indicate the same.
output shutdown and reset. When 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
voltages below 5.5 V are not be guaranteed.
DECIPHERING FAULT TYPE
system fault has occurred. With eight outputs having open
load, overcurrent, overtemperature, and overvoltage 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 33291L 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.
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.
Sometimes both a delay period greater than 70 µs to
An undervoltage V
An undervoltage condition at the V
The 33291L SO pin can be used to determine what kind of
If for a specific output the initial SI command bit were logic
DD
PWR
condition will result in the global
. Proper operation at V
FUNCTIONAL DESCRIPTION
DD
PWR
goes below the
PWR
is between 5.5 V
pin will not cause
PWR
33291L
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