33291 Datasheet, PDF (20/26 Pages) Freescale Semiconductor, Inc – Eight-Output Switch with Serial Peripheral Interface I/O

FUNCTIONAL DESCRIPTION
useful when switching incandescent lamp loads, where high
in-rush currents experienced during startup last for 10 ms to
20 ms.
Each output of the 33291 has its own overcurrent
shutdown circuitry. Overtemperature faults and overvoltage
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. Whereas an output shutdown circuitry
measures the drain-to-source voltage, shutting down the
output if its threshold (V_THRES) will be 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 (overcurrent condition), while
voltages less than 2.5 V will not be detected as shorts.
OVERCURRENT 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 overcurrent 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 µs
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, 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 33291 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 µs to
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
33291
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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
An undervoltage V_DD condition will result in the global
shutdown of all outputs. The undervoltage threshold is
between 2.5 V and 3.5 V. When V_DD goes below the
threshold, all outputs are turned OFF and the Serial Output
data register is reset to indicate the same.
An undervoltage condition at the VPWR 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 33291 SO pin can be used to determine what kind of
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 33291 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, when calling for the output to be programmed on,
the next word command entered into the corresponding bit
returns with a logic high on SO. An output overcurrent fault
would be indicated. An overcurrent fault is always reported by
the SO output and is independent of the logic state existing
on the SFPD pin. When the SFPD pin is in a logic high state,
an overcurrent condition will be reported on the SO pin.
However, limiting output current is in effect and the output is
permitted to operate if the overcurrent condition does not
drive output into an overtemperature fault. An
overtemperature fault will shut down the specific output
effected for the duration of the overtemperature condition.
Overcurrent and overtemperature faults are often related.
Turning the effected output switches OFF and waiting for
some time to allow the output to cool down should make
these types of faults go away. Soft overcurrent faults can
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Freescale Semiconductor