33291L datasheet(19/26 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33291L

FUNCTIONAL DESCRIPTION

Grounding the SFPD pin will enable the short fault

protection shutdown circuitry. Consider a load short (output

short to supply) occurring on an output before, during, and

after output turn ON. When the CS signal rises to the high

logic state, the corresponding output is turned ON and a

delay timer is activated. The duration of the delay timer is

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.

If the SFPD pin is at 5.0 V (or VDD), an output will not be

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.

Each output of the 33291L 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 (VTHRES) will be exceeded.

Short fault detection is accomplished by sensing the

output voltage and comparing it to VThres. The lowest VThres

requires a voltage of 2.5 V to be sensed. For an enabled

output, with VDD = 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 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.

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

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 VDD condition will result in the global

shutdown of all outputs. The undervoltage threshold is

between 2.5 V and 3.5 V. When VDD 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 VPWR 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 VPWR. Proper operation at VPWR

voltages below 5.5 V are not be guaranteed.

DECIPHERING FAULT TYPE

The 33291L 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 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.

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.

33291L Datasheet(PDF) 19 Page - Freescale Semiconductor, Inc

33291L Datasheet(HTML) 19 Page - Freescale Semiconductor, Inc

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