FUNCTIONAL DEVICE OPERATION
PROTECTION AND DIAGNOSTIC FEATURES
resistance limiting the internal clamp current according to the
specification.
Table 5 summarizes the various methods for resetting the
device from the latched Fail-safe Mode.
The Watchdog timeout is a multiple of an internal oscillator
and is specified in the Table 15, page 24. As long as the WD
bit (D15) of an incoming SPI message is toggled within the
minimum watchdog timeout period (WDTO), based on the
programmed value of the WDR, the device will operate
normally. If an internal watchdog timeout occurs before the
WD bit, the device will revert to a Fail-safe mode until the
device is reinitialized.
If the FSI pin is tied to GND, the Watchdog fail-safe
operation is disabled.
LOSS OF V
DD
If the external 5.0 V supply is not within specification, or
even disconnected, all register content is reset. The outputs
can still be driven by the direct inputs IN0:IN3. The 33874
uses the battery input to power the output MOSFET-related
current sense circuitry and any other internal logic providing
fail-safe device operation with no VDD supplied. In this state,
During the Fail-safe Mode, the outputs will be ON or OFF
depending upon the resistor RFS connected to the FSI pin,
regardless of the state of the various direct inputs and modes
(Table 6).
the watchdog, under-voltage, over-voltage, over-temperature
(latched) and over-current circuitry are fully operational with
default values.
Table 6. Output State During Fail-safe Mode
RFS (kΩ)
High Side State
FAULT MODE
0 (shorted to ground)
Fail-safe Mode Disabled
All HS OFF
This 33874 indicates the faults below as they occur by
driving the FS pin to logic [0]:
6.0
15
• Over-temperature fault
HS0 ON
• Over-voltage and under-voltage fault
• Open load fault
• Over-current fault (high and low)
HS1:HS3 OFF
30 (open)
HS0 and HS2 ON
HS1 and HS3 OFF
The FS pin will automatically return to logic [1] when the
fault condition is removed, except for overcurrent,
overtemperature (in case of latching configuration) and in
some cases of undervoltage.
In the Fail-safe Mode, the SPI register content is retained
except for over-current high and low detection levels, timing
and latched over-temperature which are reset to their default
value (SOCL, SOCH, and OCTL and OT_latch_[0:3] bits).
Then the watchdog, over-voltage, over-temperature, and
over-current circuitry (with default value) are fully operational.
The FS pin reports all faults. For latched faults, this pin is
reset by a new Switch ON command (via the SPI or direct
input IN).
The Fail-safe Mode can be detected by monitoring the
WDTO bit D2 of the WD register. This bit is logic [1] when the
device is in Fail-safe Mode. The device can be brought out of
the Fail-safe Mode by transitioning the WAKE and RST pins
from logic [1] to logic [0] or forcing the FSI pin to logic [0].
Fault information is retained in the fault register and is
available (and reset) via the SO pin during the first valid SPI
communication (refer to Table 17, page 26).
PROTECTION AND DIAGNOSTIC FEATURES
• in Fail-safe Mode: the FSI input must be grounded and
then set to its nominal voltage to switch ON the outputs.
OVER-TEMPERATURE FAULT (LATCHING OR
NON-LATCHING)
The over-temperature fault (one for each output) is
reported by SPI. If the over-temperature is latched, the SPI
reports OTF_s = [1] and OCLF_s = [1]. In case of non-
latched, OTF_s = [1] only is reported.
The 33874 incorporates over-temperature detection and
shutdown circuitry for each output structure.
The over-temperature is latched per default and can be
unlatched through SPI with OT_latch_[0:3] bits.
The fault bits will be cleared in the status register after either
a valid SPI read command or a power on reset of the device.
An over-temperature fault condition results in turning OFF
the corresponding output. To remove the fault and be able to
turn ON again the outputs, the failure must be removed and:
• in Normal Mode: the corresponding output must be
commanded OFF and ON again in case of over-
OVERCURRENT FAULT (LATCHING)
The 33874 has eight programmable over-current low
detection levels (IOCL) and two programmable over-current
temperature latched (OT_latch bit = 0).
• in Normal Mode: the corresponding output turns ON
automatically if the temperature is below TSD-TSD(HYS) in
case of unlatched over-temperature (OT_latch bit = 1).
high detection levels (IOCH) for maximum device protection.
The two selectable, simultaneously active over-current
33874
Analog Integrated Circuit Device Data
Freescale Semiconductor
19
FREESCALE [ Freescale ]
