33811 Datasheet, PDF (14/18 Pages) Freescale Semiconductor, Inc – Solenoid Monitor Integrated Circuit (IC)

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
3) Step 3 demonstrates an 8 bit SPI transfer. The same
exchange is performed, however, only 8 SCLK pulses are
issued and only 8 bits of data are exchanged.
4) When the transfer is complete, only the eight bits of
solenoid status has been transferred to the MCU. The data in
the 33811’s SPI register lower 8 bits has been overwritten
with the data from the MCU’s SPI register.
5) Step 5 shows the same scenario as step 3 however,
before the transfer, the RESET pin is brought to a logic 1.
This causes all data out of the SO pin to be a logical 1.
6) This step shows the contents of the MCU SPI register
after the transfer. All bits are logical 1 because the RESET
pin was held high for the duration of the transfer
WAVEFORM DETECTION ALGORITHM
Three Stage Current Waveform
An operational solenoid, once activated, produces a
current waveform that consists of three distinct regions. The
three regions are categorized by their relationship to a “dip”
in the current that occurs when the solenoid armature moves
within the coil. The regions are labeled the “Pre-Dip”, “Dip”,
and “Post-Dip” regions. At this point, it should be noted that
the 33811 does not monitor this current directly. It monitors
the voltage across the low-side MOSFET driver. When the
MOSFET is turned on, it can be thought of, to a first
approximation, as a resistor with value R_DS. Hence, any
current variation through the solenoid, appears on the
MOSFET drain, as a voltage variation, as is predicted by
Ohm’s law. The 33811 is designed to monitor the voltage
across the MOSFET and determine if the solenoid is
operational or faulty based on the voltage waveform that is
produced.
Activation of the Solenoid
If the solenoid is not activated, the low side MOSFET
driver is turned off, so almost the entire supply voltage
appears across the MOSFET. When the MOSFET is
activated, the voltage across it drops from the supply voltage
to a voltage that depends on the instantaneous current flow
through the solenoid and the R_DS of the MOSFET. This
dramatic voltage swing from the supply voltage, to near
ground, triggers a timer in the 33811 IC. The time value of this
timer is labeled T_BEGIN and is 400 to 600µs in duration. If the
voltage is still close to ground after T_BEGIN, then the solenoid
is deemed to be activated and the waveform detection
algorithm is started.
The PRE-DIP Region
After dropping to near ground, the voltage across the
MOSFET starts to increase as the current through the
solenoid begins to increase. The “Pre-Dip” region consists of
a positive slope region, leading to a voltage maxima or peak,
followed by a negative slope region. The 33811 monitors this
voltage ramp up by sampling the voltage every 72µs and
comparing it to the previous sample. If the new sample
exceeds the previous sample by 0.5mV or more, the
sampling comparator’s output is auto-zeroed to the new
voltage level by adjusting the reference voltage to the input
voltage and the sampling continues. At some point the
voltage will reach a peak and the slope of the voltage curve
will turn from positive to negative. The 33811 will continue
sampling the voltage as it begins to descend but will not auto-
zero the comparator until at least three consecutive samples
of 3mV in magnitude have been detected.
The DIP Region
Once the three descending samples have been detected,
sampling and auto-zeroing will continue to try to determine
the next inflection point. This next inflection point will be the
Dip which is caused by the successful travel of the solenoid’s
armature. If a Dip is not discovered within the total time
window of 56 mS. then the solenoid will be said to be faulty
and the appropriate SPI register fault bit will be set to a logic
1. If the inflection point is discovered then sampling will
continue.
The Post-DIP Region
After the Dip has occurred the waveform detection
algorithm will continue sampling the voltage for the remainder
of the 56mS. time window. If the voltage is still increasing
after three sample times, then the solenoid is deemed to be
operational and the appropriate SPI register fault bit is
cleared to a logic 0. The waveform detection logic is then
reset back to a state where it look for the next solenoid
activation event.
33811
14
Analog Integrated Circuit Device Data
Freescale Semiconductor