33810_10 Datasheet, PDF (20/35 Pages) Freescale Semiconductor, Inc – Automotive Engine Control IC

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
• Max dwell time
Soft Shutdown is designed to prevent an ignition spark
while turning off the external IGBT. The Low-voltage Clamp
is activated to provide the mechanism for a soft shutdown.
GAIN SELECT BIT
The ignition coil current comparators are used to compare
the programmed NOMI and MAXI DAC value with voltage
across the external current sense resistor. When selecting a
gain of two, the ignition coil current sense resistor must be
reduced from 40mΩ to 20mΩ.
OVERLAPPING DWELL ENABLE BIT
Overlapping dwell occurs when two or more ignition mode
drivers are commanded ON at the same time. In this
condition, with the Overlapping Dwell Bit enabled the MAXI
DAC threshold value is increased as a percentage of the
nominal programmed value. The percent increase is
determined by bit 5 through bit 7 of the DAC Command.
Table 9. Overlapping Dwell Compensation
DAC Command
Bits<b7,b6,b5>
000
Overlap Compensation
(%)
0%
001
7%
010
15%
011
24%
100
35% (default)
101
47%
110
63%
111
80%
MAXIMUM DWELL ENABLE BIT
Bit 8, the Maximum Dwell Enable bit allows the user to
enable the Maximum Dwell Gate Turnoff Feature. When the
Max Dwell bit is programmed as logic 0 (disabled) the device
will not perform a Low-voltage Clamp due to Max Dwell (See
Figure 9).
MAXIMUM DWELL GATE TURN OFF FEATURE
In automotive ignition systems, dwell time is defined as the
duration of time that an ignition coil is allowed to charge. The
MC33810 starts the measure of time from the gate drive ON
command. If the dwell time is greater than the Max Dwell
Timer setting (Table 10), the offending ignition gate driver is
commanded OFF. The Max Dwell Gate Turn Off Feature may
be disabled via bit 8 of the Spark Command. When the
feature is disabled, the Max Dwell fault bits are always logic
0. The Max Dwell Timer feature pertains to Ignition Mode only
and does not affect gate drivers configured as general
purpose gate drivers.
The Max Dwell gate turn off signal is a logically ANDed
with the Soft Shutdown bit to activate a Low-voltage Active
Clamp (See Figure 9).
Table 10. Maximum Dwell Timer
Spark Command
Bit<b11,b10,b9>
000
001
010
011
100
101
110
111
MAX Dwell Timer
MaxDwell (ms)
2
4
8
16
32 (default)
64
64
64
DAC COMMAND (DIGITAL TO ANALOG
CONVERSION COMMAND)
The DAC Command is an ignition mode command that
sets the nominal ignition coil current (NOMI) and maximum
ignition coil current (MAXI) DAC values. Bits 0 through 4 set
the NOMI threshold value and, bits 8 through 11 set the MAXI
threshold values. The DAC command and default values are
listed in the SPI Command Summary Table 20. The NOMI
output is used by the MCU as a variable in dwell and spark
control algorithms.
NOMI DAC BITS
The NOMI output signal is generated by comparing the
external current sense resistor differential voltage (Resistor
Sense Positive, Resistor Sense Negative) with the SPI
programmed NOMI DAC value. When the NOMI event
occurs, the NOMI output pin is asserted (High). The NOMI
output is only a flag to the MCU and it’s output does not affect
the gate driver.
When using a 20 mΩ resistor as the current sense resistor,
the gain select of the differential amplifier connected to RSP
and RSN, should be set to a gain of 2, via the SPI Command
Message Spark Command (Command 0100, hex 4), Control
bit 6 =1.
When using a 40mΩ resistor as the current sense resistor,
the gain select of the differential amplifier connected to RSP
and RSN, should be set to a gain of 1, via the SPI Command
Message Spark Command (Command 0100, hex 4), Control
bit 6 =0. This is also the default value.
The NOMI output provides a means to alert the MCU when
the ignition coil primary current equals the value programmed
into the NOMI DAC.
In V10 Mode, the NOMI pin is reconfigured as a MAXI
input pin from a third MC33810 device in the system. In this
mode a NOMI input has effectively the same control as an
internal MAXI signal. Further information is provided in the
V10 Mode application section of this data sheet.
33810
20
Analog Integrated Circuit Device Data
Freescale Semiconductor