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Appendix A: Hardware Reference Manual
10.4.2 Small Engine Control IC
10.4.2.1
Functional Description
The Small Engine Control IC (SECIC) has the ability to control a core set of loads specific to a small
engine. For the KIT33812ECUEVME, a single cylinder application is targeted with the following loads
identified as critical: fuel injector driver, ignition coil pre-driver, system voltage regulator, relay driver, lamp
driver, system watchdog, voltage reset generator, and diagnostic communication physical layer. Control
signals from the MCU are interpreted by the SECIC to drive the critical loads for the small engine system.
Fuel delivery through a fuel injector is performed by direct connection to the injector driver. Spark control
for a transistor controlled, inductive (TCI) ignition is accomplished through a pre-driver output paired with
an external IGBT. System voltage is created by an integrated 5V pre-regulator that uses an external
pass-transistor to off-load power dissipation. Relay and lamp driver output directly control loads. An
optional system watchdog is integrated with a flexible programming time and optional hardware disable.
Power on control and system voltage monitoring is integrated and capable of providing a system reset for
loss of power and brownout conditions. Communication to external electronic modules, such as a
diagnostic tool, is provided by a single wire ISO-9141 transceiver.
10.4.2.2
10.4.2.2.1
5 Volt Regulator Pre-driver
Design Criteria
This functional block steps down the system battery voltage to the 5 volt level required by the MCU,
sensors, and other circuit elements. The VCCREF pin provides the base current drive output for an
external PNP pass transistor (FZT753SMD recommended). The output current is typically 5.0 mA and it
current limits at typically 20 mA. Through the control of the pass transistor, a 5.0 V system voltage is
maintained. This 5.0 V output must provide enough current for the internal components of the ECU and
allow for some additional current for external components such as sensors. The 5.0 V supply must also
provide high tolerance so that it can be used as a reference for analog signal measurements.
10.4.2.2.2
Implementation Recommendations
The recommended PNP pass transistor has a minimum beta of 50 at 2.0 amps and could easily provide
up to 2.0 amps output if it were not for the maximum power dissipation rating of the package which limits
it to 2.0 Watts. This can be optimized for an application by taking into account temperature and voltage
requirements and selecting a pass transistor with appropriate beta and power dissipation specifications.
With a typical battery input voltage of 14 V, the maximum current the voltage regulator can supply using
the FZT753SMD, and still be within the 2 Watt rating of the transistor, is 222 mA. If more current were
needed, a larger package PNP transistor could be employed. Additionally protection of the 5.0 V can be
implemented using a current regulating circuit instead of a single PNP transistor. While this adds
components, it allows conditions such as short to ground to reduce the impact of the fault condition.
REVO hardware for the reference design uses the FZT789A, which has a higher beta of 175 at 2.0 amps.
Future revisions may make use of the lower gain of the FZT753SMD for lower sensitivity.
The VCCSENS pin is the feedback input for the voltage regulator. It ensures that the output of the pass
transistor remains at a constant 5 Volts even though the load current required is changing. The
VCCSENSE pin requires a capacitor to ground to maintain stability of the regulator. The minimum
recommended value of this capacitor is 2.2 μF. but the user can choose to increase this value depending
on the ripple voltage other requirements, such as loss of power hold time. The MC33812 requires a
reverse battery and transient voltage protected supply. A simple silicon diode in series with the VPWR pin
can protect the circuit against an inadvertent reverse connected battery. For transient voltage protection,
a TVS (transient voltage suppressor) diode from the VPWR pin to ground is recommended. Also the
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Freescale Semiconductor
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