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CA3228E Datasheet(PDF) 7 Page - Harris Corporation |
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CA3228E Datasheet(HTML) 7 Page - Harris Corporation |
7 / 9 page 10-37 CA3228 to be able to supply vacuum, the gate output must be low. If the output from the Control Amplifier exceeds 0.573VCC, vacuum is supplied to the servo unit. If the output of the Control Amplifier is between 0.573VCC and 0.427VCC the vacuum is held in the servo unit and vehicle speed is maintained. If the output from the Control Amplifier drops below 0.427VCC or if the gate output is high, the servo unit vacuum is vented. Overspeed Detector Comparator The Overspeed Detector circuit is used when the following sequence of events occur: A speed is set in memory, the vehicle is manually accelerated (foot pedal) to a higher speed and then the ACCEL switch is activated. During vehicle acceleration VS voltage is greater than the VM voltage into the memory update comparator. When the ACCEL command is given, the capacitor at pin 15 rapidly charges to within 60mV of VS before switching the compara- tor output low and starting the fixed acceleration rate from the present vehicle speed. The 60mV of offset is required to insure that the output of the overspeed detector is low under normal operating conditions. Hysteresis is also designed into the comparator to eliminate noise problems which may prevent the chip from going into the Acceleration mode. End of Resume Comparator The Resume Comparator is used when the following sequence of events occurs: A speed is set in memory, the brake applied, causing the vehicle to go to a lower speed, and the RESUME switch is activated. Activation of the RESUME switch causes a fixed accelera- tion rate from the lower speed until the capacitor voltage at pin 15 is equal to the VM voltage. A filter circuit contained in the output of the resume comparator insures that noise doesn’t reset the comparator until VPIN actually equals VM. Align Voltage Source (Pin 17) The Align Voltage Source is a X1 buffer with an output of 0.5VCC. Brake Input Comparator (Pin 12) When the Brake Input exceeds 0.55VCC, the chip will go into the Standby mode from Cruise. Minimum Speed Lockout Assures that the system remains in a Standby mode if vehicle speed VS is below 0.183VCC. It causes the system to revert to the Standby mode if VS drops below 0.183VCC in the Cruise mode. Digital Filter for Redundant Brake and Minimum Speed Lockout A 4 bit shift register with an all ‘1’s output decode is used to filter transients and electromagnetic interference. The filter prevents false signals from putting the system into Standby from Cruise. Ramp Oscillator (Pin 5) The Ramp Oscillator at pin 5 nominally varies between amplitudes of 4.1V and 6.1V. The discharge rate is approximately 4X the charge rate. With a capacitor of 0.001 µF on pin 5, the nominal oscillator frequency is 50kHz. |
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