Many existing vehicular control modules include an analog driver circuit, frequently referred to as the variable force solenoid (VFS) driver circuit, used in conjunction with automatic transmission variable force solenoids. In some embodiments, the VFS driver is a separate analog integrated circuit located apart from the digital input/output (I/O) chip. As is known, the analog driver circuit essentially provides a variable duty cycle output signal to the solenoid, in response to a binary command word from a microprocessor. For example, the pulse width modulated (PWM) signal may have a range of 5-15 kHz with about 5%-95% duty cycle. Typically, the duty cycle is 5%-55%.
Fixed PWM has associated problems, because of hysteresis effects in the solenoid. To remedy this problem, and to improve linearity, a "dither" signal is added to the PWM signal. For example, the VFS driver circuit typically adds a low frequency (e.g. 200 Hz) signal that is approximately 10% of the desired solenoid current.
U.S. Pat. No. 5,079,498, issued to Cleasby et al., discloses a combination analog/digital (i.e. hybrid) PWM generator with dither for current control. The reference teaches the use of analog circuits and digital circuits to generate a square wave PWM voltage drive to a load. The resulting current is sensed and a corresponding voltage is fedback and compared with two zone boundary voltage levels, which are spaced equally above and below a signal representing the desired output current. Based on the comparison (i.e. when the voltage feedback reaches either boundary level), a switching circuit is switched from ON to OFF. When dither is incorporated, the zone width is based on stored half-zone width values which are selected based on the dither phase. The Cleasby et al. reference however, does not teach use of an entirely digital circuit to generate a dither signal that is proportional to the desired output signal.
As future generation vehicular control modules are developed and control of the vehicle operation becomes more precise, the number of VFS driver circuits required is expected to increase. Additionally, the trend in future generation controllers is to employ all digital I/O chips. Continued use of conventional analog solenoid drivers, and hybrid drivers such as that described in the '498 reference, will present problems due to the difficulty of mixing analog and digital technology on the same silicon. If a conventional digital PWM channel is used to drive the VFS solenoid with the dither being provided by the microprocessor, the microprocessor will be burdened with a large overhead, considering the 200 Hz modulation and number of channels required to drive the solenoids.
It is, therefore, desirable to utilize a system which provides a PWM signal implemented entirely in digital circuitry and generated without microprocessor intervention. It is also desirable to utilize a system which provides a PWM signal with low frequency modulation dither that is proportional to the desired output signal.