I. Field of Invention
The present invention is directed to a trigger circuit, and more particularly to a Schmitt trigger circuit that anticipates a voltage crossing.
II. Description of the Related Art
A Schmitt trigger is a class of triggering circuit. Among other things, the Schmitt trigger serves as a signal conditioning device or as a threshold detector for switching purposes. As a signal conditioner, the Schmitt trigger takes an input voltage signal, for example a noisy input signal, and conditions the input voltage signal by limiting the output voltage signal to a logic-high or a logic-low. As a threshold detector, the Schmitt trigger may be used as a switch that is activated when an input signal crosses zero volts.
Generally, a conventional Schmitt trigger circuit changes output voltage level in response to a change in the input voltage. For example, a conventional Schmitt trigger changes output states from a logic-high to a logic-low after the input voltage makes a zero crossing and then reaches a first threshold voltage and changes output states from a logic-low to a logic-high after the input voltage makes another zero crossing and then reaches a second threshold voltage. FIG. 9 illustrates an exemplary wiring scheme for a conventional Schmitt trigger circuit of the non-inverting type.
Referring to FIG. 9, the conventional Schmitt trigger circuit 1000 includes a resistor (R1) 1010, a feed-back resistor (R2) 1015, an operational amplifier 1020, a voltage source (Vsource) 1005, and a voltage output (Vout) 1030.
Hysteresis diagrams (also referred to as voltage transfer curves) illustrate the operation of Schmitt trigger circuits by indicating when a Schmitt trigger changes logic states at the output (e.g., Vout) in response to a change in the source voltage (Vsource). The hysteresis diagram for a conventional Schmitt trigger of the non-inverting type is shown at FIG. 4. The hysteresis drawing for a conventional Schmitt trigger of the inverting type is also shown at FIG. 7.
Referring to FIGS. 4 and 9, the conventional Schmitt trigger circuit of FIG. 9 has two threshold voltages. As the source voltage (Vsource) 1005 approaches the first threshold (V1T) voltage, the source voltage (Vsource) 1005 increase in value as it crosses zero. During this time, the voltage output (Vout) 1030 is at a low voltage state (VL)(see, e.g., FIG. 4 at 420, 450). When the source voltage (Vsource) 1005 reaches the first threshold voltage (V1T), the voltage output (Vout) 1030 transitions to a high output voltage state (VH) (see, e.g., FIG. 4 at 460 and 470).
Alternatively, when the source voltage (Vsource) 1005 begins to decrease in value (see, e.g., FIG. 4 at 480), the voltage output (Vout) 1030 remains at a high voltage state (FIG. 4 at 440) until the source voltage (Vsource) 1005 crosses zero and reaches the second threshold voltage (V2T). When the source voltage (Vsource) 1005 reaches the second threshold voltage (V2T) (see, e.g., FIG. 4 at 430), the voltage output (Vout) 1030 transitions to a low voltage state (VL) (see, e.g., FIG. 4 at 430 and 410). Accordingly, the voltage output (Vout) 1030 of a conventional Schmitt trigger circuit changes output voltage states only after the source voltage (Vsource) 1005 crosses zero and reaches the appropriate threshold voltage (e.g., V2T, V1T).
Accordingly, the present invention is directed to a Schmitt trigger circuit, preferably an anticipatory Schmitt trigger circuit that transitions the output voltage in anticipation of the input voltage crossing a predetermined value.
An object of the present invention is to provide an improved Schmitt trigger that overcomes or minimizes one or more of the limitations and disadvantages of the presently available Schmitt trigger circuits. Among other things, the invention switches the output value in anticipation of the input voltage crossing a predetermined value (e.g., zero volts). By triggering or switching the anticipatory Schmitt trigger circuit in anticipation of crossing a predetermined value, the present invention overcomes one or more of the limitations of past and related approaches.
Additional features and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and advantages of the invention may be realized and attained by the system and method particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a trigger circuit including, for example, a comparator; a feedback resistor connected to the comparator; an input resistor connected to the comparator, to the feedback resistor, and to a voltage source; a pull-up resistor connected to the feedback resistor and to the output of the comparator; a plurality of voltage divider resistors, wherein at least one of the plurality of voltage divider resistors is connected to the output of the comparator and to an input of the comparator; and a capacitor connected in parallel to at least one of the plurality of voltage divider resistors.
In another embodiment of the present invention, there is provided a trigger circuit including a comparator with a plurality of inputs and an output; an input resistor connecting one of the inputs of the comparator with an input voltage; a feedback resistor connecting one of the inputs of the comparator to a pull-up resistor and connecting one of the inputs of the comparator to the output of the comparator; and a first voltage divider resistor connected to a capacitor, to one of the inputs of the comparator, and to a second voltage divider resistor, wherein the second voltage divider resistor further connects to the output of the comparator, wherein the product of the resistance values for the first voltage divider resistor and the feedback resistor exceeds the product of the resistance values for the input resistor and the second voltage divider resistor when the trigger circuit is of the non-inverting type.
In yet another embodiment of the present invention, there is provided a method of triggering a circuit including, for example, switching the output voltage from a high value to a low value, when an input to the trigger circuit decreases from a first value and when the input voltage crosses an upper threshold voltage, wherein the switching occurs in anticipation of crossing a predetermined voltage value; and changing the output voltage from the low value to the high value, when the input to the trigger circuit increases from a second value and when the input voltage crosses a lower threshold voltage, wherein the changing occurs in anticipation of crossing the predetermined voltage value.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as described.