Timing control circuit

#The invention relates generally to timing control circuits and in particular to a timing control circuit providing a wide dynamic range. Timing circuits are used in a number of applications such as pulse width modulation (PWM), voltage controlled oscillators (VCO), etc. FIG. 1 is a schematic diagram of an existing timing circuit 10 used in a PWM application. As known in the art, a PWM signal is a series of pulses. A controllable current source 12 is used to charge a capacitor 14. When the capacitor 14 reaches a certain voltage, established by a reference voltage applied to the inverting terminal of operational amplifier 16, operational amplifier 16 changes states causing a pulse to end.

#A drawback to the circuit of FIG. 1 is that it is difficult to provide a large dynamic range. For a given value of capacitor 14, a very large current is needed to generate narrow pulse widths (i.e., rapid charging of capacitor 14). On the other hand, very small current is needed to provide wide pulse widths (i.e., slow charging of capacitor 14). Unfortunately, controlling small currents can be difficult leading to unstable, wide pulse widths.

#If capacitor 14 is increased in value to allow larger currents to be used for larger pulse widths, this creates a detriment in generating narrow pulse widths. If capacitance of capacitor 14 is increased, then larger currents are need to generate small pulse widths. Thus, it is difficult to provide a wide dynamic range pulse widths with the existing timing circuit.

#Referring now to the drawings, wherein like elements are numbered alike in the several FIGURES:

#FIG. 1 is a schematic diagram of a conventional timing circuit;

#FIG. 2 is a schematic diagram of a timing circuit in an embodiment of the invention;

#FIG. 2 is a schematic diagram of a timing circuit 100 in an embodiment of the invention. The circuit includes a controllable current source 102 which charges capacitor 104 and charges capacitor 110 through resistor 108. Capacitor 104 is in parallel with series connected resistor 108 and capacitor 110. One terminal of capacitor 104 is connected to a first terminal of a trigger circuit 106 and the other terminal is connected to ground. In the embodiment of FIG. 2, the trigger circuit 106 is implemented using an operational amplifier but it is understood that other devices may be used. The output of trigger circuit 106 may be applied to a signal generator in various applications such as PWM and VCO to trigger the end of a pulse. A reset terminal 110 provides for discharge of capacitors 104 and 110 through diodes 105 and 111, respectively.

#One terminal of resistor 108 is connected to the first terminal of trigger circuit 106 and one terminal of capacitor 110 is connected to ground. Thus, the charge across capacitors 104 and 110 dictate the voltage at the first terminal of trigger circuit 106. When the voltage on the first terminal of trigger circuit 106 reaches a threshold, the trigger circuit changes states thereby ending the pulse. The threshold voltage is established by a reference voltage applied to the second terminal of the trigger circuit.

#By selecting appropriate values for capacitor 104, capacitor 110 and resistor 108, a wide dynamic range may be achieved. In general, capacitor 104 should have a capacitance lower than the capacitance of capacitor 110. In an exemplary embodiment of the invention, capacitor 104 is 100 pf, capacitor 110 is 4700 pf and resistor 108 is 10 k ohms. Accordingly, when a small pulse width is desired, a large current is applied. Resistor 108 limits the charge of capacitor 110 and the effective capacitance at the first terminal is substantially capacitor 104. Capacitor 104 reaches the reference voltage quickly causing trigger circuit 106 to change states and produce a narrow pulse. When a large pulse width is desired, a small current is applied which resistor 108 does not significantly impede. Thus, both capacitor 104 and capacitor 110 may be charged by smaller currents. This renders the effective capacitance the parallel combination of capacitor 104 and capacitor 110. A longer time is needed to charge capacitor 104 and capacitor 110 to the reference voltage resulting in a wide pulse width. Accordingly, more current may be used to achieve wide pulse widths as compared to using just capacitor 104 alone. This avoids using diminutive currents that are difficult to control.