Abstract:
A one-shot current circuit generates a current for a desired period during an input signal transition, the desired period during an input signal transition being proportional to the edge rate of the input signal. The circuit includes a MOS transistor device which selectively conducts current between an input terminal and a current generating circuit. The current generating circuit can be a bipolar transistor having its base coupled to the input terminal and a main current path between a circuit output and a supply voltage.

Description:
This is a continuation of application Ser. No. 08,565,696, filed Nov. 30, 1995 which is now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention is directed to a current one-shot circuit and more particularly to a BiCMOS current one-shot circuit. 
     A conventional current one-shot circuit is shown in FIG. 1. Current one-shot circuit 100 includes an input terminal 10, a capacitor C1, a resistor R1, and a bipolar transistor Q1. The collector of transistor Q1 is coupled to a first supply voltage V cc  and the emitter of transistor Q1 is coupled to a second supply voltage 14, for example, ground. Resistor R1 is coupled in series with a capacitor C1 between second supply voltage 14 and the base of transistor Q1. Capacitor C1 is coupled between resistor R1 and an input voltage terminal 10. Under static conditions, the base of transistor Q1 is held to ground through resistor R1. However, when an input voltage, V in , makes a low-to-high transition, the base voltage of transition Q1 is pulled up through capacitor C1. If V in  rises far enough and fast enough so that the base of Q1 is pulled up to about 0.7 volts, transistor Q1 turns on and pulls current through its collector. Transistor Q1 remains on until resistor R1 discharges the base voltage below about 0.7 volts. Accordingly, transistor Q1 provides a current &#34;one-shot&#34; during each low-to-high transition of V in . 
     There are several problems associated with the above current one-shot circuit. First, the RC time constant which controls the duration of the current one-shot is not linear. In fact, the relationship of the current duration to the input voltage is an inverse non-linear relationship whereby a shorter edge rate causes a longer one-shot duration. Also, the RC based current one-shot circuit requires a significant amount of silicon. Accordingly, it is desirable to provide a current one-shot circuit which requires less silicon and varies linearly with the input signal. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a BiCMOS current one-shot circuit. It is another object of the invention to provide a current one-shot circuit which utilizes less silicon than conventional current one-shot circuits. 
     It is yet a further object of the invention to provide a current one-shot circuit which proportionally tracks the input voltage. 
     In a preferred embodiment of the invention, a current one-shot is provided which includes an MOS transistor network to selectively provide a current path between the input terminal and the base of a bipolar transistor in order to selectively conduct current through the bipolar transistor. 
     Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. 
     The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the invention, reference is had to the following description taken in connection with accompanying drawings, in which: 
     FIG. 1 shows a current one-shot circuit in accordance with the prior art; 
     FIG. 2 shows a current one-shot circuit in accordance with a first embodiment of the invention; 
     FIG. 3 shows a current one-shot circuit in accordance with a second embodiment of the invention; 
     FIG. 4 shows show a current one-shot circuit in accordance with a third embodiment of the invention; 
     FIG. 5 shows a current one-shot circuit in accordance with a fourth embodiment of the invention; 
     FIG. 6 shows a current one-shot circuit in accordance with a fifth embodiment of the invention; and 
     FIG. 7 shows a current one-shot circuit in accordance with a sixth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is had to FIG. 2 which shows a current one-shot circuit 200. One shot circuit 200 includes an input terminal 202, an inverter U1, NMOS transistors N1 and N2 and a bipolar transistor Q2. Input terminal 202 is coupled to an input of inverter U1 and a control electrode of transistor N2. Transistor N1 has a first conducting electrode coupled to input terminal 202, a second conducting electrode coupled to the base of transistor Q2 and a control electrode coupled to an output of inverter U1. Transistor N2 has a first conducting electrode coupled to the base of transistor Q2 and a second conducting electrode coupled to a second supply voltage 214, for example, ground. Transistor Q2 has an emitter coupled to second supply voltage 214 and a collector coupled to an output terminal 216. In a static environment when V in , is either high or low, the base of transistor Q2 is discharged to ground or to V in  (low) via transistors N2 and N1, respectively. When V in  is high, transistor N1 is off, the output of inverter U1 is low and transistor N2 is on. When V in  is at a logic low level, transistor N1 is on, the output of inverter U1 is high and transistor N2 is off. When V in  undergoes a low-to-high transition, transistor N1 provides a conductive path from input terminal 202 to the base of transistor Q2. When V in  equals about 0.7 volts, transistor Q2 begins to turn on and the voltage at the base of transistor Q2 is clamped a diode drop above ground. As V in  continues its low-to-high transition, both transistors N1 and N2 (which starts to turn on as soon as V in  exceeds the NMOS threshold voltage) conduct more current. The sizing of transistors N1 and N2 is set so that the desired proportion of current enters the base of Q2. In a preferred embodiment, transistor N1 is generally larger than transistor N2. 
     When V in  exceeds the threshold voltage of the inverter U1, the output of inverter U1 goes low, turning transistor N1 off. Transistor N2 then discharges the base of Q2 to ground, turning transistor Q2 off. Thus, during this low-to-high transition, transistor Q2 produces a one-shot current. 
     Another embodiment of the invention is shown in FIG. 3. Current one-shot circuit 300 operates during the high-to-low transition of V in . Current one-shot circuit 300 includes an input terminal 302, PMOS transistors P1 and P2, an inverter U2 and a bipolar transistor Q3. Input terminal 302 is coupled to an input of an inverter U2 and to the control electrode of transistor P1. Transistor PI has a first conducting electrode coupled to a first supply voltage V cc  and a second conducting electrode coupled to the base of transistor Q3. Transistor P2 has a first conducting electrode coupled to the base of transistor Q3, a second conducting electrode coupled to input terminal 302. Transistor Q3 has an emitter coupled to first supply voltage V cc  and a collector coupled to an output terminal 316. 
     In a static environment, when V in  is either low or high, the base of transistor Q3 is held to V cc  or V in  and is off. When V in  is high, transistor P1 is off, the output of inverter U2 is low and transistor P2 is on causing the input voltage V in  to appear at the base of transistor Q3. 
     When V in  is low, transistor P1 is on, the output of inverter U2 is high and transistor P2 is off, thus causing V cc  to appear at the base of transistor Q3. 
     When V in  begins a high-to-low transition, transistor P2 provides a conductive path between the input terminal 302 and the base of transistor Q3. Q3 begins to turn on when V in  =V cc  -0.7, and the voltage at the base of transistor Q3 is clamped to a diode drop below V cc . As V in  continues its transition, transistors P1 and P2 continue to conduct more current since transistor P1 turns on when V in  falls below its threshold voltage. 
     When V in  falls below the threshold of inverter U2, the output of inverter U2 goes high, turning off transistor P2. Transistor P1 turns on, discharging the base of transistor Q3 to V cc , turning transistor Q3 off. Thus, during this high-to-low transition, transistor Q3 produces a one-shot current. 
     Reference is now had to FIG. 4 which shows a current one-shot circuit in accordance with a third embodiment of the invention. Current one-shot circuit 400 is essentially the same circuit as that of FIG. 2 except that a resistor R2 is substituted for transistor N2 to provide the base pull-off of transistor Q3. 
     Likewise, FIG. 5 shows a current one-shot circuit 500 which is essentially the same circuit as that shown in FIG. 2 except that the control electrode of transistor N2 is coupled directly to first supply voltage V cc  to provide the base pull-off of Q3. 
     Reference is now had to FIG. 6 which shows a current one-shot circuit 600 which is similar to the one-shot circuit shown in FIG. 2 except that the control electrode of transistor N1 is coupled to an external voltage terminal 610 for receiving external voltage signal V ext . FIG. 7, similarly, shows a circuit 700 similar to the circuit of FIG. 3 with an external voltage signal V ext . 
     It will thus be seen that the objects set forth above among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.