IGFET Clock Generator Circuit Employing MOS Boatstrap Capacitive Drive

This invention relates to semiconductor circuits and more particularly to a delay circuit of the type useful clock generators in semiconductor memory devices or the like.

In semiconductor memory devices of the type described in Electronics, Sept. 26, 1978, pp. 109+, or pending application Ser. No. 944,822, and now U.S. Pat. No. 4,239,993, clock generators are employed which have output driver circuits for driving capacitive loads between a low level, usually ground, and a high level of the supply voltage or a one-threshold drop from the supply voltage. The latter type is usually faster in operation than the former, i.e., introduces a smaller increment of delay. In some instances the clock output should go to the supply but the initial transition should be a small delay increment, so neither type of clock driver is suitable.

It is therefore the principal object of this invention to provide an improved clock driver circuit, particularly one which introduces a small delay increment but yet may be pumped to a full supply voltage level.

In accordance with an embodiment of the invention, a clock driver circuit includes a push-pull type output stage with two transistors having a clock .PHI. and its complement .PHI. as gate inputs. An output node is pulled to a full supply voltage level by a pump transistor connecting the output node to the supply and having a delayed clock coupled to its gate. Another transistor with the supply voltage on its gate connects the output node to the gate of the pump transistor.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to the detailed description which follows, read in conjunction with the accompanying drawings, wherein:

FIG. 1 is an electrical schematic diagram of a circuit according to the invention; and

FIG. 2 is a graphic representation of voltage vs. time for voltages appearing in the circuit of FIG. 1.

With reference to FIG. 1, a circuit according to the invention is illustrated. A pair of series transistors 10 and 11 is connected between a Vdd supply line 12 and ground. The gates 13 and 14 of the transistors 10 and 11 are connected to .PHI. and .PHI. clocks, respectively, the waveforms of which are shown in FIG. 2. An output node 15 at the juncture of the source-to-drain paths of the transistors 10 and 11 will have a voltage thereon which is labelled as .PHI.' in FIG. 2. This voltage initially goes to a level of Vdd-Vt, one threshold voltage down from the supply, but then is pumped up to the supply level. The pump circuit consists of a transistor 16 connected between the output node 15 and the supply 12, with the gate node 17 of this transistor 16 being coupled by a capacitor 18 to a .PHI.d clock which is also seen in FIG. 2. A transistor 19 having its gate connected to Vdd line 12 couples the node 17 to the output node 15.

In operation, when .PHI. goes high the output .PHI.' on node 15 will follow with a Vt delay of about two nanosecond. At this point, .PHI.' goes to Vdd-Vt; as .PHI.' goes high so will the node 17. When .PHI.d goes high (after .PHI.' has closely approached Vdd-Vt) the node 17 will be boosted to higher than Vdd, driving the transistor 16 into the linear region and pulling .PHI.' on up to Vdd. When .PHI. goes high it will begin discharging .PHI.', and when .PHI.' falls to a Vt below Vdd the voltage on the node 17 will discharge through the transistor 19, cutting off the transistor 16.

It is necessary to ratio the transistors 11 and 16 in order to allow .PHI.' to be discharged. The transistor 11 must be much larger than transistor 16. Otherwise the transistor 16 will keep .PHI.' at such a high level that the node 17 would not be discharged. Alternatively, .PHI.d may be discharged before .PHI.' is discharged as seen in FIG. 2 (dotted). In this case the transistors 11 and 16 need not be ratioed.

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.