Logic Level Shifter for 3 Volt CMOS to 5 Volt CMOS or TTL

This invention relates generally to digital circuitry, and more particularly to logic level shifters for digital circuitry.

Portable computers are becoming very popular because they are easy to transport, can be used virtually anywhere, and because they are approaching the computing power of their desk-top brethren. Portable computers are also becoming smaller and lighter in weight, which generally means that the batteries which power them are also becoming smaller. As a result, the new generation of small, light, but powerful portable computers require low-power, yet high speed, circuitry.

Because portable computers are usually battery powered, it is very important that they draw as little power as possible. In consequence, the circuitry of virtually all portable computers is implemented in complementary metal oxide semiconductor (CMOS) technology, which is noted for its low power requirements. Also, the microprocessor or controller at the heart of the portable computer often operates internally on a lower voltage than the rest of the computer's circuitry to reduce its static power consumption. Unfortunately, lowering the voltage also reduces the speed at which the microprocessor or controller operates, which means that other portions of the circuitry must not unduly add to the delay of signals being produced by the microprocessor or controller.

In FIG. 1, a microprocessor or controller 10 can utilize an internal or "core" operating voltage of three volts d.c. but must drive external or "ring" logic at standard ISA (Industry Standard Architecture) voltages of about five volts d.c. This requires a logic level shifter 12 which couples the core logic to a pad driver 14 and, from there, to a lead 16 of the microprocessor or controller 10. The logic level shifter 12 must be capable providing sufficient current to the pad driver 14. The pad driver 14 usually drives a capacitive load C.sub.L of about 200 picofarads.

A prior art logic level shifter 12' is illustrated in FIG. 2. The channel of a p-channel metal oxide field effect transistor (MOSFET) 18 and the channel of a n-channel MOSFET 20 are coupled together in a typical series CMOS configuration between a source of the ring voltage Vr and ground. The gates of the MOSFETS 18 and 20 are coupled to the input of the logic level shifter 12', and the node 22 between the channels of the MOSFETS is coupled to the pad driver 14. Typically, the relative size of the MOSFETS are selected so that the trip point between a logic 0 and a logic 1 are skewed towards the lower end of a voltage range. For example, if the voltage range is 0-3 volts, the trip point would be made less than 1.5 volts, while if the voltage range is 0-5 volts, the trip point would be made less than 2.5 volts. This skewing of the trip point enables the input signal "IN" to have a swing of 0 to 3 volts while generating an output signal that has a swing of 0 to 5 volts.

A problem with the prior art logic level shifter 12' of FIG. 2 is that it exhibits a relatively high static current. For example, when the input signal IN is at 3 volts, the output will be at 0 volts because the n-channel MOSFET 20 is on, but the p-channel MOSFET will not be shut entirely off. Therefore, current will flow from Vr to ground through MOSFETS 18 and 20. In consequence, there is a considerable static current drain through the level shifter 12', which can quickly deplete the batteries of a portable computer.

The present invention is a low power, high speed logic level shifter which is particularly well adapted for use in portable computers. The logic level shifter is preferably implemented in two inverting stages, where the first stage shifts the input "core" logic levels to a lower logic level range, and where the second stage shifts the lower logic level range to an output or "ring" logic range. Feedback is preferably used to virtually eliminate the static current in one or both of the stages.

A method of the present invention shifts a first range of voltages downwardly to a lower range of voltages and then shifts the lower range of voltages upwardly to a second range of voltages. One or both of the shifting steps may include a logic level inversion. The second range of voltages can be sensed to reduce the static current consumed by the process.

The present invention is preferably implemented in CMOS technology. Since the static current is virtually eliminated, the result is a very low power logic level shifter for battery-powered portable computers. The circuitry of the present invention is not significantly slower than the prior art logic level shifter of FIG. 2, and is quite suitable for use with standard 8 megahertz microcomputer busses.

These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following specification of the invention and a study of the several figures of the drawing.

FIG. 1 is a diagram illustrating a typical prior art environment for a logic level shifter;

FIG. 2 is a schematic of a prior art logic level shifter; and

FIG. 3 is a schematic of a logic level shifter in accordance with the present invention.

The environment of the present invention and a prior art logic level shifter were discussed with reference to FIGS. 1 and 2, respectively. In FIG. 3, a logic level shifter 12" in accordance with the present invention includes a first stage 24 and a second stage 26. The first stage 24 has a first input 28 and a first output 30, and the second stage 26 has a second input 32 coupled to the first output 30, and a second output 34.

The first stage 24 is preferably coupled to two power supply levels corresponding to the core voltage level Vc and to the ring voltage level Vr. In this preferred implementation, Vc is about 3 volts d.c. and Vr is about 5 volts d.c. As will be discussed subsequently, it is also possible to provide different voltage levels for Vc and Vr to cause the circuitry to provide different functions. The first stage 24 includes a p-channel MOSFET 36 and three n-channel MOSFETS 38, 40, and 42. The channel of MOSFET 36 is coupled between Vc and a node 44, and the channel of MOSFET 38 is coupled between node 44 and a reference voltage (ground). The gates of MOSFETS 36 and 38 are coupled to first input 28. The channel of MOSFET 40 is coupled between Vr and a node 46, and the channel of MOSFET 42 is coupled between node 46 and ground. The gate of MOSFET 40 is coupled to node 44, and the gate of MOSFET 42 is coupled to first input 28. Node 46 is coupled to first output 30.

The second stage 26 is powered by a single power supply level Vr and includes two p-channel MOSFETS 48 and 50 and an n-channel MOSFET 52. The channel of MOSFET 48 is coupled between Vr and the second input 32, and the gate of MOSFET 48 is coupled to the second output 34. The MOSFET 48 serves as a feed-back transistor to greatly reduce static current drain. The channel of MOSFET 50 is coupled between Vr and a node 54, and the channel of MOSFET 52 is coupled between node 54 and ground. The gates of MOSFETS 50 and 52 are coupled to second input 32, and node 54 is coupled to the second output 34.

The logic level shifter 12" consumes essentially no static current. When the input signal at first input 28 is at logic 0, the signal at a point A is raised to a logic 1 (3 volts) and the MOSFET 40 is turned on. This causes the signal at a point B shift downwardly to voltage lower than Vc. In the present embodiment the lower voltage level is about 2.5 volts, although a lower voltage within the range of 1.5 to 2.5 volts is also suitable. This turns on the MOSFET 52, pulling the second output 34 low to a logic level 0. MOSFET 50 is partially turned on, but is overpowered by MOSFET 52. When the second output 34 is a logic level 0, the MOSFET 48 is turned on, pulling the voltage level at point B to Vr (5 volts). The 5 volt level at point B MOSFET 50 completely turns off that transistor preventing any static current drain through the MOSFETS 50 and 52. The 5 volt level also completely shuts off MOSFET 40 because both sides of its channel are at the same voltage level. This prevents any static current from flowing through MOSFETS 40 and 42. The feedback MOSFET 48 therefore virtually eliminates static current in both the first stage 24 and the second stage 26 when the input logic level is 0.

When the first input 28 is a logic level 1 (3 volts) the signal at point B is a logic level 0. This causes the MOSFET 50 to be turned on hard and the MOSFET 52 to be completely shut off. This causes the second output voltage to go to 5 volts, shutting off the feedback MOSFET 48. Since MOSFET 38 is turned on, MOSFET 40 is turned off hard and there is virtually no static current flowing through MOSFETS 40 and 42. With MOSFET 50 turned off hard, there is virtually no static current flowing through MOSFETS 50 and 52. MOSFET 36 is also turned off hard, so there is virtually no static current flowing through MOSFETS 36 and 38. In consequence, there is only negligible static current flowing through either of stages 24 or 26 when the input logic level is 1.

It should be noted that the first stage 24 causes a first logic level inversion, and the second stage 26 causes a second logic level inversion. The second output 34 is therefore of the same polarity as the first input 28, even though the logic levels have been shifted upwardly. It is also possible to make the logic level converter 12" with non-inverting stages with essentially the same result, or to make the converter 12" with only one inverting stage, in which case there would be a polarity inversion between the first input 28 and the second output 34. For example, if the gate connections of MOSFETS 40 and 42 are swapped, i.e. the gate of MOSFET 40 is attached to first input 28 and the gate of MOSFET 42 is attached to node 44, first stage 24 is a non-inverting stage. Since second stage 26 is still an inverting stage, the logic level converter 12" will be an inverting logic level converter.

The following characteristics are typical of the logic level converter 12' when it is implemented in one micron CMOS technology. The dynamic estimates are based upon an assumption that there are clean current spikes on Vr only, that the circuit is operating at 8 megahertz, that industry standard bus cycles are used. The estimates also assume that the level shifter 12" drives a pad driver 14 which drives a capacitive load C.sub.L of approximately 200 picofarads.

A standard cell pad driver 14 with a 200 pf load is attached to the second output 34 of the logic level shifter. The following power characteristics were noted:

With a rising edge at the first input 28 (i.e. the input signal is changing from a logic level 0 to a logic level 1) the power consumed is:

With a falling edge at the first input 28 (i.e. the input signal is changing from a logic level 1 to a logic level 0) the power consumed is:

The worst case time delays for the logic level shifter 12" are as follows:

These time delays are well within the specifications for a standard microcomputer 8 megahertz bus.

It should be noted that the power inputs Vc and Vr of the first stage 24 and the power input Vr of the second stage permits a certain amount of programming of the functionality of the logic level shifter 12". For example, if Vc=Vr=5 volts and the first input 28 operates in the 0-5 volt range, the logic level shifter serves as a low-power, high-speed, ISA compatible buffer. As another example, if Vc=Vr=3 volts, the logic level shifter serves as a low-power, high-speed buffer for 0-3 volt logic devices. Since some devices operate at 3 volts, others at 5 volts, and some at both 3 and 5 volts, it is useful to have a the same circuit be able to serve as a 3 or 5 volt buffer or as a 3-to-5 volt logic level shifter.

While this invention has been described in terms of several preferred embodiments, it is contemplated that alterations, modifications and permutations thereof will become apparent to those skilled in the art upon a reading of the specification and study of the drawings. It is therefore intended that the following appended claims include all such alterations, modifications and permutations as fall within the true spirit and scope of the present invention.