LEVEL CONVERSION CIRCUIT

Abstract: In an ECL to CMOS converter circuit an ECL input signal is applied directly to the source electrode of a MOS transistor (MP1), and the gate electrode of the MOS transistor (MP1) is independently regulated by connection to a reference voltage that is connected to a current sink formed by a D.C. path to the negative CMOS voltage supply terminal. The drain electrode of the MOS transistor (MP1) is connected to the input of a CMOS inverter (MP3, MN3) to provide the necessary logic level shift. Another MOS terminal (MN1) provides a D.C. signal path by connecting the input of the CMOS inverter (MP3, MN3) to the negative CMOS voltage supply terminal.

Description: - 1 - LEVEL CONVERSION CIRCUIT Technical Field This invention relates to level conversion circuits for converting ECL logic level signals to CMOS logic level signals. Background Art In order to couple an ECL circuit to a CMOS circuit, the difference between the output voltages from the first circuit and the input voltages needed by the second circuit, must be generated by some form of converter circuit. In ECL circuits, the logic lever "1" will approach -0.8 volts while the logic level "0" will approach -1.68 volts. For CMOS circuits, the logic level "1" will approach the power supply value, which is generally -3 to -5 volts, while the logic level "0" will be near the reference or ground level voltage. An ECL to MOS converter circuit is known from U.S. Patent No. 4,563,601. In the known converter circuit, an ECL input signal is applied to the gate electrode of a P-channel transistor which is connected in series with an N-channel transistor having a control voltage applied to its gate electrode. The control voltage is generated by a voltage generation circuit which is of complex construction, utilizing a large number of devices. An output connection to the junction of the circuit path between the P-channel transistor and the N-channel transistor is connected to the input of a CMOS inverter which provides the CMOS level output signal. It is stated in the U.S. Patent that the known circuit provides high speed performance and low power consumption, while being relatively stable to statistical variation of devices and the fluctuation of temperature and power source voltages. - 2 - Disclosure of the Invention It is an object of the present invention to provide an ECL to CMOS converter circuit which ensures that the ECL level is converted to the CMOS level at a constant threshold value. Therefore, according to the invention, there is provided a level conversion circuit for converting ECL logic level signals to CMOS logic level signals, characterized by: a first MOS transistor, of first conductivity type, having source, drain, and gate electrodes, the source electrode being connected to receive the ECL logic level signals; a second MOS transistor, of first conductivity type, having source, drain, and gate electrodes, the drain electrode being connected to the gate electrode and the source electrode being connected to a biasing voltage, and the drain and gate electrodes being further connected to the gate electrode of said first MOS transistor; first and a second power terminal having voltage levels thereat corresponding to the high and low CMOS logic levels, respectively; a current sink connected between the drain and gate electrodes of said second MOS transistor and said second power terminal; a third MOS transistor, of second conductivity type, having source, drain, and gate electrodes, the drain electrode being connected to the drain electrode of the first MOS transistor, the source electrode being connected to said second power terminal and the gate electrode being connected to said first power terminal; and a complementary inverter circuit connected between said first and second power terminals, said inverter circuit having an input connected to the drain of said first MOS transistor, and an output adapted to provide the CMOS logic level signals in response to the ECL logic level signals at the source of said first MOS transistor. - - It will be appreciated that in a level conversion circuit according to the invention, the connection of the ECL level input signal to the source of an MOS gate and the independent regulation of the gate voltage enable the establishment of a constant threshold value for logic level conversion. A further advantage is that only a small number of devices are utilized, thereby minimizing propogation delays and enabling high speed circuit operation. Brief Description of the Drawing One embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of the preferred embodiment of the invention. Fig. 2 is a circuit diagram of a current sink which may be used in conjunction with the circuit of Fig. 1. Fig. 3 is a alternate circuit embodiment of a current sink that may be used in conjunction with the circuit of Fig. 1. Fig. 4 is a schematic diagram illustrating the interconnection of the invention shown in Fig. 1 and a standard type of driver circuit used in a high speed system environment. Best Mode for Carrying Out the Invention Referring to Fig. 1, a P-channel MOS transistor MPl is connected between an input terminal and an output terminal by a CMOS inverter comprised of a P- channel MOS transistor MP3 and an N-channel MOS transistor MN3. The gate electrode of the transistor MPl is connected to a reference potential VREF. A P- channel MOS transistor MP2 has its source electrode connected to a biasing voltage VBB, which in the preferred embodiment of the invention is -1.29 volts. The gate electrode of transistor MP2 is connected to its drain electrode and to the gate electrode of transistor MPl. In addition, the drain electrode of transistor MP2 is connected to a current sink. The current sink functions to provide a D.C. path to a CMOS voltage potential VEE, which in the preferred embodiment is -5.2 volts. Two implementations of a current sink are shown in Figs. 2 and 3. An N-channel MOS transistor MNl provides a D.C. path from the drain electrode of transistor MPl to the voltage potential VEE by connecting the drain electrode of transistor MNl to the drain electrode of transistor MPl and the gate electrode of transistor MNl to a CMOS potential VCC, which in the preferred embodiment is 0 volts. The inverter switches between the voltage levels VCC and VEE in response to the level of the signal on the gate electrodes of transistors MP3 and MN3. The source and drain electrodes of transistors MN3 and MP3, respectively, are connected to the output terminal of the circuit. The source electrode of MP3 is connected to the potential VCC, and the drain electrode of transistor MN3 is connected to the potential VEE. The reference voltage VBB is applied such that transistor MPl switches at the input voltage threshold for ECL logic signals. Therefore, the voltage required is: V V + V ref bb th(p) where, V , is equal to 1/2 ( V + V ) bb il ih V , is the threshold voltage of transistor MPl th(p) V , is the ECL low level input voltage, and ie - 5 - V , is the ECL high level input voltage, ih If transistor MP2 is sized the same as transistor MPl, process variations and temperature fluctuations effecting MPl will also influence MP2 equally. The determining ratio for the sizes of MPl and MNl, using first order approximations, is described by: u I V V I Wn / Ln p | bb ih 1 Wp / Lp u V - V n sg(n) th(n) where W is the channel width of transistor MPl is the channel length of transistor MPl W is the channel width of transistor MNl n is the channel length of transistor MNl n ECL reference voltage [ECL -> -1.29v] bb V ECL high level input voltage [ECL -> -0.8v] ih V is the source to gate voltage of transistor MNl sg(n) V is the threshold voltage of the transistor MNl th(n) u is the hole mobility of transistor MPl - 6 - u , is the electron mobility of transistor MNl n Ref: "Principles of CMOS VLSI Design", Neil Weste, Kamran Eshraghian, Addison Wesley Publ. Co. Pg. 39. In operation, of the circuit of Fig. 1, for the state when the voltage level of the logic signal on the input is above VBB, transistor MPl is on, thereby pulling the node labeled with a circled A, at the juncture of the drains of transistors MPl and MNl, towards the input voltage level. This counters the effect of transistor MNl which is trying to pull the node A voltage to the level of VEE. With the MPl/MNl size ratio chosen, as disclosed above, the node A voltage will be high. The voltage at the circuit output will therefore be low due to the action of the inverter. When the logic level of the input signal falls below VBB, transistor MPl will turn off. At that time, transistor MNl will provide a discharge path for node A to VEE. Node A will then be low and the voltage at the circuit output will therefore be high. Referring now to Fig. 2, one form of a current sink that may be used with the circuit embodiment of Fig. 1 is a resistance path from the drain electrode of the transistor MP2 to the potential VEE. In the preferred embodiment, the resistor in the resistance path has a value of 3.9K. A second version of a current sink, disclosed in Fig. 3 and is comprised of an N-channel MOS transistor MN4 having its drain electrode connected to the drain electrode of transistor MP2 and its source electrode connected to the potential VEE. A turn on gate potential is generated for transistor MN4 by a circuit connected between the voltage potential VCC and VEE. The circuit is comprised of a P-channel MOS transistor MP4 and two serially connected N-channel MOS transistors MN5 and MN6. The gate electrodes of transistors MN5 and MN6 are shorted to their respective drain electrodes. The gate electrode of transistor MP4 is connected to the voltage potential VEE. Referring now to Fig. 4, which is a schematic diagram illustrating the interconnection of the preferred embodiment of the invention shown in Fig. 1 and a standard type of driver circuit used in a high speed system environment where 50-100 ohm line drivers are used, the voltage VIN is directed to the base of a PNP transistor, which is connected in circuit as a differential (comparator) amplifier. The reference voltage, against which VIN is compared, is the voltage level VBB. The difference in the value of the voltage level between VBB and VIN is amplified and buffered by an NPN transistor driving a 100 ohm coaxial cable and a 100 ohm load which is connected to a -2 volt potential VTT. The present invention, although particularly adapted to convert ECL logic levels to CMOS logic levels, may also be used to convert other signals of the same potential ranges.

Claims: CLAIMS : 1. A level conversion circuit for converting ECL logic level signals to CMOS logic level signals, characterized by: a first MOS transistor (MPl), of first conductivity type, having source, drain, and gate electrodes, the source electrode being connected to receive ECL logic level signals; a second MOS transistor (MP2), of first conductivity type, having source, drain, and gate electrodes, the drain electrode being connected to the gate electrode and the source electrode being connected to a biasing voltage, and the drain and gate electrodes being further connected to the gate electrode of said first MOS transistor (MPl); first and a second power terminals (Vcc , VEE) having voltage levels thereat corresponding to high and low CMOS logic levels, respectively; a current sink connected between the drain and gate electrodes of said second MOS transistor (MP2) and said second power terminal (VEE) ? a third MOS transistor (MNl), of second conductivity type, having source, drain, and gate electrodes, the drain electrode being connected to the drain electrode of said first MOS transistor (MPl), the source electrode being connected to said second power terminal (V E) an< the gate electrode being connected to said first power terminal (Vcc)/ an< a complementary inverter circuit (MP3, MN3) connected between said first and said second power terminals ( cc, V EE) ' said inverter circuit having an input connected to the drain of said first MOS transistor (MPl) , and an output adapted to provide CMOS logic level signals in response to the ECL logic level signals at the source of said first MOS transistor (MPl). 2. A level conversion circuit according to claim 1, characterized in that said current sink includes a fourth MOS transistor (MN4), of second conductivity type, having source, drain, and gate electrodes, the drain being connected to the drain and gate of said second MOS transistor (MN2), and the source being connected to said second power terminal; fifth and sixth MOS transistors (MN5, MN6), of second conductivity type, having source, drain, and gate electrodes, the gate and drain electrodes of said fifth MOS transistor (.MN5) being connected to the gate electrode of said fourth MOS transistor (MN4), the drain and gate of said fifth MOS transistor (MN5), and the source of said sixth MOS transistor (MN6) being connected to said second power terminal (V E)» an< 3 a seventh MOS transistor (MP4), of first conductivity type, having source, drain, and gate electrodes, the gate electrode being connected to said second power terminal (V E)/ the source electrode being connected to said first power terminal (Vcc) ani -- the drain being connected to the gate of said fourth MOS transistor (MN4) 3. A level conversion circuit according to claim 1, characterized in that said current sink includes a resistor (RgiNK) connected between the drain electrode of said second MOS transistor (MP2) and said second power terminal (VEE) • 4. A level conversion circuit according to claim 1, characterized in that said complementary inverter circuit includes: a P-channel MOS transistor (MP3), having source, drain, and gate electrodes; and an N-channel MOS transistor (MN3), having source, drain, and gate electrodes, the gate electrode being connected to the gate electrode of said P-channel MOS transistor (MP3) and the drain of said first MOS transistor (MPl), the drain and source of said P- channel and said N-channel MOS transistors (MP3, MN3), respectively, being connected together to provide an output, the source and drain of said P-channel and said N-channel MOS transistors (MP3, MN3), respectively, being connected to said first and said second power terminals (Veer VEE)-- respectively. 5. A level conversion circuit according to claim 1, characterized in that the size of said first MOS transistor (MPl) is equal to the size of said second MOS transistor (MP2). 6. A level conversion circuit according to claim 1, characterized in that the biasing voltage is fixed at one-half the sum of the voltage levels of the ECL low logic level signal and the ECL high logic level signal.

Inventors: SANWO IKUO JIMMY, SUTHAR MUKESH BHOGILAL, SANWO, IKUO, JIMMY, SUTHAR, MUKESH, BHOGILAL
Applicants: NCR CO, NCR CORPORATION
Application year: 1988
Main classification: H03K19
IPC codes: H03K5, H03K19