Abstract:
An improved CMOS voltage level translator circuit having an interface stage, an intermediate stage and an output stage is presented. The inventive circuit is characterized by low crossover current in the output and intermediate stages while maintaining minimal delay response when translating a lower potential signal into a higher potential signal. The improved translator circuit may be used in applications such as during EEPROM programming where control signals with normal voltage TTL voltage swing of V CC  and V SS  need to interface with the EEPROM row decoders which require a much higher voltage swing of V CC  &#39; (&gt;V hd CC) and V SS .

Description:
FIELD OF THE INVENTION 
     This invention relates to a CMOS voltage level translator circuit formed in a semiconductor integrated circuit (IC) and in particular a circuit that may be used in various memory devices, such as dynamic random access memories (DRAMs) or erasable programmable read only memory families (EPROMs, EEPROMs, etc.). 
     BACKGROUND OF THE INVENTION 
     There are many instances on advanced ICs where voltage level translator circuits are needed to interface between circuit blocks with different power supplies, such as on DRAMs, EPROMS, EEPROMs, etc., where control signals with a normal swing (i.e. 5 V) between V CC  and V SS  need to interface with row decoders which have a much higher voltage swing of V CC  &#39; (where V CC  &#39; is &gt;V CC ) and V SS  during cell programming. 
     Also, in order to maintain device reliability for future IC generations with sub-micron devices having a feature size smaller than 0.6 μ, these devices will be required to operate under a supply voltage of &lt;4 V. Yet, voltage level translators may still be required to interface with circuit blocks having a supply voltage of &gt;4 V that is generated either internally or externally, for device performance enhancement. 
     Thus, it will become very important to have a voltage level translator with minimum propagation delay for maximum speed, as well as one with minimal power dissipation, in order to promote long term operating reliability while providing optimum circuit performance. 
     In FIG. 1a, a CMOS Voltage Converter described in U.S. Pat. No. 4,958,091, by Gregory N. Roberts, is shown. In addition, FIGS. 1b and 1c diagram a computer simulation of the circuit action of FIG. 1a in comparison with computer simulations of the present invention which is shown in FIG. 3a. The output of the present invention (FIG. 3a) is labeled OUT1 and the output of FIG. 1a is labeled OUT2. 
     As can be seen from the circuit simulation of FIG. 1b, for an input with a high to a low transition, the propagation delay for both circuits is essentially the same. However, during a low to high transition, shown in FIG. 1c, the circuit of the present invention is considerably faster. The speed of the present invention is attributed to node C being able to rise fully to V CCR , thus allowing Q 11  to fully turn on while partially turning off Q 9 , which allows the present invention to discharge the output faster. The lack of discharge speed in FIG. 1a, is due to series transmission gate, Q5, between nodes E and F that allow node F to only rise to V CCR   -V   t  initially, thus causing a weaker turn on of Q 11 . Due to the weak turn on of Q 11  and Q 9  always being fully on instead of partially off, as well as a weaker turn off of Q 8 , the output is discharged at a slower rate which also causes a considerably higher crossing current at the output stage, as compared to a lower crossing current of the present invention. 
     A high-voltage translator circuit disclosed in an article submitted by McConnell et. al., entitled &#34;An Experimental 4-Mb Flash EEPROM with Sector Erase,&#34; IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 26, NO. 4, pp. 484-491, APRIL 1991, has been reproduced in FIG. 2a in order to show computer simulation comparisons between this circuit and the present invention. 
     As can be seen from the circuit simulation of FIG. 2b, the power dissipated for the circuit of FIG. 2a, represented by curve VCCP2, is substantially higher than the power dissipated by that of the present invention, represented by curve VCCP1, during a high to low transition. This difference in power dissipation is due to the intercoupling employed in the present invention versus that of FIG. 2a. By connecting the gate of Q 6  to the input of the inverter stage and the gate of Q 9  to node C in the present invention, versus having these gates grounded, as in FIG. 2a, the crossing current is reduced, thereby reducing the power dissipated by the present invention as compared to the power dissipated by the circuit of FIG. 2a. The low crossing current of the present invention is due to the partial turning off of p-channel transistors Q 6  and Q 9  versus being always fully on during transition, as in the circuit of FIG. 2a. 
     SUMMARY OF THE INVENTION 
     The voltage level translator circuit of the present invention is an improvement to the circuit of FIG. 2a. The voltage level translator provides the interface between circuit blocks with different power supplies where control signals with a normal swing (i.e. 5 V) between V CC   and V SS  need to interface with other circuitry (i.e. row decoders) which have a much higher voltage swing of V CC  &#39; (where V CC  &#39; is &gt;V CC ) and V SS  during operation. 
     In order to maintain device reliability for future IC generations with sub-micron devices, the present invention allows a device to operate under a supply voltage of &lt;4 V, while at the same time provides the interface to circuit blocks having a supply voltage of &gt;4 V that may be generated either internally or externally. 
     The present invention&#39;s improvement on the circuitry of FIG. 2a, not only maintains excellent speed operation but also has the significantly added benefit of low power dissipation, which in turn increases long term circuit reliability when implemented in semiconductor devices of various applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1a is a schematic of a CMOS voltage converter, which may be viewed as related art; 
     FIG. 1b is a computer simulation comparing the circuit response of FIG. 1a to the present invention (shown in FIG. 3a) when the input transitions from a high to a low voltage; 
     FIG. 1c is a computer simulation of the operation of the present invention (shown in FIG. 3a) when the input transitions from a low to a high voltage; 
     FIG. 2a is a reproduced schematic of a high-voltage translator circuit which may be viewed as relevant art to the present invention; 
     FIG. 2b is a computer simulation comparing the circuit power dissipation of FIG. 2a to the present invention (shown in FIG. 3a) as the input signal transitions from a low to a high voltage and high to low voltage; 
     FIG. 3adepicts the preferred embodiment of a voltage level translator which is an improvement over the circuit of FIG. 2a; 
     FIG. 3b depicts a second embodiment of a voltage level translator circuit of the present invention; and 
     FIG. 4 is a cross-sectional view of a PMOS transistor which may be used in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3a, gives a schematic representation for the preferred embodiment of the present invention of an improved voltage level translator 1, comprising an input interface stage 2, an intermediate stage 3 and an output stage 4. Although not part of the invention, a typical input inverter stage 5 is shown coupled to interface stage 2. 
     Input stage 5 and capacitors C 1  and C 2  are present in order to produce the computer simulations of FIGS. 1b, 1c and 2b and should not be construed as being necessary components of the present invention. Therefore, attention is directed mainly towards voltage level translator 1 (or simply translator 1) presented in the following description. 
     Interface stage 2 is comprised of a CMOS inverter, formed from PMOS transistor Q3 and NMOS transistor Q4. The sources of Q3 and Q4 are coupled in typical series fashion between voltage potentials V CCR  and V SS , respectively, with their gates being coupled together, thereby functioning as the input node of level translator 1. The drains of Q3 and Q4 are then coupled together at node C to provide an inverted output at the necessary nodes in level translator 1. 
     Intermediate stage 3 is comprised of PMOS transistors Q5 and Q6 and NMOS transistor Q7 all coupled in series. Q5 is coupled between supply voltage V CCP1  and the source of Q6 at its source and drain, respectively, while the drain of Q6 and the drain of Q7 couple together at node B to provide an input coupled to the gate of Q8 in output stage 4. The gate of Q6 couples to the input node of the inverter stage 2. Q7&#39;s source couples to node C, while its gate couples to V CCR . 
     Output stage 4 is comprised of a series coupled network of drive PMOS transistors Q8 and Q9 and NMOS transistors Q 10  and Q 11 . The source of Q8 couples to potential V CCP1  while its drain couples to the source of Q9. Q9&#39;s drain couples with the drain of Q10 at node OUT1 to provide output drive to external circuitry desired by one skilled in the art to work in conjunction with level translator 1. OUT1 is also coupled back to the gate of Q5 of intermediate stage 3. The source of Q10 couples to the drain of Q11, the gate of Q10 couples to either potential V CCR  or V CCP1  (as shown in FIG. 3b) and the source of Q11 couples to potential V SS . The gates of Q 9  and Q 11  couple to node C. To complete the intercoupling of translator 1, as shown in FIG. 4, n-well 41, formed in p-substrate 40, is connected to V CCP1  at node 46 to provide the proper back bias to all PMOS transistors in intermediate stage 3 and output stage 4. 
     For a general understanding of circuit operation assume for sake of illustration that the threshold voltage (vt.) for all n-channel devices is approximately equal to 0.8 V and -0.8 V for all p-channel devices (typical threshold voltages range from 0.6 V to 1.2 V). Further assume that series transistors in their respective stages are matched. Further assume that V CCR  is 3.6 V, V CCP1  is 6.0 V and V SS  is 0 V in an ideal state. Representative cases of an input signal transitioning from a high (3.6 V for sake of illustration) to a low (0 V) and from a low to a high are discussed. 
     The significance of several of the intercouplings, which are the main focus of the present invention, will be brought to light in the following circuit operation discussion. 
     High to Low Input Transitions 
     In this discussion, refer to FIG. 1b and assume OUT1 is initially at 0 V and inverter stage 1 is used to receive input signal IN. As FIG. 1b shows, the circuit is in a steady state at T 0  with an input signal at 3.6 V. At T 1 , IN begins to transition from its 3.6 V level and reaches V SS  at T 2 . The output of inverter stage 1 follows IN and provides IN&#39; (node A) to the gates of Q3, Q4 and Q6. As IN&#39; transitions from 0 V to 3.6 V, Q3 turns off and the threshold voltage of Q4 is overcome thereby, causing Q4 to turn on thereby pulling node C down to 0 V. At the same time IN&#39; is presented to the gate of Q6 causing it to turn partially off. This coupling of IN&#39; to the gate of Q6 is one of significance as the partially turning off of Q 6  reduces the crossing current of intermediate stage 3. The presence of 0  V on node C causes Q11 to turn off and Q9 to turn on. Because the gate of Q7 is held at 3.6 V, Q7 conducts and transfers 0 V to the gate of Q8 which causes Q8 to conduct. A current path is now established from the 6.0 V supply to OUT1 thus driving OUT1 to 6 V at T 3 , thereby translating the lower 3.6 V incoming signal to a higher signal of 6 V which is required by the external circuitry. 
     Low to High Input Transitions 
     Referring to FIG. 1c, OUT1 is now at 6 V (at T 3 ) and inverter stage 1 is still in place to receive input signal IN for this discussion. The circuit is in a steady state at T 3  with an input signal at 0 V. At T 4 , IN begins to transition from its 0 V level and reaches 3.6 V at T 5 . The output of inverter stage 1 follows IN and provides IN&#39; to the gates of Q3, Q4 and Q6. As IN&#39; transitions from 3.6 V to 0 V, Q4 turns off and the threshold voltage of Q3 is overcome thereby, causing Q3 to turn on and drive node C to 3.6 V. At the same time, Q6 is biased on and will only provide current to node B when Q 5  turns on. Because the gate of Q7 is held at 3.6 V, Q7 conducts and drives node B to 3.6 V-vt. which causes Q8 to turn partially off. The presence of 3.6 V on node C causes Q9 to turn partially off and Q11 to turn on. With the gate of Q10 held at 3.6 V a current path is now established between drive transistors Q10 and Q11 from the 0 V supply to OUT1 thus driving OUT1 to 0 V at T 6 . 
     By connecting the gate of Q9 to node C, the switching current between the PMOS output drivers (Q8 and Q9) and NMOS output drivers (Q10 and Q11) is minimized, while circuit response time is maintained. This connection is one of the significant improvements of the present invention. 
     Sizing selected for PMOS and NMOS transistors should be such that sufficient drive is provided to OUT1 for a specific load, whether the output is a high or a low. 
     It is to be understood that although the present invention has been described with reference to a preferred embodiment, various modifications (such as NMOS and PMOS transistor ratios and sizing, varying circuit operating potentials or even developing a bipolar transistor version) known to those skilled in the art, may be made to the circuit presented herein without departing from the invention as recited in the several claims appended hereto.