Abstract:
The power consumed by a voltage translator circuit, such as a TTL-to-CMOS buffer, is substantially reduced by changing the supply voltages provided to the input inverter. By reducing the supply voltage provided to the source of the p-channel transistor of the input inverter, the lowest logic-high TTL voltage applied to the gate turns off the p-channel transistor and turns on the n-channel transistor of the input inverter. By increasing the supply voltage provided to the source of the n-channel transistor of the input inverter, the highest logic-low TTL voltage applied to the gate turns off the n-channel transistor and turns on the p-channel transistor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to extremely low power voltage translator circuitry and, more particularly, to an ultra low power TTL-to-CMOS buffer. 
     2. Description of the Related Art 
     A voltage translator circuit is a level shifter that shifts voltages from one logic level to another logic level. A TTL-to-CMOS buffer, in particular, is a device that converts TTL logic levels into CMOS logic levels. With TTL logic levels, a logic high is represented by voltages that fall within a range from VIH (MIN) to VIH (MAX), such as +2.0V to VCC (e.g., +5.0V). In addition, a logic low is represented by voltages that fall within a range from VIL (MIN) to VIL (MAX), such as ground to +0.8V. On the other hand, with CMOS logic levels, a logic high is represented by VCC, and a logic low is represented by ground. 
     FIG. 1A shows a schematic diagram that illustrates a conventional TTL-to-CMOS buffer  100 . As shown in FIG. 1A, TTL-to-CMOS buffer  100  includes a first inverter  110  and a second inverter  112  that is connected in series with first inverter  110 . Inverter  110  is typically implemented as a standard inverter, while inverter  112  is typically implemented with a Schmitt trigger type of arrangement for good hysteresis characteristics. 
     FIG. 1B shows a schematic diagram that illustrates first inverter  110 . As shown in FIG. 1B, inverter  110  includes a p-channel transistor P 1  and an n-channel transistor N 1 . P-channel transistor P 1  has a source connected to a power supply node PSN to receive a power supply voltage VCC, a drain connected to an output node NOUT, and a gate connected to an input node NIN. N-channel transistor N 1  has a source connected to ground, a drain connected to the output node NOUT, and a gate connected to the input node NIN. 
     In operation, p-channel transistor P 1  turns on and conducts when the source-to-drain voltage VSD is greater than zero (e.g., VSD&gt;0), and the gate-to-source voltage VGS is less than the threshold voltage VTP of the transistor (e.g., VGS&lt;VTP). N-channel transistor N 1  turns on and conducts when the drain-to-source voltage VDS is greater than zero (e.g., VDS&lt;0), and the gate-to-source voltage VGS is greater than the threshold voltage VTN of the transistor (e.g., VGS&gt;VTN). 
     One of the advantages of inverter  110  is that when an input voltage VIN on the input node NIN is at CMOS levels, no current is dissipated. For example, when the input voltage VIN is at ground, p-channel transistor P 1  is turned on and n-channel transistor N 1  is turned off. Similarly, when the input voltage VIN is at VCC, p-channel transistor P 1  is turned off and n-channel transistor N 1  is turned on. 
     One of the disadvantages of inverter  110 , however, is that when the input voltage VIN is at TTL levels, a substantial amount of current can be dissipated as transistors P 1  and N 1  are often both turned on. For example, when a logic high is represented by an input voltage VIN of +2.0V, the threshold voltage VTP is −1.0V, the threshold voltage VTN is +0.7V, and VCC is +5.0V, both transistors P 1  and N 1  are turned on. (For transistor P 1 , VGS=2.0−5=−3.0. Since −3.0V is less than the threshold voltage VTP of −1.0V, transistor P 1  is turned on. For transistor N 1 , VGS=2.0−0=2.0. Since 2.0V is greater than the threshold voltage VTN of 0.7V, transistor N 1  is turned on.) 
     Similarly, when a logic low is represented by an input voltage VIN of +0.8V, both transistors P 1  and N 1  are again turned on. (For transistor P 1 , VGS=0.8−5=−4.2. Since −4.2V is less than the threshold voltage VTP of −1.0V, transistor P 1  is turned on. For transistor N 1 , VGS=0.8−0=0.8. Since 0.8V is greater than the threshold voltage VTN of 0.7V, transistor N 1  is turned on.) 
     Since transistors P 1  and N 1  can both be turned on at the same time, the strength of n-channel transistor N 1  is typically set to insure that when the input voltage VIN is greater than VIH (MIN), n-channel transistor N 1  overpowers p-channel transistor P 1  so that the voltage on the output node NOUT is pulled down to ground. In addition, the strength of n-channel transistor N 1  is also set to insure that when the input voltage VIN is less than VIL (MAX), p-channel transistor P 1  overpowers n-channel transistor N 1  and the voltage on the output node NOUT is pulled up to VCC. 
     In almost every conventional TTL-to-CMOS buffer or a level shifter, an inverter, with its input at one logic level, and its power supplies at another logic level, is present. In a TTL-to-CMOS buffer, since a substantial amount of current is dissipated when the input voltage VIN is at TTL levels and transistors P 1  and N 1  are both turned on, there is a need for an inverter that operates on TTL levels, and dissipates little or no current throughout the range of operation. This is also applicable for a generalized level shifter. 
     SUMMARY OF THE INVENTION 
     Conventionally, a substantial amount of current is dissipated when the voltage input to a TTL-to-CMOS buffer via an input inverter is at TTL levels. This is because the TTL levels turn on both the p-channel and n-channel transistors of the input inverter. The present invention reduces the amount of current dissipated at TTL levels by insuring that only one of the two transistors is on when the input voltage is at a TTL level. The present invention is also applicable in the case of other low power level shifter circuits. 
     A translator circuit in accordance with the present invention includes an inversion stage that outputs an inversion signal in response to an input signal. The inversion signal has a logic high equal to a first voltage which is less than an upper supply voltage, and a logic low equal to a second voltage which is greater than a lower supply voltage. 
     The circuit also includes a logic-low translation stage that outputs a translation signal in response to the inversion signal. The translation signal has a logic high equal to a second voltage which is less than the first voltage, and a logic low equal to the lower supply voltage. The circuit of the present invention further includes a logic-high translation stage that outputs an output signal in response to the translation signal. The output signal has a logic high equal to the upper supply voltage, and a logic low equal to the lower supply voltage. 
     The present invention also includes a method for operating a translator circuit. The method includes the step of outputting an inversion signal from an inversion stage in response to an input signal. The inversion signal has a logic high equal to a first voltage which is less than an upper supply voltage, and a logic low equal to a second voltage which is greater than a lower supply voltage. 
     The method also includes the step of outputting a translation signal from a logic-low translation stage in response to the inversion signal. The translation signal has a logic high equal to a second voltage which is less than the first voltage, and a logic low equal to the lower supply voltage. The method further includes the step of outputting an output signal from a logic-high translation stage in response to the translation signal. The output signal has a logic high equal to the upper supply voltage, and a logic low equal to the lower supply voltage. 
     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic diagram illustrating a conventional TTL-to-CMOS buffer  100 . 
     FIG. 1B is a schematic diagram illustrating first inverter  110 . 
     FIG. 2 is a schematic illustrating a TTL-to-CMOS buffer  200  in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 shows a schematic that illustrates a TTL-to-CMOS buffer  200  in accordance with the present invention. As described in greater detail below, buffer  200  dissipates substantially less current than conventional buffers throughout the range of operation by changing the supply voltages provided to the input inverter that receives the TTL logic levels. 
     As shown in FIG. 2, buffer  200  includes a low-power inversion stage  210  that inverts an input signal IN to output an inversion signal S 1  which has a logic state opposite to that of the input signal IN. Stage  210  includes a first voltage drop circuit  212  that has a number of transistors TA which are connected between a power supply node PSN and a first reduced power supply node RCC 1 . Each of the transistors TA has an associated voltage drop which, in combination, define a first reduced voltage VR 1  on the reduced power supply node RCC 1 . 
     For example, FIG. 2 shows two n-channel diode-connected transistors TA 1  and TA 2  connected between the power supply node PSN and the first reduced power supply node RCC 1 . Transistor TA 1  has a drain and a gate connected to the power supply node PSN, a source, and a first threshold voltage drop VTH 1 . Transistor TA 2  has a drain and a gate connected to the source of transistor TA 1 , a source connected to the first reduced power supply node RCC 1 , and a second threshold voltage drop VTH 2 . The combined threshold voltage drops VTH 1  and VTH 2  define the first reduced voltage VR 1 . (VCC−VTH 1 −VTH 2 =VR 1 .) 
     The value of the first reduced voltage VR 1  can be changed by adding or subtracting transistors TA, or changing the voltage drops of the transistors TA. Other circuit elements that provide a voltage drop may also be used in place of, or in combination with, the diode-connected transistors TA of circuit  212 . 
     Stage  210  also includes an inverter that is connected to receive the input signal IN, and to output the inversion signal S 1 . The inverter includes a p-channel transistor P 11  and an n-channel transistor N 11 . P-channel transistor P 11  has a source connected to the first reduced power supply node RCC 1 , and a drain connected to an inversion node N 1  to output the inversion signal S 1 . In addition, transistor P 11  also has a gate connected to an input node NIN to receive the input signal IN, and a first p-channel threshold voltage VTP 1 . 
     N-channel transistor N 11  has a drain connected to the inversion node N 1  to output the inversion signal S 1 , and a source connected to an increased ground node NID. In addition, transistor N 11  has a gate connected to the input node NIN to receive the input signal IN, and a first n-channel threshold voltage VTN 1 . 
     Stage  210  further includes a voltage drop circuit  214  that has a number of transistors TB which are connected between the increased ground node NID and ground. Each of the transistors TB has an associated voltage drop which, in combination, define an increased ground voltage VL on the increased ground node NID. 
     For example, FIG. 2 shows one n-channel diode-connected transistor TB 1  connected between the increased ground node NID and ground. Transistor TB 1  has a drain and a gate connected to the source of transistor N 11 , a source connected to ground, and a third threshold voltage VTH 3 . The threshold voltage drop VTH 3  defines the increased ground voltage VL. (0+VTH3=VL.) 
     The value of the increased ground voltage VL can be changed by adding transistors TB, or changing the voltage drop of the transistor TB. Other circuit elements that provide a voltage drop may also be used in place of, or in combination with, the diode-connected transistor TB of circuit  214 . 
     In operation, the first reduced voltage VR 1  is set such that when the voltage of the input signal IN on the input node NIN is equal to VIH (MIN), the gate-to-source voltage VGS of transistor P 11  is greater than the threshold voltage VTP 1 . When the gate-to-source voltage VGS of transistor P 11  is greater than the threshold voltage VTP 1 , transistor P 11  is turned off. 
     Similarly, the increased ground voltage VL is set such that when the voltage of the input signal IN on the input node NIN is equal to VIL (MAX), the gate-to-source voltage VGS of transistor N 11  is less than the threshold voltage VTN 1 . When the gate-to-source voltage VGS of transistor N 11  is less than the threshold voltage VTN 1 , transistor N 11  is turned off. 
     For example, assume that the voltage of the input signal IN and the voltage VIH (MIN) are +2.0V, the threshold voltage VTP 1  is −1.0V, the threshold voltage VTN 1  is +0.8V. Further assume that a power supply voltage VCC on the power supply node PSN is +5.0V, transistors TA 1  and TA 2  each have threshold voltage drops of 1.2V and 1.0V respectively, and transistor TB 1  has a threshold voltage drop of 0.7V. (The threshold voltages of transistors TA 1 , TA 2  and N 11  are higher than that of transistor TB 1  because of the body effect, and each of them shows a corresponding increase.) 
     In this example, transistor P 1  is turned off and transistor N 1  is turned on. (RCC 1 =5−1.2−1.0=+2.8, and VL=0+0.7=+0.7. For transistor P 1 , VGS=2.0−2.8=−0.8. Since −0.8V is not less than the threshold voltage VTP 1  of −1.0V, transistor P 11  is turned off. For transistor N 11 , VGS=2.0−0.7=+1.3. Since 1.3V is greater than the threshold voltage VTN 1  of 0.8V, transistor N 11  is turned on.) 
     Similarly, assume that the voltage of the input signal IN and the voltage VIL (MAX) are +0.8V. In this example, transistor P 11  is turned on and transistor N 11  is turned off. (For transistor P 11 , VGS=0.8−2.8=2.0. Since −2.0V is less than the threshold voltage VTP 1  of −1.0V, transistor P 11  is turned on. For transistor N 11 , VGS=0.8−0.7=0.1. Since 0.1V is less than the threshold voltage VTN 1  of 0.8V, transistor N 11  is turned off.) 
     Thus, the inversion signal S 1  has a logic high equal to the first reduced voltage VR 1  when the voltage of the input signal IN is between VIL (MAX) and ground. In addition, the inversion signal S 1  has a logic low equal to the increased ground voltage VL when the voltage of the input signal IN is between VIH (MIN) and VCC. 
     One of the advantages of the present invention is that stage  210  significantly reduces the current dissipated from TTL signal levels since only one of the two transistors P 11  and N 11  are on when the input signal IN is at TTL levels. Stage  210  dissipates current in the order of nanoamperes when the input signal IN is in the TTL range of operation, ground to VIL (MAX) and VIH (MIN) to VCC. Further, stage  210  dissipates current in the order of microamperes when the input signal IN is in the narrow transition range VIL (MAX) to VIH (MIN), and dissipates no current in some parts of the transition range VIL (MAX) to VIH (MIN). This transition range, however, is not a part of the operation specifications. 
     As further shown in FIG. 2, buffer  200  also includes a logic-low translation stage  220  that outputs a translation signal S 2  in response to the inversion signal S 1 . The translation signal S 2  has a logic state that is the same as the logic state of the inversion signal S 1 . As described in greater detail below, the translation signal S 2  has a logic high equal to a voltage which is less than the first reduced voltage VR 1 , and a logic low equal to ground. Thus, stage  220  outputs the translation signal S 2  with a logic low that is equal to a CMOS logic low. 
     Stage  220  includes a voltage drop circuit  222  that has a number of transistors TC that are connected between the power supply node PSN and a second reduced power supply node RCC 2 . Each of the transistors TC has an associated voltage drop which, in combination, define a second reduced voltage VR 2  on a second reduced power supply node RCC 2 . 
     For example, FIG. 2 shows two n-channel diode-connected transistors TC 1  and TC 2  connected between the power supply node PSN and the second reduced power supply node RCC 2 . Transistor TC 1  has a drain and a gate connected to the power supply node PSN, a source, and a fourth threshold voltage drop VTH 4 . Transistor TC 2  has a drain and a gate connected to the source of transistor TC 1 , a source connected to the second reduced power supply node RCC 2 , and a fifth threshold voltage drop VTH 5 . The combined threshold voltage drops VTH 4  and VTH 5  define the second reduced voltage VR 2 . (VCC−VTH 4 −TH 5 =VR 2 .) 
     Stage  220  also includes an inverter that is connected to receive the inversion signal S 1 , and to output a first intermediate signal SIM 1  which has a logic state opposite to the inversion signal S 1 . The inverter includes a p-channel transistor P 21  and an n-channel transistor N 21 . P-channel transistor P 21  has a source connected to the second reduced power supply node RCC 2 , and a drain connected to a first intermediate node NIM 1  to output the first intermediate signal SIM 1 . In addition, transistor P 21  also has a gate connected to the inversion node N 1  to receive the inversion signal S 1 , and a second p-channel threshold voltage VTP 2 . 
     N-channel transistor N 21  has a drain connected to the first intermediate node NIM 1  to output the first intermediate signal SIM 1 , and a source connected to ground. In addition, transistor N 21  has a gate connected to the inversion node N 1  to receive the inversion signal S 1 , and a second n-channel threshold voltage VTN 2 . 
     Stage  220  further includes a voltage drop  224  that has a number of transistors TD which are connected between the inversion node N 1  and a translation node N 2 . For example, FIG. 2 shows one n-channel diode-connected transistor TD 1  connected between the inversion node N 1  and the translation node N 2 . Transistor TD 1  has a drain and a gate connected to the inversion node N 1 . Transistor TD 1  also has a source connected to the translation node N 2 , and a sixth threshold voltage VTH 6 . Other circuit elements that provide a voltage drop may alternately be used in place of, or in combination with, the diode-connected transistors of circuits  222  and  224 . 
     Stage  220  additionally includes an n-channel control transistor MPD that has a drain connected to the translation node N 2 , and a source connected to ground. Transistor MPD also has a gate connected to the first intermediate node NIM 1 , and a seventh threshold voltage VTH 7 . 
     In operation, when the voltage of the inversion signal S 1  is equal to the increased ground voltage VL (representing a logic low), transistor P 21  is turned on and transistor N 21  is turned off. When transistor P 21  is turned on, the voltage of the first intermediate signal SIM 1  is pulled high which, in turn, turns on transistor MPD. When transistor MPD is turned on, the voltage of the translation signal S 2  on the translation node N 2  is pulled to ground. This, in turn, turns off transistor TD 1 . 
     When the voltage of the inversion signal S 1  is equal to the first reduced voltage RCC 1  (representing a logic high), transistor P 21  is turned off and transistor N 21  is turned on. The second reduced voltage RCC 2  is ideally the same as the maximum voltage of the inversion signal S 1  to insure that transistor P 21  is turned off when the inversion signal S 1  equal to the reduced voltage VR 1 . Thus, two transistors TC are preferably used when two transistors TA are used. 
     When transistor N 21  is turned on, the voltage of the first intermediate signal SIM 1  on the first intermediate node NIM 1  is pulled low which, in turn, turns off transistor MPD. When transistor MPD is turned off, the voltage of the translation signal S 2  on node N 2  is pulled to one diode drop less than the voltage of the inversion signal S 1  on the inversion node N 1 . Thus, the voltage of the translation signal S 2  is equal to VCC− 3 VTH (VTH 1 , VTH 2 , and VTH 6 ). (The voltage of the translation signal S 2  (VCC− 3 VTH) must be greater than the threshold voltage VTH 7  of transistor MPD.) 
     As further shown in FIG. 2, buffer  200  also includes a logic-high translation stage  230  that inverts the inversion signal S 2  to output an output signal OUT which has a logic state opposite to that of the inversion signal  52 . As described in greater detail below, the output signal OUT has a logic high equal to the power supply voltage VCC, and a logic low equal to ground. Thus, stage  230  outputs the output signal OUT with a logic high that is equal to a CMOS logic high, and a logic low that is equal to a CMOS logic low. 
     Stage  230  includes a voltage drop  231  that has a number of transistors TE that are connected between the power supply node PSN and a third reduced power supply node RCC 3 . Each of the transistors TE has an associated voltage drop which, in combination, define a third reduced voltage VR 3  on the third reduced power supply node RCC 3 . 
     For example, FIG. 2 shows three n-channel transistors TE 1 , TE 2 , and TC 3  connected between the power supply node PSN and the third reduced power supply node RCC 3 . Transistor TE 1  has a drain and a gate connected to the power supply node PSN, a source, and an eighth threshold voltage drop VTH 8 . Transistor TE 2  has a drain and a gate connected to the source of transistor TE 1 , a source, and a ninth threshold voltage drop VTH 9 . 
     Transistor TE 3  has a drain and a gate connected to the source of transistor TE 2 , a source connected to the third reduced power supply node RCC 3 , and a tenth threshold voltage drop VTH 10 . The combined threshold voltage drops VTH 8 , VTH 9 , and VTH 10  define the third reduced voltage VR 3 . (VCC−VTH 8 −VTH 9 −VTH 10 =VR 3 .) Other circuit elements that provide a voltage drop may alternately be used in place of, or in combination with, the diode-connected transistors of circuit  231 . 
     Stage  230  also includes an inverter that is connected to receive the translation signal S 2 , and to output a second intermediate signal SIM 2  which has a logic state that is opposite to signal S 2 . The inverter includes a p-channel transistor P 31  and an n-channel transistor N 31 . P-channel transistor P 31  has a source connected to the reduced power supply node RCC 3 , and a drain connected to a second intermediate node NIM 2  to output the second intermediate signal SIM 2 . In addition, transistor P 31  also has a gate connected to the translation node N 2  to receive the translation signal S 2 , and a third p-channel threshold voltage VTP 3 . 
     N-channel transistor N 31  has a drain connected to the second intermediate node NIM 2  to output the second intermediate signal SIM 2 , and a source connected to ground. In addition, transistor N 31  has a gate connected to the translation node N 2  to receive the translation signal S 2 , and a third n-channel threshold voltage VTN 3 . 
     Stage  230  further includes an output circuit  232  that receives the translation signal S 2  and the second intermediate signal SIM 2 . Circuit  232  outputs the output signal OUT with a logic high equal to the power supply voltage VCC when the voltage of the translation signal S 2  is low, and a logic low equal to ground when the voltage of the translation signal S 2  is high. 
     Output circuit  232  includes a pair of p-channel transistors P 41  and P 42  which each have a source connected to the power supply node PSN. Transistor P 41  has a gate connected to an output node NOUT, and a drain connected to a third intermediate node NIM 3 . Transistor P 42  has a gate connected to the third intermediate node NIM 3 , and a drain connected to the output node NOUT. 
     Output circuit  232  also includes a pair of n-channel transistors N 41  and N 42  which each have a source connected to ground. Transistor N 41  has a gate connected to the second intermediate node NIM 2 , and a drain connected to the third intermediate node NIM 3 . Transistor N 42  has a gate connected to the translation node N 2 , and a drain connected to the output node NOUT. 
     In operation, when the voltage of the translation signal S 2  has been pulled to one diode drop less than the voltage of the inversion signal S 1 , transistor P 31  is turned off and transistor N 31  is turned on. When transistor N 31  is turned on, the voltage of the second intermediate signal SIM 2  is pulled low which turns off transistor N 41 . 
     The third reduced voltage RCC 3  is ideally the same as the maximum voltage of the translation signal S 2  to insure that transistor P 31  is turned off when the translation signal S 2  is equal to the VCC−3VTH. Thus, three transistors TE are preferably used when three transistors (2−TA and 1−TD) are previously used. 
     In addition, transistor N 42  is also turned on which, in turn, pulls the voltage of the output signal OUT on the output node NOUT to ground. Further, the low on the output node NOUT turns on transistor P 41  which, since transistor N 41  is off, charges up the third intermediate node NIM 3 . The increased voltage on the third intermediate node N 3  turns off transistor P 42 . 
     On the other hand, when the voltage of the translation signal S 2  has been pulled to ground, transistor P 31  turns on and transistor N 31  turns off. When transistor P 31  turns on, the voltage of the second intermediate signal SIM 2  is pulled high which, in turn, turns on transistor N 41 . 
     When transistor N 41  turns on, the voltage on the third intermediate node NIM 3  is pulled low which, in turn, turns on transistor P 42 . When transistor P 42  turns on, the voltage of the output signal OUT is pulled up to the power supply voltage VCC. The logic high of the output signal OUT turns off transistor P 41 . In addition, the translation signal S 2  turns off transistor N 42 . 
     One of the advantages of stages  220  and  230  is that the power dissipation is only dynamic. As a result, once the translation and output signals S 2  and OUT are latched, there is no current dissipation. Another advantage is that stages  220  and  230  incorporate hysterisis (the logic level of the output signal OUT changes states at different points on the rising and falling edges of the input signal IN). Thus, a TTL-to-CMOS buffer has been described that significantly reduces the current dissipated by the buffer over the entire range of operation. 
     The present invention is particularly suitable for low power applications where power consumption is more important than speed (buffer  200  is slower than conventional TTL-to-CMOS buffers). In addition, testing results have shown that buffer  200  operates reliably beyond the typical VCC range of 4.4 to 5.5V (buffer  200  should be simulated over the entire range of expected VCC operation to insure operation). 
     It should be understood that various alternatives to the embodiment of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.