# Patent

## Title
Low-swing level shifter

## Abstract

In general, in one aspect, the disclosure describes an apparatus for shifting a low swing signal. The apparatus includes a first pair of transistors to receive a first input signal and a second input signal and to generate a first output signal that is a shifted version of the first input signal. The apparatus further includes a second pair of transistors to receive the first input signal and the second input signal and to generate a second output signal that is a shifted version of the second input signal.

## Background
BACKGROUND
Voltage signals have an upper limit and a lower limit and a voltage swing therebetween. Circuits may be designed to work with high or low voltages, may be designed for high or low swings, may be designed to work near a saturation region or in the saturation region. The lower the swing the faster that processing can occur. Often the swing of a signal is sufficient but the upper or lower parameters of the signals need to be adjusted. For example, the signal may need to be shifted up or down so that a transistor receiving the signal operates in the saturation region. Shifting a signal entails maintaining the swing (absolute voltage drop) of the signal while moving upper and lower limits of the signal.
Devices for shifting the current may be complex or may be based on current drawn by a load connected to the shifting device. Relying of the current drawn by the load requires excess power consumption. If the load is modified then the current drawn may be modified and the voltage shift may change accordingly.



BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the various embodiments will become apparent from the following detailed description in which:
FIG. 1A illustrates an exemplary comparison of ideal signal values versus an incoming signal, according to one embodiment;
FIG. 1B illustrates exemplary shifted incoming signals, according to one embodiment;
FIG. 2 illustrates an exemplary differential signal and shifted versions thereof, according to one embodiment;
FIG. 3 illustrates an exemplary net differential input signal, according to one embodiment;
FIG. 4A illustrates an exemplary level shifter for a differential input signal that is always high, according to one embodiment;
FIG. 4B illustrates exemplary differential input voltage signals being shifted by the level shifter of FIG. 4A, according to one embodiment;
FIG. 5 illustrates an exemplary CML D-latch, according to one embodiment;
FIG. 6A illustrates an exemplary level shifter for a differential input signal that is always low, according to one embodiment; and
FIG. 6B illustrates exemplary differential input voltage signals being shifted by the level shifter of FIG. 6A, according to one embodiment.



DETAILED DESCRIPTION
Transistors may be used in circuits for many purposes and the circuits may be designed to operate in many different ways. For example, the circuits may be designed to work with high or low voltages, may be designed for high or low swings (difference between upper and lower value), may be designed to work near a saturation region or in the saturation region. Often the swing of a signal is sufficient but the upper or lower parameters of the signals need to be adjusted. For example, the signal may need to be shifted up or down so that a transistor receiving the signal operates in the saturation region.
Shifting a signal entails maintaining the swing (absolute voltage drop) of the signal while moving upper and lower limits of the signal. For example, a signal ranging from 0.5V to 4.5V would have a swing of 4.0V and could be shifted up 0.5V so that the range of the signal was from 1.0V to 5.0V and still had a swing of 4.0V. The reasons that a signals need to be shifted can vary.
A signal may be shifted to account for differences between an ideal signal and a generated signal. That is, the generated signal may not meet the parameters of the ideal signal and may need to be shifted up or down to ensure a particular parameter is met. Using the example above, the 0.5V to 4.5V signal may be the actual signal generated for an ideal 0.0V to 5.0V signal. A circuit receiving the signal may be triggered by an upper or lower value of the ideal signal so the signal may be shifted up or down based on parameters associated with the circuit receiving the signal.
FIG. 1A illustrates an exemplary comparison of ideal signal values versus an exemplary incoming signal 100. The ideal values include a lower value 110 (VIL) and an upper value 120 (VIH). The incoming signal 100 also has a lower value 130 (VRL) and an upper value 140 (VRH). However, the VRL 130 may not be as low as the VIL 110 and the VRH 140 may not be as high as the VIH 120. For example, ideal signal values may include a VIL 110 of 0.0 V and a VIH 120 of 5.0 V, while an incoming signal may have a VRL 130 of 0.2 V and a VRH 140 of 4.8 V. Ideal signal values of VIL 110=−2.5 V and VIH 120=2.5 V may have a VRL 130 of −2.3 V and a VRH 140 of 2.3 V. The swing (VSW) 150 of the incoming signal 100 is from the VRL 130 to the VRH 140.
If a circuit is designed to perform (or for optimum performance) at an ideal signal value and the realistic value drifts to far from the ideal value, the performance of the circuit may be affected. For example, if a circuit is designed to be activated at a VIL 110 of 0.0V and the VRL 130 is 0.2 V, the operation of the circuit may be degraded in some fashion. The incoming signal 100 may be shifted down so that VRL 130 was closer to VIL 110. Likewise, if VIH 120 is critical to the operation of the circuit the incoming signal 100 may be shifted up prior to applying to the circuit.
FIG. 1B illustrates exemplary shifted incoming signals 160, 170. The incoming signal 160 has been shifted up so that an upper value (VRHS) 180 it is at or near the VIH 120 and a lower value (VRLS ) 185 is accordingly shifted up an equal amount. The incoming signal 170 has been shifted down so that a lower value (VRLS) 190 it is at or near the VIL 110 and an upper value (VRHS) 195 is accordingly shifted down an equal amount. For example, an incoming signal having a VRL 130 of 0.2 V and a VRH 140 of 4.8 V may be shifted down by 0.2 V so that the VRLS 190 becomes 0.0 V (equal to VIL 110) and the VRHS 195 becomes 4.6 V or may be shifted up by 0.2 V so that the VRLS 185 becomes 0.4 V and the VRHS 180 becomes 5.0 V (equal to VIH 120). An incoming signal having a VRL 130 of −2.3 V, and a VRH 140 of 2.3 V may be shifted down by 0.2 V so that the VRLS 190 becomes −2.5 V (equal to VIL 110) and the VRHS 195 becomes 2.1 V or may be shifted up by 0.2 V so that the VRLS 185 becomes −2.1 V and the VRHS 180 becomes 2.5 V (equal to VIH 120).
A signal having a particular swing may be shifted so that it can be used at a different offset voltage (voltage around which the signal is centered). For example, a signal having a particular swing and offset voltage may be shifted up or down so that it can be used more efficiently by another circuit. For example, if a signal has an offset voltage of 1.0V and a swing of 0.4V (range from 0.8 to 1.2V) and another circuit operates most efficiently with an offset voltage of 5.0V the signal may be shifted up 4.0V so that the offset is 5.0V and the signal ranges from 4.8 to 5.2V.
FIG. 2 illustrates an exemplary differential signal 200 and shifted versions 240, 270 thereof. Each of the differential signal 200, 240, 270 includes VDiff− 205, 245, 275 and VDiff+ 210, 250, 280 (the compliment of VDiff− 205, 245, 285). The differential signal 200 is centered around an offset voltage VOff 215 and has a voltage swing (VSW) 220. Accordingly, the VDiff− 205 and VDiff+ 210 range from a high voltage (VH) 225 (VOff 215+VSW/2) to a low voltage (VL) 230 (VOff 215−VSW/2). The shifted signal 240 is shifted up by VUP so that it centered around an offset voltage VOff 255 (VOff 215+VUP) but still has the same VSW 220. Accordingly, the VDiff− 245 and VDiff+ 250 range from a VH 260 (VOff 255+VSW/2) to a VL 265 (VOff 255−VSW/2). The shifted signal 270 is shifted down by VDOWN so that it centered around an offset voltage VOff 285 (VOff 215−VDOWN) but still has the same VSW 220. Accordingly, the VDiff− 275 and VDiff+ 280 range from a VH 290 (VOff 285+VSW/2) to a VL 295 (VOff 285−VSW/2).
For example, signals 205, 210 having a VOff 215 of 2.5V and a VSW 220 of 1.0 V (range from 2.0V to 3.0V) may be shifted up by 2.0V so that signals 245, 250 have a VOff 255 of 4.5V and range from 4.0V to 5.0V. Likewise, the signals 205, 210 may be shifted down 2.0V so that signals 275, 280 have a VOff 285 of 1.0V and range from 0.0V to 1.0V. As noted, the VSW 220 stays the same when the signal is shifted, regardless of it is shifted up or down.
FIG. 3 illustrates an exemplary net differential input signal 300. The signal 300 is the same before and after being shifted (either up or down). The net differential signal 300 is the difference between the two signals making up the differential signal (VDiff+−VDiff−). Accordingly, the net differential signal 300 has a VOff 310 of 0.0V when the VDiff+ and the VDiff− cross. The net differential signal 300 ranges from a VH 320 of +VSW to a VL 330 of −VSW. For the example noted directly above, the net differential signal 300 for the incoming signal and each of the shifted signals would have the VOff 310 of 0.0V and range from a VH 320 of 1.0V to a VL 330 of −1.0V.
FIG. 4A illustrates an exemplary level shifter 400 for a differential signal, wherein each leg of the differential signal is always high. The level shifter 400 includes four NMOS transistors 410, 420, 430, 440. All NMOS transistors 410, 420, 430, 440 are used because the incoming signal is always high and would not activate a PMOS transistor. The transistors 410, 420 have their sources tied to GND 450 and the transistors 430, 440 have their drains tied to VCC 460. The drains of transistors 410, 420 are tied to the sources of transistors 430, 440 respectively. A gate of the transistors 420, 430 receive a first leg of the differential input signal (VIN+) 470 and a gate of the transistors 410, 440 receive a second leg (VIN−) 480, that is the compliment of the VIN+ 470. The transistors 410, 430 in combination with one another produce a first leg of the differential output signal (VOUT+) 490. The transistors 420, 440 in combination with one another produce a second leg (VOUT−) 495 that is the compliment of the VOUT+ 490.
The transistors 420, 430 are turned ON when the VIN+ 470 is at or near it's high point (VCC 460) and VIN− 480 is at or near it's low point (VCC 460−VSW). The transistors 410, 440 are turned ON when the VIN− 480 is high and the VIN+ 470 is low. The fact that the transistors 430, 440 are NMOS means that there will a voltage drop Vt) across the transistors 430, 440 when the transistor 430, 440 are ON. Accordingly, the transistors 430, 440 will not pass VCC but instead will pass VCC−Vt. Accordingly, VOUT+ 490 and VOUT− 495 will at a minimum be shifted down by Vt so that they range between VCC−Vt and VCC−Vt−VSW. By varying the relative size of the transistors 430, 440 with respect to the transistor 410, 420, the offset voltage of the output signals 490, 495 can be shifted down further with respect to the input signals 470, 480. The output signals 490, 495 could be possibly be shifted down a maximum amount so that they range from GND to GND+VSW.
FIG. 4B illustrates an exemplary differential input voltage signal being shifted by a level shifter (e.g., level shifter 400 of FIG. 4A). Each leg of the input signals includes a high voltage (VIH), a low voltage (VIL) and a voltage swing (VSW) therebetween. The VIH is equal to VCC and the VIL is equal to VCC−VSW. The input signals are provided to the level shifter 400 that has the upper voltage (VCC) 460 and the lower voltage (GND) 450. The output signals maintain the same VSW as the received signals 470, 480 however upper and lower values of the output signals (VOH, VOL) can be varied between VCC−Vt and GND. That is, the output signals can be shifted up down a minimum amount so that VOH is equal to VCC−Vt and the VOL is equal to VCC−Vt−VSW. Likewise, the output signals can be shifted down a maximum amount until the VOL is equal to the GND and the VOH is equal to GND+VSW.
For example, assume the incoming signals had a VIH of 3.5V, a VIL of 2.5V, and the transistors 430, 440 have a Vt of 0.2V. The incoming signals can be shifted down a minimum amount of Vt so that the output signals swing between 2.3V and 3.3V, a maximum amount so that the output signals range between 0.0V and 1.0V, or anywhere in between.
The level shifter 400 takes in a low-swing differential signal, and outputs a level-shifted version of that signal. The output signal can drive large capacitive loads and the signal swing is very accurately buffered. The level-shifter 400 may be used in CML circuits. While CML logic signals typically operate at signal swings of VCC to VCC−VSW, in certain situations they operate better when the clock signals are shifted down so that transistors can operate in the saturation region. The level shifter 400 shifts down the signal, while maintaining the signal's swing, and consuming less power than other types of buffers. The level shifter 400 is useful in circuits which are implemented using CML logic.
FIG. 5 illustrates an exemplary CML D-latch 500. The CML D-latch 500 includes transistors 510, 520 for receiving a differential clock signal. The clock signals received by the transistors 510, 520 typically operate at signal swings of VCC to VCC−VSW. However, the circuit 500 operates better when the clock signals are shifted down so that the transistors 510, 520 operate in the saturation region. A low swing level-shifter (e.g., 400 of FIG. 4A) would be placed before the circuit 500 in order to lower the offset of the clock signal, and hence enable the D-latch to operate better.
FIG. 6A illustrates an exemplary level shifter 600 for a differential signal, wherein each leg of the differential signal is always low. The level shifter 600 includes four PMOS transistors 610, 620, 630, 640. The PMOS transistors 610, 620, 630, 640 are used because the incoming signal maintains is always low and would not activate an NMOS transistor. The transistors 610, 620 have their drains tied to GND 650 and the transistors 630, 640 have their sources tied to VCC 660. The transistors 610, 620 have their sources tied to drains of the transistors 630, 640 respectively. A gate of the transistors 610, 640 receive a first input (VIN+) 670 and a gate of the transistors 620, 630 receive a second input (VIN−) 680, that is the compliment of the VIN+ 670. The transistors 620, 640 in combination with one another produce a first output (VOUT+) 690. The transistors 610, 630 in combination with one another produce a second output (VOUT−) 695 that is the compliment of the VOUT+ 690.
The transistors 610, 640 are turned ON when the VIN+ 670 is at or near it's low point (GND 650) and VIN− 680 is at or near it's high point (GND 650+VSW). The transistors 620, 630 are turned ON when the VIN− 680 is low and the VIN+ 670 is high. The fact that the transistors 610, 620 are PMOS means that there will a voltage drop (Vt) across the transistors 610, 620 when the transistor 610, 620 are ON. Accordingly, the transistors 610, 620 will not pass GND but instead will pass GND+Vt. Accordingly, VOUT+ 690 and VOUT− 695 will at a minimum be shifted up by Vt so that they range between GND+Vt and GND+Vt+VSW. By varying the relative size of the transistors 630, 640 with respect to the transistor 610, 620, the offset voltage of the output signals 690, 695 can be shifted up further with respect to the input signals 670, 680. The output signals 690, 695 could possibly be shifted up a maximum amount so that they range from VCC to VCC−VSW.
FIG. 6B illustrates an exemplary differential input voltage signal being shifted by a level shifter (e.g., level shifter 600 of FIG. 6A). Each of the input signals 670, 680 includes a high voltage (VIH) and a low voltage (VIL) and a voltage swing (VSW) therebetween. The input signals 670, 680 are provided to the level shifter 600 that has the upper voltage (VCC) 660 and the lower voltage (GND) 650. The output signals 690, 695 will maintain the same VSW as the received signals 670, 680 however upper and lower values of the output signals (VOH, VOL) can be varied between the VDD and the GND+Vt. That is, the output signals 690, 695 can be shifted up until the VOH is equal to VDD and the VOL is equal to VDD−VSW. Likewise, the output signals 690, 695 can be shifted down until the VOL is equal to the GND+Vt and the VOH is equal to GND+VSW+Vt.
For example, assume the incoming signals had a VIH of 1.0V, a VIL of 0.0V, VCC is 3.5V and the transistors 610, 620 had a Vt of 0.2V. The incoming signals can be shifted up a minimum amount of Vt so that the output signals swing between 0.2V and 1.2V, a maximum amount so that the output signals range between 2.5V and 3.5V, or anywhere in between.
The various embodiments described herein could be utilized in a computer system. As one skilled in the art would recognize a computer system includes processor(s) and memory and may interface to periphery, networks, the Internet, and other computer systems. The computer system may include a single die with the processor(s) and memory or may include a processor die and off die memory (e.g., a memory die). The various embodiments may be implemented as part of the memory or part of the processor(s).
Although the various embodiments have been illustrated by reference to specific embodiments, it will be apparent that various changes and modifications may be made. Reference to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment” appearing in various places throughout the specification are not necessarily all referring to the same embodiment.
Different implementations may feature different combinations of hardware, firmware, and/or software. It may be possible to implement, for example, some or all components of various embodiments in software and/or firmware as well as hardware, as known in the art. Embodiments may be implemented in numerous types of hardware, software and firmware known in the art, for example, integrated circuits, including ASICs and other types known in the art, printed circuit broads, components, etc.
The various embodiments are intended to be protected broadly within the spirit and scope of the appended claims.

## Claims

1. An apparatus comprising two parallel stacks of transistors, coupled between two voltage sources, to receive a differential voltage input signal and generate a differential voltage output signal, wherein the two parallel stacks include
a first stack of transistors includes a first transistor coupled to a first voltage source and receiving a first leg of a differential voltage input signal and a second transistor coupled to a second voltage source and receiving a second leg of the differential voltage input signal, wherein the first stack generates a first leg of a differential voltage output signal; and
a second stack of transistors including a third transistor coupled to the first voltage source and receiving the second leg of the differential voltage input signal and a fourth transistor coupled to the second voltage source and receiving the first leg of the differential voltage input signal, wherein the second stack generates a second leg of the differential voltage output signal,
wherein the differential voltage output is a shifted down version of the differential voltage input signal and has substantially same swing as the differential voltage input signal, wherein the down shift is at least a threshold voltage associated with the transistors, wherein the first leg of the differential voltage output signal is the first leg of the differential voltage input signal minus at least the threshold voltage of the first transistor and the second leg of the differential output voltage signal is the second leg of the differential input signal minus at least the threshold voltage of the third transistor, and wherein the differential voltage output signal is shifted further downward based on ratio of widths of the second and the fourth transistors to the first and the third transistors respectively.

2. The apparatus of claim 1, wherein the downshift is independent of load the differential voltage output signal is applied to.