Voltage mode differential driver and method

A differential driver includes a switching module and first and second voltage controlled voltage sources. The switching module has a plurality of switches each controlled by an input signal, a first voltage input and a second voltage input, and a signal output. The first voltage controlled voltage source is connected to the first voltage input. The first voltage controlled voltage source has a low impedance. The second voltage controlled voltage source is connected to the second voltage input. The second voltage controlled voltage source also has a low impedance. The switching circuit outputs an output signal having an output voltage and current controlled by the first and second voltage controlled voltage sources. The output signal is based upon the input signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to input/output (I/O) interface circuitry for high speed data communications applications. More specifically the invention relates to low voltage differential signaling (LVDS) drivers, for use in the fields of communications, video and other integrated circuits that demand very high data transfer rates.

2. Description of the Related Art

Differential drivers are well known. Differential drivers are used in many input/output (I/O) applications such as in communications, video and integrated circuits that may demand high data transfer rate. Differential drivers are used in integrated circuits (IC) for on-chip communications between circuits, chip-to-board, off-chip communications, etc.

Low-voltage differential signaling (LVDS) technology was developed in order to provide a low-power and low-voltage alternative to other high-speed I/O interfaces specifically for point-to-point transmissions, such as those used in a network devices within data and communication networks. LVDS drivers can be implemented to overcome some deficiencies with previous I/O interface circuitry. However, the LVDS standard provides strict specifications for signal input and output characteristics, such as common mode voltage, differential voltage, etc.

In conventional I/O designs, high-speed data rates are accomplished with parallel I/O structures, each I/O device typically having a limited bandwidth. As bandwidth increases, more I/O devices are required to achieve the increased bandwidth. Over the years, bandwidth has increased substantially leading to massive parallelism in I/O designs in ICs. As a result, these parallel I/O structures occupy more and more space on ICs. This complicates the design of the circuits because there is less available space on the chip. The use of parallel structures also creates a need for additional supporting power supplies because of the numerous extra pads, current sources, etc. necessary in a parallel structure. Thus, most existing I/O drivers are not power efficient.

In portable devices, such as laptop computers, the power coming from the battery, low power allows for longer operating time. In the case where power is not restricted, such as in a desk top PC, power consumption is also important in IC. For example, if a CPU consumes more power, it will require an expensive package for the IC and possibly an additional cooling fan. Therefore, lower power means lower cost to the system.

A prior art LVDS driver is shown in FIG.1. The metal oxide silicon (MOS) transistor100is represented with a circle at the gate indicating that it is a P-type MOS (PMOS) transistor. Transistors101,110,111,120and121are N-type (NMOS) transistors. The driver includes two current sources100and101, and four current switching NMOS transistors110,111,120, and121. PMOS transistor100provides current from VDD to the top switching transistors110and121. A bias voltage Vb1controls the amount of current following through the transistor100. The bottom NMOS sinks current from the switching transistors120and111to ground (GND). A second bias, voltage Vb2, controls the current following through the transistor101. Biasing this circuit is fairly easy, and bias voltages are typically provided using current mirrors.

In normal operation, only one group of switching can be on. In the case when transistors110and111are ON and120and121are OFF, the current from the current source100flows through the switching transistor100and follows to the load resistor130. A voltage drop develops on the terminal of the resistor130. Since, in this case, the current follows from bottom node132to top node131, the bottom node132has a higher potential than the up node131. The current on the top node131is sunk by current source101through the switching transistor111. The current source101should sink the same amount of current as provided by current source100, to get the common mode voltage correctly.

In the opposite case, when transistors110and111are OFF and transistors121and121are ON, current will create a voltage drop of a reversed polarity on the load resistor130. In this case, the top node131has a higher potential than the bottom node132.

There are two major drawbacks in this circuitry for high speed IC applications. First, operating speed is limited due to the high impedance design. Node Vhigh and node Vlow are high impedance nodes with relatively large parasitic capacitance, and therefore, are slow to respond. In high speed switching, these nodes also cause the common mode voltage to drift. A poorly designed current source, as an example, could have an impedance above a few kilo-ohms. Moreover, a well designed current source will have much higher impedance. Moreover, a well designed current source, such as cascoded current source, will have much high impedance.

Second, in a high speed serial interconnection, termination at the driver side may be required for good signal integrity. This circuit does not include terminal resistors, and therefore, has poor signal integrity at high speeds.

FIG. 2shows another prior art implementation of an LVDS driver that has built-in termination resistors. The operation of the circuit is very similar to the first circuit, except the load is now shared with the resistors150and151. The impedances at the current source100and101are very high and can be neglected compared to the termination resistor. To terminate the source properly, resistors150and151need to be half the resistance of the resistor130. For a typical application, resistor130is 100 ohms. Thus, resistors150and151need to be 50 ohms each. In this design, the same amount of current will follow into resistors150and151. The advantage of adding resistors150and151is that the impedance at Vhigh and Vlow are reduced for high speed operation. Also, since this reduces reflection in the transmission line, signal integrity is improved. However, the current efficiency of this driver is 50% because only 50% of the current generated flows to the load. Thus, this circuit design is deficient for having a low current efficiency.

In view of the deficiencies in the prior art, there is a need for new and improved systems and methods for driving LVDS in modern I/O applications.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a differential driver is provided. The differential driver includes a switching module and first and second voltage controlled voltage sources. The switching module has a plurality of switches each controlled by an input signal, a first voltage input and a second voltage input, and a signal output. The first voltage controlled voltage source is connected to the first voltage input. The first voltage controlled voltage source has a low impedance. The second voltage controlled voltage source is connected to the second voltage input. The second voltage controlled voltage source also has a low impedance. The switching circuit outputs an output signal having an output voltage and current controlled by the first and second voltage controlled voltage sources. The output signal is based upon the input signal.

According to another embodiment of the present invention, a method of driving a signal is provided. The method includes a step of providing a switching module having a first and second voltage input, a signal input, and a signal output. The signal input is connected to a plurality of switches in order to control an operation of the switches. The signal output is connected to the first and second voltage inputs via the plurality of switches. The method also includes a step of providing a first voltage controlled voltage source having a first voltage output having a low impedance. The method also includes a step of providing a second voltage controlled voltage source having a second voltage output having a low impedance. The method also includes a step of connecting the voltage output of the first voltage controlled voltage source to the first voltage input of the switching module. The method also includes a step of connecting the voltage output of the second voltage controlled voltage source to the second voltage input of said switching module.

According to another embodiment of the present invention, a differential driver is provided. The differential driver includes a switching means and first and second voltage controlled voltage source means. The switching means is for switching a plurality of switches in order to produce a signal output based on an input signal, a first and second voltage input. The first voltage controlled voltage source means is for generating a first low impedance voltage output as the first voltage input to the switching means. The second voltage controlled voltage source means is for generating a second low impedance voltage output as the second voltage input to the switching means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3is an illustration of a voltage mode differential driver according to an embodiment of the present invention. The differential driver includes two Voltage Controlled Voltage Sources (VCVS)210and211, which provide DC voltages to the nodes Vhigh and Vlow, respectively. The output impedance for voltage controlled voltage source210is modeled by a resistor202and is configured to be a low impedance in the range of a few hundred ohms, preferable around 30 ohms for a differential load of 100 ohms. Similarly, the output impedance for the voltage controlled voltage source211is modeled by resistor203and is similarly configured to be a low impedance. VCVS210and211are biased by bias voltages Vb1and Vb2, respectively output by a bias generator215.

The differential driver also includes a switching circuit having a plurality of switches for switching an output voltage (signal) based on an input signal. The switching circuit may include a pair of voltage inputs, at nodes Vhigh and Vlow, and series of switches. In this example, the switching circuit includes two pair of switching transistors110a,111aand120a,121a,which act as the switches. A signal output, Vout1and Vout2, are output from the switching circuit to nodes131and132across a resistive load130. Load130may typically be 100 ohms, but can vary depending on the application. Load130may be a differential load, and accordingly, may be grounded in the middle of the load. As a differential load, load130would include 50 ohms above the ground and 50 ohms below the ground. Output node131is connected to the drains of transistors110aand120a,and output node132is connected to the drains of transistors121aand111a.

In normal operation, only a single pair of switching transistors will be ON while the other is OFF, in order to allow a current to flow from VCVS210through the load130to VCVS211. In the present embodiment, switching transistors110aand121aare PMOS transistors, and switching transistors120aand111aare NMOS transistors. PMOS transistors are used on the top of the switching circuit because the common mode voltage is around half of the supply voltage, or about 1.2V. The PMOS transistors could have a smaller size than that of NMOS transistors. If NMOS were used, the size would be very big comparatively, because of the Vgs required to turn the switching transistor completely ON. If NMOS transistors were used, then there is a risk of common mode voltage drifting, which will place the common mode voltage outside of the LVDS standard for LVDS applications.

An input signal is used to control the switching of the switching circuit. In order to turn the pair110a,111aON and pair120a,121aOFF simultaneously, input signal IN+ is sent to transistor110awhile an inverted signal IN− is sent to transistor111ato turn both transistors ON. Similarly, IN+ is sent to transistor120awhile inverted signal IN− is sent to transistor121ato turn them both OFF. Therefore, the gates of transistors110aand120amay be connected, while the gates of121aand111amay be connected. In a preferred embodiment of the present invention, the differential driver is used in LVDS applications. In LVDS applications, IN+ may be 2.5V while IN− may be 0V.

The voltage drop across node Vhigh and Vlow is calculated by:
Vhigh−Vlow=I*(R(110a)+R(111a)+Rload),

where R(110a) and R(111a) are the ON resistance of transistors110aand111a,and I is the current required to be flowing through the load resistor. Of course, the current I at the load may be set to comply with the LVDS standard. The biasing and configuration of the VCVS210and211may likewise be adjusted in accordance with the LVDS standard or to obtain a desired output Vout2−Vout2. Accordingly, the voltage supplied at Vhigh and Vlow are calculated to provide the proper common mode voltage and current output to the load. Because this circuit has very low impedance at Vlow, and Vhigh, it is capable of high speed operation with high efficiency.

The switching circuit may also provides gain, and the differential driver may act as an amplifier to amplify the incoming signal. As described above, the gates of transistors110aand120a,hereafter referred to as G0, may be tied together, and the gates of transistors111aand121a,hereafter referred to as G1, may be tied together. The gain may be calculated by:
A(v/v)=[V(131)−V(132)]/[V(G0)−V(G1)]=[gm(110a)+gm(120a)]*R(130),

where gm(110a) and gm(120a) are the transconductance of transistors110aand120a.As an example, when there is a current of around 3 mA flowing into the load resistor130, then gm(110a) or gm(120a) is typically around 15 millisiemens (mS). Thus,
A(v/v)=(15mS+15mS)*100 ohms=3V/V.

Here is an example when the incoming signal is 50 mV and the output signal would be 150 mV. Note that this is a small signal gain. In a preferred embodiment, the incoming signals are large signal (as opposed to small signal). The voltage swing across G0and G1may be 0 to 2.5V. Therefore, the output would be 7.5V if the output is not limited by the power supply. The results are that these four transistors are working in triode region (with gm much smaller than 15 mS), where the resistance between source and drain may be around 10 ohms. When the transistors are OFF, the resistance across drain and source are infinity, and when the transistors are ON, the resistance across the drain and source is only about 10 ohms. Therefore, the transistors may be referred to or replaced by switches.

An advantage of the embodiment illustrated inFIG. 3is that it allows for low power consumption with high speed operation through the use source followers at nodes Vhigh and Vlow. The impedance looking into the source follower is significantly lower than if a current source were used. Thus, the nodes Vhigh and Vlow can be operated at high speed.

FIG. 4is an illustration of voltage mode differential driver according to another embodiment of the present invention. The differential driver includes VCVS210and VCVS211, which provide DC voltages to the nodes Vhigh and Vlow, respectively, and a switching circuit.

VCVS210includes a transistor300, which is implemented by an NMOS transistor in a source follower configuration. Similarly, VCVS211includes transistor301, which is implemented by a PMOS transistor in a source follower configuration. Source followers provide significantly lower impedance looking into the source follower, i.e., at nodes Vhigh and Vlow, than that of a current source, such as shown inFIGS. 1-2. For example, in an LVDS application, a typical driver current I flowing to the load is 3 mA. When a 3 mA current flows in the transistors300or301, the impedance looking into source is very low, and can be well below 50 ohms. Resistors190and191may be added to VCVS210and211to protect the differential driver from excessive current in the case of shorted terminals.

The switching circuit includes a pair of voltage inputs, connected at nodes Vhigh and Vlow, and two pair of switching transistors110a,111aand121a,122a,which act as the switches. In the present embodiment, switching transistors110aand121aare PMOS transistors, and switching transistors120aand111aare NMOS transistors. PMOS transistors are used on the top of the switching circuit because the common mode voltage is around half of the supply voltage, which is about 1.2V. The PMOS transistors could have a smaller size than that of NMOS. If NMOS were used, the size would be very big comparatively, because of the Vgs required to turn the switching transistor completely ON. Moreover, if NMOS transistors were used, then there is a risk of common mode voltage drifting, which will place the common mode voltage outside of the LVDS standard for LVDS applications. Vout1and Vout2are output from the switching circuit to nodes131and132across a resistive load130, which may be a differential load as described above. Load130may typically be 100 ohms, such as in LVDS applications. Output node131is connected to the drains of transistors110aand120a,and output node132is connected to the drains of transistors121aand111a.

The source of transistor300is connected to node Vhigh, which provides voltage to the top of switching transistors110aand121a.The gate of transistor300is connected to bias voltage Vb1, the drain is connected to VDD through resistor190, and the P-well is also connected node Vhigh. In normal process the substrate (P-well) is connected to ground by default. However, it is preferred that the substrate be connected to the source (node Vhigh) to reduce the body effect and lower the threshold voltage of the transistor. The lowering of the threshold voltage allows the MOSFET to be smaller for the same amount of current. Therefore, the area of the IC can be smaller and cost can be lower.

From gate to source, transistor300can have a voltage gain of 0.8 V/V-1.0 V/V, and also will have some DC level shifting (Vt+Vdsat in this case). Transistor300has a low impedance given by 1/gm, where gm is the transconductance of the source follower300. The low impedance at node Vhigh allows faster response time at the node, and therefore, allows for better high-speed switching output from the differential driver at load130.

Transistor301may be a PMOS transistor. The source of transistor301is connected to Vlow and provides solid low impedance voltage for the switching transistors120aand111a.The gate of transistor301is connected to bias voltage Vb2, the drain is connected to ground (GND) through resistor191, and the N-well is preferably connected to source (node Vlow). Connecting the N-well to the source lowers the threshold voltage and provides more “headroom” for operating at low supply voltage.

Similar to that described above with reference toFIG. 3, in normal operation, only one pair of switching transistors are switched ON at a time. An input signal is used to control the switching of the switching circuit. In order to turn the pair110a,111aON and pair120a,121aOFF simultaneously, input signal IN+ is sent to transistor110awhile an inverted signal IN− is sent to transistor111ato turn both transistors ON. Similarly, IN+ is sent to transistor120awhile inverted signal IN− is sent to transistor121ato turn them both OFF. Therefore, the gates of transistors110aand120amay be connected, while the gates of121aand111amay be connected. In a preferred embodiment of the present invention, the differential driver is used in LVDS applications. In LVDS applications, IN+ may be 2.5V while IN− may be 0V.

In the case where transistors110aand111aare ON and121aand120aare OFF, the transistor300provides a voltage Vhigh to drive a current through the MOS switch110ato the load resistor130, then through transistor111ato the source of transistor301. Similarly, when110aand111aare OFF and121aand120aare ON, the transistor300provides a voltage Vhigh to drive current through the MOS switch121ato the load resistor130, then through transistor120ato the source of transistor301.

Using source followers to provide voltage to the switching circuit requires proper biasing, especially in a low voltage or LVDS applications. The bias voltage Vb1may be determined as follows:
Vb1=(Vhigh+Vtn+Vdsat),

where Vtn is the threshold voltage of the NMOS transistor300, and Vdsat is the overdrive voltage for the NMOS transistor300when conducting a certain amount of current. Since the current flows from bottom node132to top node131, the bottom node132has a higher potential than the up node131. The PMOS transistor301provides a low voltage at node Vlow to sink current from the load resistor130through the transistor111. The voltage drop at the load resistor (130), the required voltage difference of Vhigh and Vlow can be calculated as followed:
Vhigh−Vlow=I*[R(121a)+R(130)+R(120a)],

where R(121a), R(130) and R(120a) are the ON resistance of transistors121a,130and120arespectively. Transistors110aand121amay be provided to have identical sizes, and so may transistors120aand111a,and are preferably 20 ohms for a differential load of 100 ohms. Thus, the common mode voltage is calculated by
(Vhigh+Vlow)/2,

since the resistance of transistors121aand120aare designed to be the same ideally. On the opposite case, when transistor110aand111aare OFF and transistor121aand120aare ON, the voltage drop on the load resistor130will be reversed polarity. The top node131has a higher potential than the bottom node132.

Similar calculations can be made to determine the necessary bias voltage Vb2. In low voltage applications it may be desired to provide at least one voltage source greater than 1.2 volts in order to ensure that proper biasing of the circuit is obtained.

Similar to above, the driver of this embodiment may provide gain. Accordingly, the switching transistors and the power supplies may be configured to apply a small signal or large signal gain to the incoming signal. Furthermore, the switching transistors may be replaced by switches.

There are two major advantages in this implementation. First, as described above, node Vhigh and node Vlow are very low impedance nodes. Although the nodes have relatively large parasitic capacitance, they are fast to respond. Therefore, the differential driver is capable of high speed operations. Second, the driver may include built-in terminated resistance, for better signal integrity. To terminate the differential driver properly, the impedance of the driver needs to be the same as the transmission line. A typical transmission line has single ended 50 ohms impedance, thus the output impedance should be 50 ohms. Take the example when transistors110aand111aare closed. The MOS transistors110aand111ahave impedances of R(110a) and R(111a), respectively. The impedance looking into source of the NMOS transistor300is 1/gm (300). Thus, to get a total impedance of 50 ohms, one should design R(110a)=50−1/gm (300) ohms. The same can be said for the PMOS side, transistor301, and one should design R(111a)=50−1/gm (301) ohms. Therefore, if the load130is a differential load of 100 ohms, half the load (50 ohms) is mirrored by the top half of the driver (R(300)+R(110aor121a)=50 ohms) and the other half of the load is mirrored by the bottom half of the driver (R(301)+R(111aor120a)=50 ohms).

Because of the built-in termination resistance, the circuit does not need additional termination resistors in parallel with the load. Therefore the circuit inFIG. 4can achieve 100% current efficiency, without wasting current in the passive termination resistors.

It should be noted that a more linearized output impedance may be provided by added a linear resistor between the source of each VCVS and the node Vhigh and Vlow, respectively. Accordingly,FIG. 6shows linear resistors400and401added to VCVS210and211, respectively. In this case, when 1/gm is small, the linear resistors can be added to get an output impendence of R(301)+R(111aor120a)+R(400), and R(300)+R(10aor121a)+R(401).

FIG. 5is a flowchart of a method for driving a signal according to an embodiment of the present invention. The process begins at step S5-1. At step S5-2, a switching module is provided, such as described above. The switching module may be implemented via MOS transistors. The switching module may include a voltage input and output, and a signal input and output. The voltage input is connected to the switches in order to flow a current to the signal output. The switching module may be configured to receive an input signal and switch the switches to produce a signal output based on the signal input. The switching module may be configured for LVDS applications and may have built-in termination resistors as described above.

Next, at step S5-3, low impedance, voltage controlled voltage sources are provided at the voltage input and output of the switching module. Voltage controlled voltage sources may be as already described above and may include source followers.

Next, at step S5-4, the voltage controlled voltage sources are biased for the application of the driver. The biasing of the voltage controlled voltage sources can be done by a bias generator or other circuit, and may be implemented in accordance with the above-described embodiments. For LVDS applications, the biasing of the circuit should take into consideration the desired output voltage and current of the switching module, as well as all the characteristics of the source followers and the switches themselves.

Next, at step S5-5, an input signal may be provided. The input signal may be input via an input circuit to each switch, as already described above. Depending upon the configuration of the switches, the input signal may be inverted, pulled-up or pulled-down. The voltage controlled voltage source and the switching module are configured, as described above, to generate a high speed output signal based up the input signal. This output signal may be in compliance with LVDS standards.

One having ordinary skill in the art will understand that these method steps may be performed in different orders to accomplish the same result.

Embodiments of the present invention may be drawn to differential drivers such as LVDS drivers that can operate at high speed with less power because it operates with a reduced voltage swing. Due to the reduced voltage swing, which allows the LVDS driver to operate at high speeds, less parallelism is needed. Also with the differential outputs, a receiver can reject ambient common mode noise and system reflection noise. However, performance can vary significantly for LVDS drivers of different designs. Two important parameters to consider are operation frequency and power consumption.

Although the invention has been described based upon these preferred embodiments, it would be apparent to those of skilled in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention. In order to determine the metes and bounds of the invention, therefore, reference should be made to the appended claims.

For example, VCVS210and211may include an operational amplifier, and the impedance of the drive my be controlled by adding an extra termination resistor Rtt. In this case, the loop gain of the opamp and of transistor300or301, can be large, and the impedance can be very low. In this case, the output impedance is dominated by Rtt, and Rtt may be set close to 50 ohms to get good termination.

Furthermore, other active devices, such as BJTs or BiCMOS transistors may be used. In this case, transistor300could be an NPN transistor with the emitter connected to Vhigh, and transistor301may be a PNP transistor with the emitter connected to Vlow.