WO2005057831A2

WO2005057831A2 - Low-power low-voltage multi-level variable-resistor line drive

Info

Publication number: WO2005057831A2
Application number: PCT/US2004/040802
Authority: WO
Inventors: Ramin Shirani
Original assignee: Plexus Networks, Inc.
Priority date: 2003-12-05
Filing date: 2004-12-03
Publication date: 2005-06-23
Also published as: WO2005057831A3; US20050122143A1; US7221196B2

Abstract

A low-power multi-level pulse amplitude modulation (PAM) line driver (100) using variable resistors (Rd and Rt) is disclosed for transmitting digital data over controlled-impedance transmission lines (110). The driver comprises two push-pull variable resistor branches (102a, 104a, 112a, with 114a and 102b, 104b, 112b, with 114b), and a middle variable resistor branch (116). The purpose of the two push-pull branches is to generate target voltage (Vop, Von) across the line, and the middle branch ensures the effective impedance of the resistors match the line impedance (Zo). The values of the variable resistors are selected by combinational logic (driver coder logic) whose input is raw data bits (Data(n)). The driver requires a supply voltage being equal or higher than twice the maximum output signal level (2*Vswing). This supply voltage can be a voltage supplied to the chip itself or a regulated supply voltage adjusted to provide a certain voltage swing (Vswing).

Description

LOW-POWER LOW- VOLTAGE MULTI-LEVEL VARIABLE-RESISTOR LINE DRIVER

FIELD OF THE INVENTION

[001] The present invention relates generally to the field of communications, and in

particular, to line drivers utilized for communications.

BACKGROUND OF THE INVENTION

[002] Communication links require a driver to transmit information into the channel

from transmitter to receiver. In wireline communications, the line drivers typically must satify two requirements: first generate a certain voltage swing across the transmission

line, and second have an output impedance that is matched to the line characteristic

impedance to absorb signals arriving at the transmitter and avoid reflections back to the

line. In digital data transmision, information is sent using different modulations. One of

the most common types of data modulation is pulse amplitude modulation (PAM), where

each group of data bits are presented by a voltage level that is transmitted into the

channel. Data modems and lOOBase-T/lOOOBase-T Ethernet transceivers are examples of

links that use multi- level sigaling or PAM to transmit information. Information typically

needs to be transmitted over distant and lossy channels. To ensure a minimum signal level at the receive end, the driver must generate a high enough signal power at transmit

end. As a result, the power efficiency of the drivers in most communication links is of

great importance, since significant portion of transceiver power is typically burned in the driver and its related circuitry.

[003] Figure 1 shows an example of a conventional commonly-used differential line driver 10, also known as common-source or common-emitter stage. The line driver 10 comprises a tail current source 12 and a pair of switches 14a and 14b that steer the

current from one branch to the other. Each branch of the driver is terminated to a voltage

source 16 by a fixed resistor 18a and 18b that is matched to the line single-ended charateristic impedance (or half its differential impedance). The effective impedance at the output of the driver is fixed and equal to the parallel resistance of the termination

impedance and line single-ended impedance (i.e. Z₀/2||Z_o/2=Z₀/4). The amplitude of the

output signal is controlled by the amount of the current steered into either of the

equivalent parallel resistors, being Z₀/4. Accordingly, for the conventional driver, to deliver a max swing of IV (or 2V differential pk- pk) into the line, the driver current is high due to the high single ended impedance.

[004] Another example of a line driver is the H-bridge topology 20 as shown in Figure

2. This topology 20 differs from the conventional driver 10 of Figure 1 in that it has

current steering branches 22a and 22b both at the bottom and top, respectively, and has a single termination resistor 24 equal to line impedance across its outputs nodes. Thus, H-

bridge bottom branch 22a pulls the same current that its top branch 22b pushes into the

equivalent line and termination impedance (i.e. Z₀||Z_o=Z₀/2), theoretically resulting in

twice the current efficiency of the convential type in Figure 1. However, in the H-bridge design to keep the current sources in saturation, it requires twice the headroom of a

conventional source-coupled design for the current source devices, thus typically requires

a higher supply voltage.

[005] A design that solves the headroom problem of H-bridge topology is disclosed in

U.S. Patent No. 6,175,255, entitled Line Driver Circuit for Low Voltage and Low Power

Applications by Jidentra and is shown in Figure 3. In this topology the top current

sources are removed and only the top switches that do not have a headroom requirement are left in the top branches. As shown in the waveforms in Figure 3, the top voltage level

of the stage output is in fact supply voltage, and thus leaving enough voltage headroom

for the bottom current sources. However, during the rise period that one of the top

switches is shorted, the resistance at the rising output node V_op or V_on becomes very low.

Thus, the RC time constant of the rising node gets very low, resulting in a very fast

transition. The falling node, where the switch is off, has an effective impedance of the

termination resistor R_t parallel with line impedance Z₀, thus experiencing a considerably

larger time constant than the rising node. This difference in the time constants on the two output nodes results in a rather large output common-mode voltage. Common-mode

voltage is not desirable in most wireline applications, especially Ethernet over unshielded

twisted pair, as it cause the wire to radiate electromagnetic waves and cause interference that violates FCC regulations. To avoid this common-mode effect, a transformer must be used to cancel out the common-mode component of the transmitted signal. The

requirement for a transformer makes the solution more expensive and less desirable

especially for very high-speed application as the transformer cost also goes up with

frequency of operation.

[006] It should be noted that in all above mentioned designs, the driver output

amplitude can be controlled by modulating the amplitude of the current in the current

sources.

[007] Accordingly, what is desired is that a line driver circuit is provided that

overcomes the above-identified problems. The present invention addresses sucha need.

SUMMARY OF THE INVENTION

[008] A low-power multi-level pulse amplitude modulation (PAM) line driver using variable resistors is disclosed for transmitting digital data over controlled-impedance

transmission lines. The driver comprises two push-pull variable resisor branches, and a middle variable resistor branch. The purpose of the two push-pull branches is to generate

the target voltage level onto the line, and the middle branch ensures that at all times the

effective parallel impedance of the resistors matches to the line impedance. The values of

the variable resistors are selected by a driver code logic whose input is the raw data bits. Each set of raw data bits is converted to a specific analog voltage level by the driver. The driver requires a supply voltage that is equal or higher than twice the absolute

maximum output signal level. This supply votage can be the supply voltage supplied to

the chip itself or a regulated supply voltage adjusted to result in a certain voltage swing.

[009] The present invention discloses the design of a multi-level PAM driver for highspeed wireline communication, with up to four times improvement in power efficiency

over conventional drivers. Two key requirements for high-speed line drivers are first

generating the target voltage level onto the controlled-impedance line, and second being

impedance-matched to the line itself to eliminate signal reflections from the transmitter back to the line. The driver in accordance with the present invention satisfies both of these requirements at very high power efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

[010] Figure 1 illustrates a conventional source/emitter-coupled line driver.

[011] Figure 2 illustrates a conventional H-bridge push-pull line driver.

[012] Figure 3 illustrates a modified H-bridge line driver for low- voltage operation.

[013] Figure 4 A illustrates a first embodiment of a variable-resistor multi-level line driver in accordance with the present invention.

[014] Figure 4B illustrates a second embodiment of a variable resistor multilevel driver in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION

[015] The present invention relates generally to the field of communications, and in

particular, to line drivers utilized for communications. The following description is

presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various

modifications to the preferred embodiment and the generic principles and features

described herein will be readily apparent to those skilled in the art. Thus, the present

invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein.

[016] Figures 3 A and 3B illustrate two embodiments for a variable-resistor line driver

circuit in accordance with the present invention. Both of the embodiments have the

capability to generate a continuous range of output signal swings, while maintaining an effective impedance equal to line differential line impedance, Z₀. The two embodiments

differ in the way the resistor pull up and pull down switching is done.

[017] The line driver circuit 100 of Fig. 3 A comprises two pull-up variable resistors,

R_d, 102a and 102b, series switch combination 104a and 104b connected between a voltage source, V_tt, 108 and output line 110, plus two pull-down variable resistors 112a

and 112b and switches 114a and 114b connected between ground (or negative supply)

and ouput lines, plus a floating variable resisor 116 connected across the output lines

110. The line driver circuit 200 of Fig. 3B is a similar topology but it shares the variable

resistor for pull-up and pull-down path. By sharing the switching node driver 200 saves

two variable resistors, and at the same time reduces the effective RC introduced to the

output node as a result of reduced switch capacitances. On the other hand, driver 100 can generate an output voltage with a controlled common-mode voltage if the pull-up and

pull-down resistors are set differently. Each of the drivers 100 and 200 convert each

group of n data bits into an output voltage level at a fixed output impedance in this driver, a coder logic 120 and 220 is required to digitally set the appropriate value of the

variable resistors. The coder logic also sets the sign of the output voltage based on a data digital value. A voltage buffer 101 regulates the top rail voltage, Vtt, of the driver

structure to twice of the maximum signal swing, V_SWιng, expected. [018] The goal is to produce and launch a voltage into the line, positive or negative,

whose amplitude is adjustable from zero to V_SWι_ng, and the driver's power/current consumption reduces with the output swing. To explain the operation of the cell, the

following example is used. Let's consider launching a positive signal, i.e. V_Sw_ιn_g ^>V₀p-

V_on^-O, into the line. In this case, the sign signal, Sign, is high and Sign_b is low. The

current path then is as shown with the dotted line in Figures 4A and 4B. The voltage across the output line is the result of voltage division over three resistors, the two

switched resistors R_d in the path and the parallel of R_t and the line impedance Z₀. So for

V₀ut we have:

Eql

[019] So by varying the two resistor in the line driver, R and R_t, the output voltage is

adjustable from

and R_t= Infinite. However, the two resistor at the same time must ensure a effective output impedance equal to that of the line impedance, so we have: Eq2 Solving for R_t we have: ^R _d * z₀ */ = Eq3 The above equation shows that the minimum value for R_d is Z₀/2 and minimum value for R_t is Z₀. The current consumption of this driver to launch a swing of V_outinto the line is:

Eq4 where R_d and R_t are calculated based on equations (1) and (2) for a given Vout. For example, to have the maximum output swing of V_0Ut⁼V_Swιng and for a line impedance of Z_o=100Ω, we have R_d=50Ω and R_t= Infinite, leading to a maximum j _ V swing ^max 100Ω current consumption of: Eq5

[020] This drive current also scales down with lower output voltage, although not

linearly, for example for the same above line impedance but a swing of V_out ⁼V_SWιng/2, we

have: R_d=100Ω and R_t=200Ω, and thus: r _ V swing 133Ω Eq6

[021] So for a maximum swing of V_SWιng⁼l V (or 2V differential pk-pk), as in

100/lOOOBaseT, the current consumption for maximum output voltage (2V differential

pk-pk) is 10mA, and for half of that output voltage (IV differential pk-pk) is 7.5mA.

[022] The above current numbers compared to a conventional source-coupled (or

emitter-coupled) driver of Figure 1, shows the clear power advantage of the proposed

block for providing a certain output voltage. For the conventional driver, to deliver a max swing of IV (or 2V differential pk- pk) into the line, the driver current must be four times

that of the inventive design:

γ _ V swi ■ng _ A V swi ■ng drive _Zo /₄ ^' ^ Eq7 [023] The inventive design also has a power advantage over other lower power designs

such as H- bridge line driver, as shown in Figure 2. As the H-bridge driver both sources

and sinks current at the same time, its current efficiency is twice that of the conventional

source-coupled design, but still almost half the proposed design. To repeat above

example again, for delivering a max swing of IV (or 2V differential pk-pk) into the line,

the driver current must be two times that of the proposed design:

j _ V ^r swi ■ng _ r V ^r swi ■ng drive _{ZQ / 1} ^■ _Zo Eq8 [024] However, in the H-bridge design to keep the current sources in saturation, it requires twice the headroom of a conventional source-coupled design for the current

source devices, thus typically requiring a higher supply voltage. The inventive design

does not suffer from this drawback either as the resistors do not require any headroom at

all to preserve their value and thus source or sink the correct current. [025] One of the other very important advantages of the line driver in accordance with

the present invention is the fact that the current driven into the link does not vary as a

result of large variations of line voltage. The other above mentioned conventional

designs as a result of requiring current source devices are subject to channel length

modulation for the output current as a result of large output voltage variations. This property of the inventive design is very crucial for applications such as lOOOBaseT, that uses bidirectional signaling, where the output voltage is super-imposed by the incoming

signal that can be as large as 2V differential pk-pk by itself, resulting in up to 4V

differential pk-pk swing (or 2Vswing on either side).

[026] One of the main advantages of the proposed driver design in the line driver circuit

of Fig. 3 A is that the output common mode of the driver can be adjusted by having

different pull-up and pull-down resistors. So as long as the differential impedance of the

stage stays the same, meaning:

(^Rdup ^{+ R}ddown) \ \^Rt ^{= Z}o Eq9 the signal common-mode can be shifted to a higher or lower voltage than half the

regulated voltage. Such a common-mode shift comes at no trade off in this driver, but

results in a rather considerable trade off in the conventional drivers. In the source/emitter

coupled driver, common-mode shift results in more power consumption proportional to the common-mode shift. In the H-bridge driver, it increases the headroom requirement or

in some case may not leave enough headroom for the current sources to operate properly.

[027] It is also very important to note that due to the flexibility of this inventive design,

the regulated supply is restricted to twice the voltage swing level at the minimum, but

there is no limitation on its maximum the value. As an option, to do without an extra

regulated power supply for the driver, one can simply use the off-chip supply that is rated

to be always higher than twice the required output swing. For example to support an

output swing of IV single-ended, one can use the 2.5V supply, that may go as low as

10% lower due to voltage tolerances and IR drops, and still get the required output swing and termination by proper choice of the resistors in the driver.

Advantages

[028] 1. Low power: Almost 75% less power compared to the conventional source-coupled (emitter-coupled) line driver, and 50% less power compared to the H-

bridge driver.

[029] 2. Low supply voltage: Does not require a high supply voltage as there is not

much headroom requirement by the resistor structure. What limits the minimum required supply voltage for a certain output swing is ensuring the effective output impedance of

the driver is equal to the line impedance for a required output swing.

[030] 3. The driver supply voltage can assume a range of values for a given output

swing above the minimum required supply (minimum being 2*V_SWι_ng) by proper choice of

resistor values in the driver branches.

[031] 4. No output current modulation: Since the proposed design is not a current

source driver, it does not suffer from channel length modulation that results in output

current modulation as a result of large voltage variations at the output.

[032] 5. Adjustable output common-mode voltage at no trade off for extra power or headroom requirement.

[033] Although the present invention has been described in accordance with the

embodiments shown, one of ordinary skill in the art will readily recognize that there

could be variations to the embodiments and those variations would be within the spirit

and scope of the present invention. Accordingly, many modifications may be made by

one of ordinary skill in the art without departing from the spirit and scope of the

appended claims.

Claims

What is claimed is: 1. A line driver circuit comprising: a voltage source; two variable push-pull resistor branches coupled to the voltage source; and a middle variable resistor branch, coupled to the two variable push-pull

resistor branches, wherein the values of the two push-pull variable resistor branches and the middle variable resistor branch are set to provide an output target level voltage.

2. The line driver circuit of claim 1 wherein the values are set by a driver code

logic.

3. The line driver circuit of claim 2 wherein the driver code logic is capable of

adjusting the two push-pull resistor branches and the middle variable resistor branch to

maintain a fixed output impedance across a range of output voltages levels.

4. The line driver circuit of claim 1 wherein each push-pull branch comprises two

switches and at least one variable resistor coupled to the switch.

5. The line driver circuit of claim 2 wherein the driver code logic generates

control signals that determine the sign of the drive output signal based on a digital data

source.

6. The line driver circuit of claim 2 wherein a voltage regulator regulates a top rail voltage of the voltage source to twice the absolute maximum signal swing.

7. The line driver circuit of claim 2 wherein the driver code logic converts digital

data sets into specific analog voltage levels.

8. The line driver circuit of claim 1 wherein the middle variable resistor branch is

coupled across the output ports of the two variable push pull resistor branches.

9. A line driver circuit comprising: a voltage source; two variable push-pull resistor branches coupled to the voltage source, wherein

the two variable push-pull resistor branches comprise two pull-up variable resistors, a first

two switch combination coupled to the two pull-up variable resistors, two pull down variable

resistors coupled to the two pull-up variable resistors and a second two switch combination coupled to the two pull-down variable resistors; and a middle variable resistor branch, coupled to the two variable push-pull

resistor branches, wherein the values of the two push-pull variable resistor branches and the

middle variable resistor branch are set to provide a target level voltage.

10. The line driver circuit of claim 9 wherein the values are set by a driver code logic.

11. The line driver circuit of claim 10 wherein the driver code logic is capable of

adjusting the two push-pull resistor branches and the middle variable resistor branch to maintain a fixed output impedance across a range of voltages.

12. The line driver circuit of claim 10 wherein the driver code logic generates

control signals that determines the sign of the drive output signal based on a digital data source.

13. The line driver circuit of claim 10 wherein an adjustable output common

mode signal can be provided thereby.