U.S. patent application number 15/227853 was filed with the patent office on 2018-02-08 for impedance and swing control for voltage-mode driver.
This patent application is currently assigned to Xilinx, Inc.. The applicant listed for this patent is Xilinx, Inc.. Invention is credited to Kok Lim Chan, Kun-Yung Chang, Yohan Frans, Chin Yang Koay, Siok Wei Lim, Kee Hian Tan, Hongyuan Zhao.
Application Number | 20180041232 15/227853 |
Document ID | / |
Family ID | 61027238 |
Filed Date | 2018-02-08 |
United States Patent
Application |
20180041232 |
Kind Code |
A1 |
Lim; Siok Wei ; et
al. |
February 8, 2018 |
IMPEDANCE AND SWING CONTROL FOR VOLTAGE-MODE DRIVER
Abstract
A driver circuit includes a plurality of output circuits coupled
in parallel between a differential input and a differential output
and having a first common node and a second common node. Each of
the plurality of output circuits includes a series combination of a
pair of inverters and a pair of resistors, coupled between the
differential input and the differential output; first source
terminals of the pair of inverters coupled to the first common
node; and second source terminals of the pair of inverters coupled
to the second common node. The driver circuit further includes a
first voltage regulator having an output coupled to the first
common node of the plurality of output circuits; a second voltage
regulator having an output coupled to the second common node of the
plurality of circuits; and a current compensation circuit coupled
between the outputs of the first voltage regulator and the second
voltage regulator.
Inventors: |
Lim; Siok Wei; (Singapore,
SG) ; Chan; Kok Lim; (Singapore, SG) ; Tan;
Kee Hian; (Singapore, SG) ; Zhao; Hongyuan;
(Singapore, SG) ; Koay; Chin Yang; (Singapore,
SG) ; Frans; Yohan; (Palo Alto, CA) ; Chang;
Kun-Yung; (Los Altos Hills, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Xilinx, Inc. |
San Jose |
CA |
US |
|
|
Assignee: |
Xilinx, Inc.
San Jose
CA
|
Family ID: |
61027238 |
Appl. No.: |
15/227853 |
Filed: |
August 3, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03M 9/00 20130101; H03K
17/163 20130101; H03K 17/6872 20130101; H03K 19/0175 20130101; H02M
3/158 20130101 |
International
Class: |
H04B 1/04 20060101
H04B001/04; H03M 9/00 20060101 H03M009/00; H02M 3/158 20060101
H02M003/158; H03K 17/687 20060101 H03K017/687 |
Claims
1. A driver circuit, comprising: a plurality of output circuits
coupled in parallel between a differential input and a differential
output and having a first common node and a second common node,
each of the plurality of output circuits comprising: a series
combination of a pair of inverters and a pair of resistors, coupled
between the differential input and the differential output; first
source terminals of the pair of inverters coupled to the first
common node; and second source terminals of the pair of inverters
coupled to the second common node; a first voltage regulator having
an output coupled to the first common node of the plurality of
output circuits; a second voltage regulator having an output
coupled to the second common node of the plurality of circuits; and
a current compensation circuit coupled between the outputs of the
first voltage regulator and the second voltage regulator.
2. The driver circuit of claim 1, wherein the current compensation
circuit comprises: a plurality of circuits having an enable input,
a first bias input, and a second bias input, each of the plurality
of circuits having a first transistor, a second transistor, and a
third transistor serially connected to provide a current path
between the outputs of the first and second voltage regulators.
3. The driver circuit of claim 2, wherein, for each of the
plurality of circuits of the current compensation circuit, a gate
of the first transistor is coupled to receive a signal of the
enable input, a gate of the second transistor is coupled to receive
a signal of the first bias input, and a gate of the third
transistor is coupled to receive a signal of the second bias
input.
4. The driver circuit of claim 1, wherein the first voltage
regulator comprises: a first transistor coupled between a supply
voltage source and the first common node of the plurality of
outputs circuits; and a first operational amplifier having a first
input coupled to a first reference voltage source, a second input
coupled to the first common node of the plurality of output
circuits, and an output coupled to a gate of the first
transistor.
5. The driver circuit of claim 4, wherein the second voltage
regulator comprises: a second transistor coupled between a ground
source and the second common node of the plurality of outputs
circuits; and a second operational amplifier having a first input
coupled to a second reference voltage source, a second input
coupled to the second common node of the plurality of output
circuits, and an output coupled to a gate of the second
transistor.
6. The driver circuit of claim 1, further comprising: a first
capacitor coupled between the first common node of the plurality of
output circuits and a ground source; and a second capacitor coupled
between the second common node of the plurality of output circuits
and the ground source.
7. The driver circuit of claim 1, wherein the differential output
of the plurality of output circuits is coupled to a pair of
transmission lines.
8. A driver circuit, comprising: a plurality of output circuits
coupled in parallel between a differential input and a differential
output and having a first common node and a second common node,
each of the plurality of output circuits comprising: a series
combination of a pair of enable circuits, a pair of inverters, and
a pair of resistors, coupled between the differential input and the
differential output; a first transistor coupled between the first
common node and first source terminals of the pair of inverters;
and a second transistor coupled between the second common node and
second source terminals of the pair of inverters; first and second
replica output circuits coupled in series between the first and
second common nodes; and a control circuit coupled to each of:
respective gates of the first and second transistors in each of the
plurality of output circuits; and the first and second replica
output circuits.
9. The driver circuit of claim 8, further comprising: a replica
load resistor coupled in series between the first and second
replica output circuits.
10. The driver circuit of claim 9, wherein: the first replica
output circuit comprises a first replica transistor, a second
replica transistor, and a first replica resistor serially connected
to provide a current path between the first common node and the
replica load resistor; and the second replica output circuit
comprises a third replica transistor, a fourth replica transistor,
and a second replica resistor serially connected to provide a
current path between the replica load resistor and the second
common node.
11. The driver circuit of claim 10, wherein the control circuit
comprises: a first operational amplifier having a first terminal
coupled to a first switched resistor network, a second terminal
coupled to a node between the first replica resistor and the
replica load resistor, and an output terminal coupled to each of: a
gate of the first replica transistor; and gate of the first
transistor in each of the plurality of output circuits; and a
second operational amplifier having a first terminal coupled to a
second switched resistor network, a second terminal coupled to a
node between the second replica resistor and the replica load
resistor, and an output terminal coupled to each of: a gate of the
fourth replica transistor; and gate of the second transistor in
each of the plurality of output circuits.
12. The driver circuit of claim 11, wherein the first switched
resistor network is coupled between the first common node and a
reference resistor, and wherein the second switched resistor
network is coupled between the reference resistor and the second
common node.
13. The driver circuit of claim 12, wherein the first switched
resistor network comprises a first plurality of taps and a first
switch coupled between the first plurality of taps and the first
terminal of the first operational amplifier, and wherein the second
switched resistor network comprises a second plurality of taps and
a second switch coupled between the second plurality of taps and
the first terminal of the second operational amplifier.
14. The driver circuit of claim 11, wherein the second replica
output circuit further comprises: a startup circuit coupled to the
output terminal of the second operational amplifier.
15. The driver circuit of claim 8, wherein the pair of enable
circuits in each of the plurality of output circuits each comprises
a NAND gate and a NOR gate.
16. The driver circuit of claim 8, further comprising: a first
voltage regulator having an output coupled to the first common node
of the plurality of output circuits; and a second voltage regulator
having an output coupled to the second common node of the plurality
of circuits.
17. The driver circuit of claim 16, further comprising: a first
capacitor coupled between the first common node of the plurality of
output circuits and a ground source; and a second capacitor coupled
between the second common node of the plurality of output circuits
and the ground source.
18. The driver circuit of claim 8, wherein the differential output
of the plurality of output circuits is coupled to a pair of
transmission lines.
19. A method of controlling a driver circuit in a transmitter,
comprising: receiving a plurality of outputs of an equalizer in the
transmitter; coupling each of the plurality of outputs of the
equalizer to at least one of a plurality of output circuits of the
driver circuit; enabling first and second voltage regulators
coupled to the plurality of output circuits; and enabling at least
one of a plurality of current compensation circuits coupled between
the first and second voltage regulators.
20. The method of claim 19, wherein the plurality of output
circuits is coupled in parallel between a differential input and a
differential output and includes a first common node and a second
common node, wherein each of the plurality of output circuits
comprises a series combination of a pair of enable circuits, a pair
of inverters, and a pair of resistors, coupled between the
differential input and the differential output; a first transistor
coupled between the first common node and first source terminals of
the pair of inverters; and a second transistor coupled between the
second common node and second source terminals of the pair of
inverters, and wherein the method further comprises: disabling at
least one of the plurality of output circuits; and adjusting a
gate-to-source voltage of each of the first transistor and the
second transistor in each of the plurality of circuits based on
feedback from first and second replica output circuits.
Description
TECHNICAL FIELD
[0001] Examples of the present disclosure generally relate to
electronic circuits and, in particular, to impedance and swing
control for a voltage-mode driver.
BACKGROUND
[0002] In serial communication systems, a large percentage of the
total power is consumed in the transmitter, which must provide for
adequate signal swing on a low-impedance channel while maintaining
an appropriate source termination. In addition, the transmitter
often includes equalization to compensate for frequency-dependent
loss in the channel. The driver circuit in the transmitter often
consumes the majority of the power of the transmitter. Driver
circuits can be implemented as current-mode drivers or voltage-mode
drivers. Voltage-mode drivers are known to consume far less power
in comparison to current-mode drivers. For example, a voltage-mode
driver can consume four times less DC power than a current-mode
driver to provide the same output swing.
[0003] A voltage-mode driver for a transmitter requires swing and
impedance control such that the swing and
common-mode/differential-mode return loss are within
specifications. One technique for output signal swing control in a
driver circuit is to use a single voltage regulator to generate a
reference voltage that sets the voltage swing. However, with a
single regulator, the common-mode will shift as the output swing of
the driver circuit changes. Such a shift in the common-mode can
cause the return loss to exceed specifications.
SUMMARY
[0004] Techniques for impedance and swing control for a
voltage-mode driver are described. In an example, a driver circuit
includes a plurality of output circuits coupled in parallel between
a differential input and a differential output and having a first
common node and a second common node. Each of the plurality of
output circuits includes a series combination of a pair of
inverters and a pair of resistors, coupled between the differential
input and the differential output; first source terminals of the
pair of inverters coupled to the first common node; and second
source terminals of the pair of inverters coupled to the second
common node. The driver circuit further includes a first voltage
regulator having an output coupled to the first common node of the
plurality of output circuits; a second voltage regulator having an
output coupled to the second common node of the plurality of
circuits; and a current compensation circuit coupled between the
outputs of the first voltage regulator and the second voltage
regulator.
[0005] In another example, a driver circuit includes a plurality of
output circuits coupled in parallel between a differential input
and a differential output and having a first common node and a
second common node. Each of the plurality of output circuits
includes a series combination of a pair of enable circuits, a pair
of inverters, and a pair of resistors, coupled between the
differential input and the differential output; a first transistor
coupled between the first common node and first source terminals of
the pair of inverters; and a second transistor coupled between the
second common node and second source terminals of the pair of
inverters. The driver circuit further includes first and second
replica output circuits coupled in series between the first and
second common nodes; and a control circuit coupled to each of:
respective gates of the first and second transistors in each of the
plurality of output circuits; and the first and second replica
output circuits.
[0006] In another example, a method of controlling a driver circuit
in a transmitter includes receiving a plurality of outputs of an
equalizer in the transmitter; coupling each of the plurality of
outputs of the equalizer to at least one of a plurality of output
circuits of the driver circuit; enabling first and second voltage
regulators coupled to the plurality of output circuits; and
enabling at least one of a plurality of current compensation
circuits coupled between the first and second voltage
regulators.
[0007] These and other aspects may be understood with reference to
the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] So that the manner in which the above recited features can
be understood in detail, a more particular description, briefly
summarized above, may be had by reference to example
implementations, some of which are illustrated in the appended
drawings. It is to be noted, however, that the appended drawings
illustrate only typical example implementations and are therefore
not to be considered limiting of its scope.
[0009] FIG. 1 is a block diagram depicting an example of a serial
communication system.
[0010] FIG. 2 is a schematic diagram depicting an output driver
according to an example.
[0011] FIGS. 3A-3B depict a schematic diagram of an output driver
according to another example.
[0012] FIG. 4 is a flow diagram depicting a method of controlling a
driver circuit in a transmitter according to an example.
[0013] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures. It is contemplated that elements of
one example may be beneficially incorporated in other examples.
DETAILED DESCRIPTION
[0014] Various features are described hereinafter with reference to
the figures. It should be noted that the figures may or may not be
drawn to scale and that the elements of similar structures or
functions are represented by like reference numerals throughout the
figures. It should be noted that the figures are only intended to
facilitate the description of the features. They are not intended
as an exhaustive description of the claimed invention or as a
limitation on the scope of the claimed invention. In addition, an
illustrated example need not have all the aspects or advantages
shown. An aspect or an advantage described in conjunction with a
particular example is not necessarily limited to that example and
can be practiced in any other examples even if not so illustrated
or if not so explicitly described.
[0015] Techniques for impedance and swing control for a
voltage-mode driver are described. In an example, a driver circuit
includes output circuits between a differential input and a
differential output. The output circuits are coupled between first
and second common nodes. Each output circuit includes a pair of
inverters and a pair of resistors coupled between the differential
input and output. First source terminals of the pair of inverters
are coupled to the first common node and second source terminals of
the pair of inverters are coupled to the second common node. First
and second voltage regulators are coupled to the first and second
common nodes. A current compensation circuit is coupled between
outputs of the first and second voltage regulators. Zero or more of
the current compensation circuits can be selectively enabled to
draw dummy current from the voltage regulators to improve return
loss, as discussed further below. Further, use of dual regulators
allows for a fixed common mode in both low- and high-swing modes.
These and further aspects are discussed below with respect to the
drawings.
[0016] FIG. 1 is a block diagram depicting an example of a serial
communication system 100. The serial communication system 100
comprises a transmitter 112 coupled to a receiver 126 over
transmission medium 160. The transmitter 112 can be part of a
serializer-deserializer (SERDES) 116. The receiver 126 can be part
of a SERDES 122. The transmission medium 160 comprises an
electrical path between the transmitter 112 and the receiver 126
and can include printed circuit board (PCB) traces, vias, cables,
connectors, decoupling capacitors, and the like. In examples, the
transmission medium 160 includes a matched pair of transmission
lines each having a characteristic impedance (Z.sub.0). The
receiver of the SERDES 116, and the transmitter of the SERDES 122,
are omitted for clarity. In some examples, the SERDES 116 can be
disposed in an integrated circuit (IC) 110, and the SERDES 122 can
be disposed in an IC 120.
[0017] In general, the transmitter 112 generates a serial data
signal from a parallel data path (serialization). The serial data
signal has a particular data rate (symbol rate). In some examples,
data bytes from the parallel data path can be encoded prior to
serialization using, for example, and 813/10B encoder or the like.
The transmitter 112 drives the serial data signal onto the
transmission medium 160 using a digital modulation technique, such
as binary non-return-to-zero (NRZ) pulse amplitude modulation
(PAM). The transmission medium 160 propagates electrical signal(s)
representing symbols of the serial data signal (e.g., logic "1" and
logic "0") towards the receiver 126.
[0018] In the example shown, the transmission medium 160 is a
differential channel. Data on the differential channel is
represented using two electrical signals ("true" and "complement"
signals). A logic "0" is represented by driving the true signal to
its lower voltage limit and driving the complement signal to its
upper voltage limit. A logic "1" is represented by driving the true
signal to its upper voltage limit and driving the complement signal
to its lower voltage limit. Thus, the logic value of each
transmitted symbol is based on the difference between the true and
complement signals, and not based on the level of either signal
individually. The peak-to-peak difference between the true signal
and the complement signal is the voltage swing (also referred to as
signal swing or swing).
[0019] The transmitter 112 includes a finite impulse response (FIR)
filter 114, a pre-driver 115, an output driver 118, and control
logic 150. The transmitter 112 is configured to equalize the serial
data signal prior to transmission over the transmission medium 160.
The FIR 114 can be used to mitigate inter-symbol interference (ISI)
caused by the transmission medium 160. The transmission medium 160
degrades the signal quality of the transmitted signal. Channel
insertion loss is the frequency-dependent degradation in signal
power of the transmitted signal. When signals travel through a
transmission line, the high frequency components of the transmitted
signal are attenuated more than the low frequency components. In
general, channel insertion loss increases as frequency increases.
Signal pulse energy in the transmitted signal can be spread from
one symbol period to another during propagation on the transmission
medium 160. The resulting distortion is known as ISI. In general,
ISI becomes worse as the speed of the communication system
increases.
[0020] The output of the FIR filter 114 is coupled to an input of
the pre-driver 115. The output of the FIR filter 114 can include a
plurality of signals, including a main-cursor signal, and one or
more pre-cursor signals, one or more post-cursor signals, or a
plurality of post-cursor and pre-cursor signals. For purposes of
clarity by example, the present description assumes the FIR filter
114 outputs one main-cursor signal, one pre-cursor signal, and one
post-cursor signal. The pre-driver 115 is configured to couple the
output of the FIR filter 114 to the output driver 118. As discussed
below, the output driver 118 is segmented and includes a plurality
of output circuits coupled in parallel to the transmission medium
160. The pre-driver 115 couples each of the main-cursor, the
pre-cursor, and the post-cursor signals to a selected percentage of
the output circuits of the output driver 118. The percentages of
output circuits driven by the main-cursor, pre-cursor, and
post-cursor signals as selected by the pre-driver 115 is controlled
by the control logic 150. The control logic 150 also controls
aspects of the output driver 118, as discussed further below.
[0021] While the SERDES 116 and the SERDES 122 are shown, in other
examples, each of the transmitter 112 and/or the receiver 126 can
be a stand-alone circuit not being part of a larger transceiver
circuit. In some examples, the transmitter 112 and the receiver 126
can be part of one or more integrated circuits (ICs), such as
application specific integrated circuits (ASICs) or programmable
ICs, such as field programmable gate arrays (FPGAs).
[0022] FIG. 2 is a schematic diagram depicting the output driver
118 according to an example. The output driver 118 includes output
circuits 208.sub.1 through 208.sub.N (where N is an integer greater
than one), voltage regulators 210.sub.1 and 210.sub.2, and current
compensation circuits 206.sub.1 through 206.sub.M (where M is an
integer greater than one). The output circuits 208.sub.1 through
208.sub.N are collectively referred to as output circuits 208; the
voltage regulators 210.sub.1 and 210.sub.2 are collectively
referred to as voltage regulators 210; and the current compensation
circuits 206.sub.1 through 206.sub.M are collectively referred to
as current compensation circuits 206.
[0023] The output circuits 208 are coupled in parallel between a
differential input 202 and a differential output (Txp, Txn). The
differential input 202 includes N differential signals output by
the pre-driver 115. Each differential signal includes a true
signal, Inp, and a complement signal, Inn. Thus, the differential
input 202 includes signals Inp.sub.1 through Inp.sub.N and signals
Inn.sub.1 through Inn.sub.N.
[0024] The output circuits 208 are coupled to common nodes
V.sub.refp and V.sub.refn. Each of the output circuits 208 includes
transistors M.sub.p1, M.sub.p2, M.sub.n1, and M.sub.n2. Each of the
output circuits 208 also includes resistors R.sub.p and R.sub.n.
The transistors M.sub.p1 and M.sub.n1 comprise p-channel field
effect transistors (FETs), such as P-type metal-oxide semiconductor
FETs (MOSFETs) (also referred to as PMOS transistors). The
transistors M.sub.p2 and M.sub.n2 comprise n-channel FETs, such as
N-type MOSFETs (also referred to as NMOS transistors). For purposes
of clarity, only the output circuit 208.sub.1 is shown in detail.
However, each of the output circuits 208.sub.2 through 208.sub.N
are configured identically with the output circuit 208.sub.1.
[0025] Sources of the transistors M.sub.p1 and M.sub.n1 are coupled
to the common node V.sub.refp. Drains of the transistors M.sub.p1
and M.sub.n1 are coupled to drains of the transistors M.sub.p2 and
M.sub.n2, respectively. Sources of the transistors M.sub.p2 and
M.sub.n2 are coupled to the common node V.sub.refn. Gates of the
transistors M.sub.p1 and M.sub.p2 are coupled together and are
coupled to receive a signal Inp of one of the input differential
signals. Gates of the transistors M.sub.n1 and M.sub.n2 are coupled
together and are coupled to receive a signal Inn of one of the
input differential signals. A first terminal of the resistor
R.sub.p is coupled to the drains of the transistors M.sub.p1 and
M.sub.p2, and a second terminal of the resistor R.sub.p is coupled
to the node Txp of the differential output. A first terminal of the
resistor R.sub.n is coupled to the drains of the transistors
M.sub.n1 and M.sub.n2, and a second terminal of the resistor
R.sub.n is coupled to the node Txn of the differential output. The
transistors M.sub.p1 and M.sub.p2 form a first inverter (M.sub.p),
and the transistors M.sub.n1 and M.sub.n2 form a second inverter
(M.sub.n). A series combination of the pair of inverters (M.sub.p,
M.sub.n) and the pair of resistors R.sub.p and R.sub.n is coupled
between the differential input 202 and the differential output
(Txp, Txn). The source terminals of the inverters are coupled
between the nodes V.sub.refp and V.sub.refn.
[0026] The voltage regulator 210.sub.1 includes an operational
amplifier A.sub.vrefp and a transistor M.sub.vrefp. The transistor
M.sub.vrefp is an n-channel FET, such as an N-type MOSFET. A
non-inverting input terminal of the operational amplifier
A.sub.vrefp is coupled to a first reference voltage source
(V.sub.ref1). An inverting input of the operational amplifier
A.sub.vrefp is coupled to the node V.sub.refp. A drain of the
transistor M.sub.vrefp is coupled to a supply voltage source
(V.sub.sup). A source of the transistor M.sub.vrefp is coupled to
the node V.sub.refp. A gate of the transistor M.sub.vrefp is
coupled to an output of the operational amplifier A.sub.vrefp.
[0027] The voltage regulator 210.sub.2 includes an operational
amplifier A.sub.vrefn and a transistor M.sub.vrefn. The transistor
M.sub.vrefn is an n-channel FET, such as an N-type MOSFET. A
non-inverting input terminal of the operational amplifier
A.sub.vrefn is coupled to a second reference voltage source
(V.sub.ref2). An inverting input of the operational amplifier
A.sub.vrefn is coupled to the node V.sub.refn. A source of the
transistor M.sub.vrefn is coupled to a ground voltage source. A
drain of the transistor M.sub.vrefn is coupled to the node
V.sub.refn. A gate of the transistor M.sub.vrefn is coupled to an
output of the operational amplifier A.sub.vrefn.
[0028] The current compensation circuits 206 are coupled in
parallel between the nodes V.sub.refp and V.sub.refn. Each of the
current compensation circuits 206 includes transistors M.sub.1,
M.sub.2, and M.sub.3. The transistor M.sub.1 is a p-channel FET,
such as a P-type MOSFET. The transistors M.sub.2 and M.sub.3 are
n-channel FETs, such as an N-type MOSFET. For purposes of clarity,
only the current compensation circuit 206.sub.1 is shown in detail.
However, each of the current compensation circuits 206.sub.2
through 206.sub.M are configured identically with the current
compensation circuit 206.sub.1.
[0029] A source of the transistor M.sub.1 is coupled to the node
V.sub.refp. A drain of the transistor M.sub.1 is coupled to a drain
of the transistor M.sub.2. A source of the transistor M.sub.2 is
coupled to a drain of the transistor M.sub.3. A source of the
transistor M.sub.3 is coupled to the node V.sub.refn. A gate of the
transistor M.sub.2 in each of the current compensation circuits 206
is coupled to a bias voltage source V.sub.1. A gate of the
transistor M.sub.3 in each of the current compensation circuits 206
is coupled to a bias voltage source V.sub.2. A gate of the
transistor M.sub.1 is coupled to receive an enable signal of an
enable input 204. The enable input 204 includes M enable signals
EN.sub.1 through EN.sub.M coupled to the M current compensation
circuits 206, respectively.
[0030] The output driver 118 further includes capacitors
C.sub.vrefp and C.sub.vrefn. The capacitor C.sub.vrefp is coupled
between the node V.sub.refp and electrical ground. The capacitor
C.sub.vrefn is coupled between the node V.sub.refn and electrical
ground.
[0031] The differential output (Txp, Txn) is coupled to a pair of
transmission lines 212.sub.P and 212.sub.n (collectively
transmission lines 212). The transmission lines 212 drive a load
resistance R.sub.L. The transmission lines 212 and the load
resistance R.sub.L are not part of the output driver 118. Rather,
the transmission lines 212 are part of the transmission medium 160
and the load resistance R.sub.L is part of the receiver 126.
[0032] In operation, each output circuit 208 includes a pair of
inverters driven by complementary input (a differential signal of
the differential input 202). Each differential signal of the
differential input 202 can be one of a main-cursor signal, a
post-cursor signal, or a pre-cursor signal. As discussed above, the
pre-driver 115 controls the number of output circuits 208 receiving
each of the main-cursor, post-cursor, and pre-cursor signals. For
example, the output circuits can receive all main-cursor signals,
some main-cursor signals and some pre-cursor signals, some
main-cursor signals and some post-cursor signals, or some
main-cursor signals, some post-cursor signals, and some pre-cursor
signals. Mixing post/pre-cursor signals with the main-cursor
signals is used to implement emphasis and de-emphasis equalization
in the transmitter 112.
[0033] The voltage regulators 210 set the swing of the output
driver 118. The differential peak-to-peak swing is
V.sub.refp-V.sub.refn. In an example, the voltage regulator
210.sub.2 can include a switch 214 configured to short the drain of
the transistor M.sub.vrefn to electrical ground. This allows the
voltage regulator 210.sub.2 to be disabled in one mode (high-swing
mode) and enabled in another mode (low swing mode). Zero or more of
the current compensation circuits 206 are selectively enabled using
the enable input 204 to draw dummy current from the voltage
regulator 210 to improve return loss, as discussed further below. A
control signal for the switch 214, and the enable input to the
current compensation circuits 206, can be generated by the control
logic 150.
[0034] With the dual regulators 210.sub.1 and 210.sub.2 in the
output driver 118, the swing and common-mode can be set
independently. For example, the common-mode can be fixed at 0.45 V.
Table 1 below illustrates characteristics of the high-swing mode
and the low-swing mode for both dual regulators and a single
regulator.
TABLE-US-00001 TABLE 1 Regulator Mode Swing V.sub.refp V.sub.refn
Common-mode Dual 0.6 V 0.75 V 0.15 V 0.45 V Dual 0.9 V 0.9 V 0 V
0.45 V Single 0.6 V 0.6 V 0 V 0.3 V Single 0.9 V 0.9 V 0 V 0.45
V
As shown in Table 1, when both regulators 210.sub.1 and 210.sub.2
are enabled, the common-mode is the target 0.45 V for the low-swing
mode (e.g., 0.6 V). If only the regulator 210.sub.1 is enabled, the
common mode is lower than the target 0.45 V (e.g., 0.3 V) for the
low-swing mode. Use of dual regulators allows for a fixed common
mode in both low- and high-swing modes. The values in Table 1 are
exemplary and the output driver 118 can be configured with other
common-mode voltages, other high-swing voltages, and
other-low-swing voltages.
[0035] In the output driver 118, equalization can be implemented by
driving a different number of the output circuits 208 with
different main/pre/post cursor signals. With the dual-regulator
approach, the swing is changed by adjusting the regulator voltage.
Thus, equalization control is independent of the swing control.
This allows for high FIR resolution even in low-swing mode.
[0036] For a voltage-mode driver, the current drawn by the output
circuits 208 can be calculated using the following relationship:
Id=(differential swing)/(external differential resistance+internal
differential resistance). In an example, each transmission line
212.sub.p and 212.sub.n has a characteristic impedance Z.sub.0 of
50 ohms (external differential resistance=100 ohms). Ideally, the
output driver 118 provides a matching impedance of 50 ohms for each
transmission line 212 (e.g., internal differential resistance=100
ohms). If the desired swing is 0.9 V, then the current drawn by the
output circuits 208 is approximately 4.5 mA. The actual current
consumption may be higher to account for transient switching
crowbar current.
[0037] For the above equation, it is noted that the current drawn
by the output circuits 208 changes with the output swing. For lower
swing, less current is drawn by the output circuits 208 from the
voltage regulator 210.sub.1. The output impedance of the voltage
regulator 210.sub.1 increases as less current is drawn from the
voltage regulator 210.sub.1. Notably, the output impedance of the
voltage regulator 210.sub.1 is the output resistance of the
transistor M.sub.vrefp (gm) divided by (1+loop gain). When the
voltage regulator 210.sub.1 supplies low current, the operational
amplifier A.sub.vrefp provides less loop gain, thereby increasing
the output impedance of the voltage regulator 210.sub.1. The output
circuits 208 see an effective impedance of the capacitor
C.sub.vrefp in parallel with the output impedance of the voltage
regulator 210.sub.1. For mid- to low-frequencies (e.g., 100 MHz),
the impedance of the capacitor C.sub.vrefp is high and thus the
output impedance of the voltage regulator 210.sub.1 is not
negligible. Thus, the decreased output impedance of the voltage
regulator 210.sub.1 due to low current draw by the output circuits
208 degrades the return loss of the output driver 118.
[0038] The current compensation circuits 206 are selectively
enabled to mitigate the increase in return loss by drawing a
constant dummy current in parallel with the output circuits 208.
Thus, at higher swing settings, less or none of the current
compensation circuits 206 can be enabled, as sufficient current is
drawn from the voltage regulator 210.sub.1. At lower swing
settings, more of the current compensation circuits 206 can be
enabled, which ensures that sufficient current is drawn from the
voltage regulator 210.sub.1 to maintain loop gain and low output
impedance.
[0039] FIGS. 3A-3B depict a schematic diagram of the output driver
118 according to another example. FIG. 3A shows a portion 118A of
the output driver 118, and FIG. 3B shows a portion 118B of the
output driver 118. Elements in FIGS. 3A and 3B that are the same or
similar to those of FIG. 2 are designated with identical reference
numerals and are described above. The output driver 118 includes
output circuits 308.sub.1 through 308.sub.N (where N is an integer
greater than one), the voltage regulators 210.sub.1 and 210.sub.2,
replica circuits 320.sub.1 and 320.sub.2, and a control circuit 350
comprising operational amplifiers A.sub.repl1, A.sub.repl2, and
resistors R.sub.ref1 through R.sub.ref11. The output circuits
308.sub.1 through 308.sub.N are collectively referred to as output
circuits 308, and the replica circuits 320.sub.1 and 320.sub.2 are
collectively referred to as replica circuits 320. In some examples,
the output driver shown in FIGS. 3A and 3B can also include the
current compensation circuits 206 described above. For purposes of
clarity, the current compensation circuits 206 are omitted from
FIGS. 3A and 3B.
[0040] As shown in the portion 118A of the output driver 118 in
FIG. 3A, the output circuits 308 are coupled in parallel between
the differential input 202 and the differential output (Txp, Txn).
The output circuits 308 are coupled to the common nodes V.sub.refp
and V.sub.refn. Each of the output circuits 308 includes
transistors M.sub.pdrv1, M.sub.pdrv2, M.sub.ndrv1, M.sub.ndrv2,
M.sub.res1, and M.sub.res2. Each of the output circuits 208 also
includes resistors R.sub.pdrv and R.sub.ndrv, and enable circuit
U.sub.p formed by NAND gate U.sub.p1 and U.sub.p2, and an enable
circuit Un formed by U.sub.n1 and U.sub.n2. The transistors
M.sub.pdrv1 and M.sub.ndrv1 comprise p-channel FETs, such as P-type
MOSFETs. The transistors M.sub.pdrv2 and M.sub.ndrv2 comprise
n-channel FETs, such as N-type MOSFETs. Sources of the transistors
M.sub.pdrv1 and M.sub.ndrv1 are coupled to a drain of the
transistor M.sub.res1. Drains of the transistors M.sub.pdrv1 and
M.sub.ndrv1 are coupled to drains of the transistors M.sub.pdrv2
and M.sub.ndrv2, respectively. Sources of the transistors
M.sub.pdrv2 and M.sub.ndrv2 are coupled to a drain of the
transistor M.sub.res2.
[0041] Gates of the transistors M.sub.pdrv1 and M.sub.pdrv2 are
coupled to outputs of the NAND gate U.sub.p1 and the NOR gate
U.sub.p2, respectively. In another example, the NAND gate U.sub.p1
and the NOR gate U.sub.p2 are replaced by a single inverter having
an output coupled to the gates of the transistors M.sub.pdrv1 and
M.sub.pdrv2. First input terminals of the NAND gate U.sub.p1 and
the NOR gate U.sub.p2 are coupled together, and are coupled to
receive one end of a differential input signal (Inp). Second inputs
of the NAND gate U.sub.p1 and the NOR gate U.sub.p2 are coupled to
a true enable signal en and a complement enable signal enb. Gates
of the transistors M.sub.ndrv1 and M.sub.ndrv2 are coupled to
outputs of the NAND gate U.sub.n1 and the NOR gate U.sub.n2,
respectively. First input terminals of the NAND gate U.sub.n1 and
the NOR gate U.sub.n2 are coupled together, and are coupled to
receive the other end of the differential input signal (Inn).
Second inputs of the NAND gate U.sub.n1 and the NOR gate U.sub.n2
are coupled to the true enable signal en and the complement enable
signal enb. The true enable signal en and the complement enable
signal enb are signals of a true enable input and a complement
enable input, respectively. The true enable input includes N true
enable signals en through en.sub.N respectively coupled to the N
output circuits 308, and the complement enable input includes N
complement enable signals enb.sub.1 through enb.sub.N respectively
coupled to the N output circuits 308.
[0042] A source of the transistor M.sub.res1 is coupled to the
common node V.sub.refp. A source of the transistor M.sub.res2 is
coupled to the common node V.sub.refn. A gate of the transistor
M.sub.res1 is coupled to an output of the operational amplifier
A.sub.repl1 (designated node V.sub.g1). A gate of the transistor
M.sub.res2 is coupled to an output of the operational amplifier
A.sub.repl2 (designated node V.sub.g2).
[0043] One terminal of the resistor R.sub.pdrv is coupled to the
drains of the transistors M.sub.pdrv1 and M.sub.pdrv2, and another
terminal of the resistor R.sub.pdrv is coupled to the node Txp of
the differential output. One terminal of the resistor R.sub.ndrv is
coupled to the drains of the transistors M.sub.ndrv1 and
M.sub.ndrv2, and another terminal of the resistor R.sub.ndrv is
coupled to the node Txn of the differential output. The transistors
M.sub.pdrv1 and M.sub.pdrv2 form a first inverter (M.sub.pdrv), and
the transistors M.sub.ndrv1 and M.sub.ndrv2 form a second inverter
(M.sub.ndrv). A series combination of the enable circuits (U.sub.p,
U.sub.n), the pair of inverters (M.sub.pdrv, M.sub.ndrv) and the
pair of resistors R.sub.pdrv and R.sub.ndrv is coupled between the
differential input 202 and the differential output (Txp, Txn). The
source terminals of the inverters (M.sub.pdrv, M.sub.ndrv) are
coupled between the nodes V.sub.refp and V.sub.refn.
[0044] As shown in the portion 118B of the output driver 118, the
replica output circuit 320.sub.1 includes transistors
M.sub.resrepl1 and M.sub.repl1 and a resistor R.sub.repl1. The
transistors M.sub.resrepl1 and M.sub.repl1 are each a p-channel
FET, such as a P-type MOSFET. A source of the transistor
M.sub.resrepl1 is coupled to the common node V.sub.refp. A drain of
the transistor M.sub.resrepl1 is coupled to a source of the
transistor M.sub.repl1. A drain of the transistor M.sub.repl1 is
coupled to one terminal of the resistor R.sub.repl1. Another
terminal of the resistor R.sub.repl1 is coupled to one terminal of
a resistor R.sub.repl.sub._.sub.load at a node V.sub.p. A gate of
the transistor M.sub.resrepl1 is coupled to the output of the
operational amplifier A.sub.repl1. A gate of the transistor
M.sub.rep1 is coupled to a ground source.
[0045] The replica output circuit 320.sub.2 includes transistors
M.sub.resrepl2 and M.sub.repl2 and a resistor R.sub.repl2. The
transistors M.sub.resrepl2 and M.sub.repl2 are each an n-channel
FET, such as a N-type MOSFET. A source of the transistor
M.sub.resrepl2 is coupled to the common node V.sub.refn. A drain of
the transistor M.sub.resrepl2 is coupled to a source of the
transistor M.sub.repl2. A drain of the transistor M.sub.repl2 is
coupled to one terminal of the resistor R.sub.repl2. Another
terminal of the resistor R.sub.repl2 is coupled to a second
terminal of a resistor R.sub.repl.sub._.sub.load at a node V.sub.n.
A gate of the transistor M.sub.resrepl2 is coupled to the output of
the operational amplifier A.sub.repl2. A gate of the transistor
M.sub.rep2 is coupled to a supply source (Vsup). The replica output
circuit 320.sub.2 also includes a startup circuit S1. The startup
circuit S1 comprises a switch coupled between the output of the
operational amplifier A.sub.repl2 and the supply source Vsup.
[0046] An inverting input of the operational amplifier A.sub.repl1
is coupled between the resistor R.sub.repl1 and the resistor
R.sub.repl.sub._.sub.load. A non-inverting input of the operational
amplifier A.sub.repl1 is coupled to a switched resistor network
322.sub.1. The switched resistor network 322.sub.1 comprises the
resistors R.sub.ref1 through R.sub.ref5 and a switch Sw1. The
resistors R.sub.ref1 through R.sub.ref5 are coupled in series
between the node V.sub.refp and the resistor R.sub.ref6. The
switched resistor network 322.sub.1 includes a plurality of taps
(e.g., 5 taps in the example). The switch Sw1 is controllable to
couple the non-inverting input of the operational amplifier
A.sub.repl1 to one of the taps.
[0047] An inverting input of the operational amplifier A.sub.repl2
is coupled between the resistor R.sub.repl2 and the resistor
R.sub.repl.sub._.sub.load. A non-inverting input of the operational
amplifier A.sub.repl2 is coupled to a switched resistor network
322.sub.2. The switched resistor network 322.sub.2 comprises the
resistors R.sub.ref7 through R.sub.ref11 and a switch Sw2. The
resistors R.sub.ref7 through R.sub.ref11 are coupled in series
between the node V.sub.refn and the resistor R.sub.ref6. The
switched resistor network 322.sub.2 includes a plurality of taps
(e.g., 5 taps in the example). The switch Sw2 is controllable to
couple the non-inverting input of the operational amplifier
A.sub.repl2 to one of the taps.
[0048] One example technique for impedance control is to provide a
pair of programmable resistors stacked in series with all output
slices of the driver array. The intent is to adjust the
programmable resistors to compensate for variations in the output
slices. However, as the programmable resistors are shared by all of
the output slices, the differential impedance will deviate from the
desired 100 ohms when some output slices are driven in the opposite
direction. Another example technique for impedance control is to
configure the output slices of the driver array to be selectively
enabled/disabled. However, such a technique alone does not
compensate for the difference in process variations of PMOS and
NMOS transistors, e.g., when PMOS is at fast corner while NMOS is
at slow corner and vice versa.
[0049] In an example, the output driver 118 provides for impedance
control that addresses these problems. Turning on/off output
circuits 308 is used to only compensate for on-chip resistor
variations. To compensate for NMOS/PMOS variations, each output
circuit 308 includes a pair of stacked programmable resistors
(described below). The impedance of the stacked programmable
resistors is controlled by two impedance control loops.
[0050] In operation, the output circuits 308 can be selectively
enabled on or off through the enable input. The enable input can be
provided by the control logic 150. If enabled, an output circuit
308 contributes to the differential output (Txp, Txn). If disabled,
the output circuit 308 does not contribute to the differential
output (Txp, Txn) (high impedance state). Turning output circuits
308 on/off provides for coarse impedance control to compensate for
variation in the on-chip resistors R.sub.pdrv and R.sub.ndrv. The
transistors M.sub.res1 and M.sub.res2 are driven to operate in the
triode region to provide programmable resistors controllable
through V.sub.g1 and V.sub.g2, respectively. The transistors
M.sub.res1 and M.sub.res2 are controlled to compensate for
variation in the transistors M.sub.pdrv1, M.sub.pdrv2, M.sub.ndrv1,
and M.sub.ndrv2. The resistance provided by the transistors
M.sub.res1 and M.sub.res2 is controlled by adjusting their
respective gate-to-source voltages using feedback control loops. A
feedback control loop that controls the transistor M.sub.res1
comprises the replica 320.sub.1 and the operational amplifier
A.sub.repl1. A feedback control loop that controls the transistor
M.sub.res2 comprises the replica 320.sub.2 and the operational
amplifier A.sub.repl2.
[0051] The operational amplifier A.sub.repl1 adjusts the
gate-to-source voltage of the transistor M.sub.resrepl1 such that
its impedance is set to a desired value. Notably, the transistor
M.sub.resrepl1 is fabricated to be a replica of the transistor
M.sub.res1. The transistor M.sub.repl1 is fabricated to be a
replica of a p-channel FET in the output circuits 308 (e.g., the
characteristics for each of M.sub.pdrv1, M.sub.pdrv2, and
M.sub.repl1 are the same or substantially similar). The resistor
R.sub.repl1 is fabricated to be a replica of an on-chip resistor in
the output circuits 308 (e.g., the characteristics for each of
R.sub.pdrv, R.sub.ndrv, and R.sub.repl1 are the same or
substantially similar). Each output circuit 308 (if enabled)
includes an internal impedance in series with one of the
transmission lines 212 formed by a series combination of
M.sub.res1, one p-channel FET (i.e., M.sub.pdrv1 or M.sub.ndrv1),
and one resistor (R.sub.pdrv or R.sub.ndrv). The replica 320.sub.1
replicates this internal impedance. The desired voltage at node
V.sub.p is selected at the non-inverting input of the operational
amplifier A.sub.repl1 and the operational amplifier A.sub.repl1
drives the node V.sub.p to that voltage by controlling the
impedance of the transistor M.sub.resrepl1. The operational
amplifier A.sub.repl1 provides the same control voltage to the gate
of the transistor M.sub.res1 in each output circuit 308.
[0052] The operational amplifier A.sub.repl2 adjusts the
gate-to-source voltage of the transistor M.sub.resrepl2 such that
its impedance is set to a desired value. The transistor
M.sub.resrepl2 is fabricated to be a replica of the transistor
M.sub.res2. The transistor M.sub.repl2 is fabricated to be a
replica of a n-channel FET in the output circuits 308 (e.g., the
characteristics for each of M.sub.ndrv1, M.sub.ndrv2, and
M.sub.repl2 are the same or substantially similar). The resistor
R.sub.repl2 is fabricated to be a replica of an on-chip resistor in
the output circuits 308 (e.g., the characteristics for each of
R.sub.pdrv, R.sub.ndrv, and R.sub.repl2 are the same or
substantially similar). Each output circuit 308 (if enabled)
includes an internal impedance in series with one of the
transmission lines 212 formed by a series combination of
M.sub.res2, one n-channel FET (i.e., M.sub.pdrv1 or M.sub.ndrv1),
and one resistor (R.sub.pdrv or R.sub.ndrv). The replica 320.sub.2
replicates this internal impedance. The desired voltage at node
V.sub.n is selected at the non-inverting input of the operational
amplifier A.sub.repl2 and the operational amplifier A.sub.repl2
drives the node V.sub.n to that voltage by controlling the
impedance of the transistor M.sub.resrepl2. The operational
amplifier A.sub.repl2 provides the same control voltage to the gate
of the transistor M.sub.res2 in each output circuit 308.
[0053] By including transistors M.sub.res1 and M.sub.res2 in each
output circuit 308, the differential output impedance of the output
driver 118 can be maintained to match the transmission medium 160
even when the main and pre/post cursor signals switch in the
opposite direction. Further, by provide two feedback control loops
for separately controlling the resistance provided by the
transistors M.sub.res1 and M.sub.res2, the output driver 118 can
compensate for different NMOS and PMOS process variations.
[0054] As shown in FIG. 3B, the two feedback control loops are
coupled together through the resistor R.sub.repl.sub._.sub.load so
that the current through the two loops can be re-used. To ensure
both loops start up properly, the startup circuit S1 can be
incorporated into the replica circuit 320.sub.2. The startup
circuit S1 can disable one loop initially so that the other loop
starts up properly. Alternatively, rather than the startup circuit
S1, a common-mode buffer can be used to decouple the two feedback
control loops by driving the midpoint of the replica load to a
common-mode voltage.
[0055] To illustrate the impedance control in more detail, consider
an example where the output driver 118 includes N=75 to 85 output
circuits 308. Typically, an on-chip resistor can change by .+-.10%
due to process variations. As discussed above, variation in the
on-chip resistors R.sub.pdrv and R.sub.ndrv is compensated for by
adjusting the number of enabled output circuits 308 (e.g., between
75 and 85 as shown in the example of Table 2).
TABLE-US-00002 TABLE 2 Each slice with +10% resistor Each slice
with -10% resistor 80 Slices Each Slice resistor + 10% with 85
slices resistor - 10% with 75 slices Transistor 20 ohms 1600 ohms
1600 ohms 18.8 ohms 1600 ohms 21.3 ohms resistance On-chip 30 ohms
2400 ohms 2640 ohms 31.1 ohms 2160 ohms 28.8 ohms resistance Total
50 ohms 4000 ohms 4240 ohms 49.9 ohms 3760 ohms 50.2 ohms
resistance
[0056] As shown in Table 2, the total output impedance can be
maintained at approximately 50 ohms for a given differential output
(assuming a 50-ohm characteristic impedance of the transmission
line) despite a .+-.10% variation in on-chip resistance by enabling
more or less of the output circuits 308. To calibrate the number of
output circuits 308 to be turned on/off, the resistance of the
on-chip resistors R.sub.pdrv and R.sub.ndrv can be sensed with a
constant current source (not shown). The control logic 150 can read
the output of the sensing operation and then enable/disable the
output circuits 308 based on values in a lookup table.
[0057] One difference between the replica output circuits 320 and
the output circuits 308 is that the load of the replica circuits
320, R.sub.repl.sub._.sub.load, is implemented with an on-chip
resistor, while the actual load for the transmitter, R.sub.load, is
a constant termination at the receiver. To avoid using an external
resistor or trimming the on-chip resistor
R.sub.repl.sub._.sub.load, the reference voltages used in the
feedback control loops can be adjusted to compensate for variation
in the on-chip replica resistor R.sub.repl.sub._.sub.load. This is
achieved by selecting a desired voltage at the non-inverting inputs
to the operational amplifiers A.sub.repl1 and A.sub.repl2. Note
that although each switched resistor network 322 is shown has
having five taps for providing five reference voltages, the
switched resistor networks 322 can have more or less than five
taps.
[0058] FIG. 4 is a flow diagram depicting a method 400 of
controlling a driver circuit in a transmitter according to an
example. The method 400 can be performed to control the output
driver 118 of the transmitter 112. The method 400 begins at step
402, where the pre-driver 115 receives the outputs of an equalizer
in the transmitter 112 (e.g., FIR filter 114). At step 404, the
pre-driver 115 couples each equalizer output to at least one of a
plurality of output circuits in the output driver 118 (e.g., output
circuits 208 or 308). Step 404 implements equalizer control
independently from swing control and impedance control. The main-,
pre-, and post-cursor signals can be coupled to any number of
output circuits in the output driver 118 to achieve the desired
emphasis or de-emphasis.
[0059] At step 406, the control logic 150 enables first and second
voltage regulators 210 coupled to the output circuits in the output
driver 118 to establish a desired swing. The voltage output from
the dual voltage regulators 210 can be set to generate a desired
peak-to-peak voltage swing at the output of the output driver 118.
In some cases, at step 410, the control logic 150 can optionally
enable one or more current compensation circuits 206 to equalize
current drawn from the current-supplying voltage regular (e.g., the
voltage regulator 210.sub.1). Step 406 implements output swing
control independent of equalizer control and impedance control.
[0060] At step 408, the impedance of the output driver is
controlled. For example, at step 412, the control logic 150
disables one or more of the output circuits in the output driver
118 to compensate for on-chip resistor variation. This provides for
a coarse impedance control. At step 414, feedback control loops in
the output driver adjust the gate-to-source voltage of stacked
transistors M.sub.res1 and M.sub.res2 in each output circuit based
on feedback from replica circuits 320 to adjust for NMOS/PMOS
transistor variation and provide for fine impedance control. As
discussed above, the feedback control loops can independently
adjust the impedance of the stacked transistors M.sub.res1 and
M.sub.res2 to independently compensate for NMOS and PMOS
variations.
[0061] While the foregoing is directed to specific examples, other
and further examples may be devised without departing from the
basic scope thereof, and the scope thereof is determined by the
claims that follow.
* * * * *