- BACKGROUND OF THE INVENTION
This invention relates to a current mode output circuit. More particularly, this invention relates to an output circuit with a discretely variable output current.
One type of data interface uses changes in the magnitude of a current to transmit data signals. In the simplest case, one state is represented by the presence of a current flow and another by the absence of current. An output circuit of this type using a switch S to connect a current driver through the output nodes to a load RL is shown in FIG. 1a.
Other conventional data interfaces, such as those used in SCSI architecture, require a current to be always present on an active interface. The direction of current flow indicates whether the transmitted signal is a logic 0 or a logic 1. A conventional current steering output buffer of this type is illustrated in FIG. 1b. The circuit contains a current driver which is connected to the output nodes by a current switch. In a first state, switch pair S A is closed, switch pair SB is open, and the current driver contributes a current I to the total load current. In a second state, switch pair SB is closed, switch pair SA is open, and the current driver contributes a current of −I to the total load current. In conventional systems, current may be turned off by opening both switch pairs SA and SB.
- SUMMARY OF THE INVENTION
As performance of integrated circuits continues to increase, the limited number of I/O (input/output) pins demands better utilization. Incorporating an output signal with more than the two conventional digital output states over a wire allows the interconnect of the I/O to carry a larger data bandwidth. Previous attempts to do this have used signalling via multiple voltage levels. However, in many circumstances, it is preferable to use current signaling instead. Further, conventional circuits generate switching noise at either of the power supply leads as the amount of power flowing into the output circuit is continuously changed to thereby vary the output signal level.
According to the present invention, an output buffer is provided that utilizes current signaling to provide a multi-level data output and draws a constant amount of current from the power supply regardless of the output level, thus reducing switching noise and startup delays. An output buffer circuit according to a first embodiment of the present invention has a current driver supplying a current I which is connectable to a load resistance. The connection may be through a current switch. The circuit also has one or more bypass resistors which can be switchable connected in parallel with the load resistance or disconnected from the circuit. When a bypass resistor is switched in parallel with the load resistor, some of the output current is diverted through the bypass resistor, thus reducing the current supplied to the load. For N switched bypass resistors, there are 2N possible resistor state combinations. By choosing appropriate resistance values for the bypass resistors, the current through the load resistor may be varied in 2 N discrete. The number of states may be increased to 2N+1 if a current switch which allows the direction of the output current to be reversed or stopped is included in the output circuit. According to the invention, a zero-current output state is achieved by closing all switches in the current switch, instead of opening them. In this state, the introduced current flows equally in both directions through the load and therefore does not contribute a net current even though the given current driver is still sourcing current. Because the effect of a current driver on the load current may be eliminated without shutting off the current, it is possible to avoid startup delay and noise which would be introduced if the current switch were disconnected from the circuit by opening all the internal switches. Preferably, N bypass resistors are provided having appropriate resistances to provide at least N equal current steps for each current direction.
In another embodiment, a plurality of current drivers is provided and connected to the load resistance in parallel. It can be appreciated that when both of the current paths in a given current switch are active (e.g., all internal switches in a particular current switch are closed), no net current flow will be introduced into the load. However, the internal switches will also introduce a resistance in parallel with the load resistor. According to this aspect of the invention, the resistance of each internal switch is chosen so that when all internal switches are closed, the current switch provides a preselected effective resistance in parallel with the load which diverts current flow from other (active) current drivers and thereby reduces the current in the load by a predefined discrete amount. The magnitude of each current source and the effective resistance of the switches in each current switch can be chosen to minimize the number of redundant output states, and therefore maximize the number of possible output current steps.
BRIEF DESCRIPTION OF THE DRAWINGS
A multiple-state current output circuit according to the present invention can be fabricated as an integrated circuit using MOS transistors and located on the same chip as its driving circuitry. The present design allows for compact circuit dimensions when compared with conventional circuits of a similar type.
The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of illustrative embodiments of the invention in which:
FIGS. 1a and 1 b are ideal representations of conventional current output circuits;
FIG. 2 is an ideal representation of an output circuit according to a first embodiment of the invention;
FIGS. 3a and 3 b are graphs of load current for various switch states and resistance values in the circuit of FIG. 2;
FIG. 4 is an ideal representation of an output circuit according to a second embodiment of the invention;
FIGS. 5a and 5 b are circuit diagrams of switchable bypass resistors according to the invention;
FIGS. 6a-6 c are ideal representations of an output circuit according to a third embodiment of the invention;
FIGS. 7a- 7 c show circuit diagrams of the preferred implementation of the circuits of FIGS. 6a according to one specific implementation of this particular embodiment; and
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 8 is a graph of the output currents for the circuit of FIGS. 7a-7 c with a varying transistor bias voltage.
FIG. 2 is an ideal representation of an output circuit according to a first embodiment of the invention having two output nodes 20 and 22 and a current driver comprised of current source 24 which supplies a current I. The output driver is connected to a receiving unit, indicated as load resistance RL 26, through the output nodes 20, 22. According to the invention, one or more bypass resistors R1 through RN are switchable connected in parallel with the load resistance RL, e.g., by means of switches S1 through SN, to form switchable bypass resistor assemblies. By closing one or more of the switches, the current IL driven through the load 26 can be varied in discrete steps between a maximum current of I and a minimum current of IL=I*RE/(RL+RE), where RE is the equivalent parallel resistance introduced when all N bypass resistors are conducting. Advantageously, the net current flow from the current driver 24 remains constant at I, even as the load current is changed. This reduces transient switching effects on the current driver and allows for high switching speeds and low output noise.
The value of bypass resistors R1
can be defined in terms of the expected load resistance RL
and preferably are chosen so that the output buffer can source current in at least N+1 substantially equal output current steps. For example, if N=3, the value of the bypass resistors can be selected to provide, for example, four equal output current steps of magnitude I, 3/4I, 1/2I, and 1/4I. Depending on various design considerations, such as switching speed, device area, etc., several switching configurations may be used. In a “single-switch” configuration, four equal steps are achieved by switching only one bypass resistor at a time to a conducting state. In tabular form, this particular “single-switch” arrangement is summarized as:
|S1 ||S2 ||S3 ||IL |
|0 ||0 ||0 ||I |
|1 ||0 ||0 ||¾ I |
|0 ||1 ||0 ||½ I |
|0 ||0 ||1 ||¼ I |
where a “1” indicates that a switch is closed, i.e., the bypass resistor is conducting, and a “0” indicates that the switch is open, i.e., the bypass resistor is non-conducting. To achieve this result, the values of the bypass resistors are:
R1=3RL R2=RL R3=1/3RL
In the general case of M equal output current steps separated by I/M in a “single-switch” configuration, N=M−1 switched bypass resistors are provided. The values of each switched bypass resistor Rn
and the resulting load current IL
when only that particular resistor is conducting (only switch Sn
is closed) is given by equation 1:
Thus, for 8 equal states, N=7 and the values of bypass resistors R1 to R7 is:
7/1RL 6/2RL 5/3RL 4/4RL 3/5RL 2/6RL 1/7RL
The output current when only bypass resistor Rn is conducting for n=1−7 is:
7/8I 6/8I 5/8I 4/8I 3/8I 2/8I 1/8I
(The final state, output of I, is achieved when all resistors are non-conducting, i.e., no switches are closed).
Other configurations may also be used to achieve equal output current steps. For example, in a “cumulative parallel” configuration, the switches are closed successively, adding more than one bypass resistor in parallel with the load resistor at a time. The four states achieved in this configuration where N=3 may be represented as:
|S1 ||S2 ||S3 ||IL |
|0 ||0 ||0 ||I |
|1 ||0 ||0 ||¾ I |
|1 ||1 ||0 ||½ I |
|1 ||1 ||1 ||¼ I |
To achieve this result, the values of the bypass resistors are configured as:
R1=3RL R2=3/2RL R3=1/2RL
It can be appreciated that even though the resistor values may be optimized to provide equal current steps for certain switch configurations, such as the single-switch or cumulative-parallel configurations discussed above, all of the 2N switch combinations can be utilized if desired. FIG. 3a is a graph of the output current IL for each of the eight possible output states using the values of the single-switch configuration example. FIG. 3b is a similar graph for the resistor values of the cumulative-parallel configuration example. In each graph, the points at equal steps of 1/4I are indicated by a box. In both graphs, it can be seen that when the bypass resistor values are configured to achieve four equal steps as discussed above, the minimum separation between current points diminishes as more resistors are switched into place. Accordingly, the number of additional detectable states may be limited by noise concerns. Therefore, it would be advantageous to introduce additional states without reducing the current point separation.
FIG. 4 shows the output driver 10 according to a second embodiment of the invention. In this invention, a current switch 15 is integrated with the driver of FIG. 2. Current switch 15 connects the current driver, here formed of matched current source 24 and current sink 25, to the output nodes 20, 22 via internal switch pairs SA and SB. Although all four internal switches in a given current switch can be independently controlled, for purposes of this discussion, switches which are normally closed together are given the same designation and are discussed in pairs. When switch pair SA is open and switch pair SB is closed, current flows out from node 22 through the load 26 and back into node 20. When switch pair SA is closed and switch pair SB is open, current flows through the load in the opposite direction.
By including a current switch in the output circuit, the number of available current output states is doubled. In addition, a zero-current state can be achieved by opening or closing the internal switch pairs SA and SB in unison. Thus, for example, by using three switched bypass resistors configured as discussed above combined with a current switch as shown, eight primary output current states at steps of 1/4I can be achieved. An additional, ninth zero-current state can also be achieved by opening or closing both switch pairs SA and SB together.
Accordingly, in this example, a three-bit digital output may be encoded in a single current signal. Advantageously, the circuit according to the present invention provides a wide and uniform separation between each of the current levels. This results in a robust output signal which can absorb a significant degree of noise, up to +/−1/8I, and still provide extractable data.
In the preferred embodiment, the conducting or non-conducting state of the bypass resistors (here shown as the state of resistor switches S1
) and the state of current switch pairs SA
is controlled by conventional logic circuitry which converts a three-bit output value into appropriate control signal combinations. If the bypass resistors R1
are configured to be used in a single-switch arrangement, as discussed above for example, the nine output logic states may be encoded as follows:
| || |
| || |
| ||Output ||S1 ||S2 ||S3 ||SA ||SB ||IL |
| || |
| ||111 ||0 ||0 ||0 ||1 ||0 ||+I |
| ||110 ||1 ||0 ||0 ||1 ||0 ||+¾ I |
| ||101 ||0 ||1 ||0 ||1 ||0 ||+½ I |
| ||100 ||0 ||0 ||1 ||1 ||0 ||+¼ I |
| ||— ||— ||— ||— ||both 1/0 ||0 |
| ||011 ||0 ||0 ||1 ||0 ||1 ||−¼ I |
| ||010 ||0 ||1 ||0 ||0 ||1 ||−½ I |
| ||001 ||1 ||0 ||0 ||0 ||1 ||−¾ I |
| ||000 ||0 ||0 ||0 ||0 ||1 ||−I |
| || |
A similar state table can easily be configured for the parallel-cumulative example discussed above. Appropriate logic circuitry can be designed directly from state tables such as this as is well known in the art.
FIGS. 5a and 5 b show preferred structures for implementing the switchable bypass resistors. In FIG. 5a, a switchable bypass resistor is implemented as two series-connected MOS transistors 100, 102 driven by a reference voltage Vref 103 which is adjusted tso that the transistors 100, 102 are either non-conducting or resistively conducting. Preferably, a capacitor 104 is connected from node 105 (the junction of transistors 100, 102) to ground. Capacitor 104 serves to provide an AC ground for common mode signals which may be induced in the circuit. Although only one MOS transistor is necessary, preferably two are used in combination with a capacitor because it is beneficial to have an AC ground at the center point.
In this embodiment, when the driving voltage Vref 103 is below the turn-on voltage for transistors 100, 102, the transistors are non-conducting. When a voltage greater the turn-on voltage (for NMOS transistors) is applied to the gates of transistors 100, 102, they conduct and have a specific resistance dependant on the geometry of each transistor and the applied voltage gate Vref 103. The proper biasing voltage to apply to a given MOS transistor so as to set it to a particular predefined resistance can be generated using feedback circuits. One such feedback circuit is described below.
According to another aspect of the invention, the output circuit is fabricated as an integrated circuit on a single chip so that all transistors in the circuit are exposed to the same fabrication conditions. A single bias voltage Vref may then be used for all of the switched bypass resistors implemented as MOS transistors by scaling various transistors relative to a “master” transistor that is biased to a known resistance. For example, if transistor 100 biased with a voltage V has a resistance R, a resistance of 2R may be achieved by applying the same biasing voltage V to a transistor with a channel width half that of transistor 100 and having the same length. Similarly, a resistance of 0.5R may be achieved by applying voltage V to a transistor having a channel twice as wide but the same length as transistor 100. Because all the transistors are fabricated with identical process conditions, errors introduced during fabrication cancel out.
FIG. 5b shows an alternative embodiment for implementing a switched bypass resistor. In this embodiment, two resistors 116, 118 are connected in series with two MOS transistors 110, 112. The transistors are driven by a biasing voltage Vbias 114. Preferably, a capacitor 120 is connected between the junction of the transistors 110, 112 (node 119) and ground. As above, in the “off” condition, the transistors 110; 112 are non-conducting. However, in the “on” state, the transistors are driven with a biasing voltage sufficient to place them in a saturated state so that their internal resistance is small when compared with resistors 116, 118. In this embodiment, the transistors are preferably driven using digital signals of appropriate magnitudes. The resistors can be fabricated using any appropriate technique and can be on-or off-chip passive devices, or even appropriately biased transistors.
FIG. 6a is an ideal representation of a third embodiment of the invention. As shown, current output circuit 50 consists of a plurality of current drivers, in this example drivers 56, 57, 58, which are connected in parallel to output nodes 52, 54, and thereby to a load resistor RL 26, representing an attached receiving device. Each of the current drivers preferably includes a matched current source and sink connected to the output nodes by a current switch. Driver 56 includes current source and sink 60, 62 which provide a current I1 through current switch 64 having switch pairs S1 and S2. Driver 57 includes current source and sink 66, 68 which provide current I2 through current switch 70 having switch pairs S3 and S4. Driver 58 includes current source and sink 72, 74 which provide current I3 through current switch 76 having switch pairs S5 and S6.
According to this aspect of the invention, the output current IL through load 26 may be varied by selectively providing various combinations of currents I1 to I3, in either direction, via the current drivers 56, 57, 58. Further according to the invention, the resistance of switch pairs S1- S6, when conducting, is set to various predefined values. Each switch thus functions as a switchable bypass resistor as discussed above. In addition, all of the internal switches in a given current switch are configured so that the net resistance for each path between the given current source and current sink is equivalent. This ensures that when all internal switches in a given current switch are closed, half the switched current flows in one direction through the load and half flows in the other direction.
In this state, no net current is introduced into the load from the particular current driver and therefore, the particular switched current can be ignored. According to the invention, however, because the closed internal switches have a predefined resistance, closing both switch pairs also introduces a predetermined effective resistance in parallel with the load. This diverts current flow from other current drivers and thereby reduces the current in the load by a particular amount. The resistances of the internal switches and thereby the effective resistance associated with each current driver is chosen to provide for a plurality of discretely selectable current levels through the load in a manner similar to that discussed above with respect to FIGS. 2-4.
Each internal switch in a switch pair S can be considered an ideal switch in series with a resistor R or a voltage-controlled resistor. For purposes of clarity, when a given switch is open, i.e., when the resistor is not conducting, only the open-switch portion is shown in the figures. When an internal switch is closed, i.e., the resistor is conducting, it is represented as the equivalent resistance. It can be appreciated that each internal switch in each of the current switches 64, 70, 76 can have a different internal resistance subject to the constraint, in this embodiment, that the two current paths in a given current switch, i.e., the path through each pair of switches, have equivalent resistances to allow for a zero net current flow state as previously discussed. For purposes of simplicity, it is assumed that each of the four internal switches comprising a given current switch 64, 70, 76 have the same internal resistance, defined as R64, R70, R76, respectively.
FIG. 6b is an illustration showing the circuit 50 with switch pairs S1, S3, and S5 closed and replaced with their equivalent switch resistances, R64, R70, R76. Because each resistance is introduced in series between a current source/sink and the load, the internal resistances do not affect the net current flow, only the voltage drop across the load. Thus, the net current flow IL through load resistor RL is I1+I2+I3. This sum is defined as the maximum output current I. (Analogously, if switch pairs S2, S4, and S6 were closed, the total current through the load would be −I).
FIG. 6c is an illustration showing the circuit 50 with switch pairs S3, and S5 closed and replaced with their equivalent switch resistances, R70, R76. In addition, current switch 64 has both switch pairs S1 and S2 closed and is therefore in a zero net current state. Therefore, current I1 is not contributing current to the load because the effect of switched current I1 is equal in both directions. Note, however, that the I1 current is still flowing, and therefore, switching noise and start-up delays associated with stopping current flow are not introduced. The total contributing current flow is I2+I3. However, as shown in the figure, the internal switch resistances from current switch 64 are now in parallel with the load resistor RL. This introduces a parallel aggregate resistance of (R64+R64)¦¦R64+R64)=R64, where“¦¦” signifies a parallel combination. This bypass resistance diverts a portion of the I2+I3 current, thereby reducing the output current IL. According to the invention, the parallel equivalent resistance (and thus each internal switch resistance) R64 is chosen to provide a particular output current in a manner similar to that discussed above for FIGS. 2-5. Preferably, the switchable resistors used in switch pairs S1-S6 are implemented as MOS transistors as shown in FIGS. 5a and 5 b, discussed above, as are the current drivers 60, 66, and 72, and the reference voltage circuit which applies the appropriate gate voltage(s).
In one configuration, for example, I1
=0.1I, I2 =)0.3
I, and I3
=0.6I and the resistances R64
, and R76
are set to 2RL
, respectively. When all three current switches 64
, and 76
are contributing current in the same direction (FIG. 6b
), the net current flow is I. When current switch 64
is set to a non-contributing state as shown in FIG. 6c
, current I1
does not contribute to the load and a resistance of R64
is placed in parallel with the load resistance RL
. Because currents I2
remain contributing, the total sourced current is 0.3I+0.6I=0.9I. RL
is in parallel with a bypass resistance equal to 2RL
and so the current sourced to the load is 0.9I*2RL
/RL =R L
)=0.9I*2/(1+2)=0.6I. Similarly, in the case where switch pairs S1
, and S5
are closed, only the current driver 58
is contributing I3 =)0.6
I and the resulting load current is 0.2I. Similar calculations can be performed for other current switch states as well. One group of switch states for this embodiment and the resulting load current is summarized in the following table, where a 1 indicates that a switch pair is closed and a 0 indicates that a switch pair is open.
| || |
| || |
| ||S1 ||S2 ||S3 ||S4 ||S5 ||S6 ||IL |
| || |
| ||1 ||0 ||1 ||0 ||1 ||0 ||I |
| ||1 ||1 ||1 ||0 ||1 ||0 ||0.6 I |
| ||1 ||1 ||1 ||1 ||1 ||0 ||0.2 I |
| ||1 ||1 ||1 ||1 ||1 ||1 ||0 |
| ||1 ||1 ||1 ||1 ||0 ||1 ||−0.2 I |
| ||1 ||1 ||0 ||1 ||0 ||1 ||−0.4 I |
| ||0 ||1 ||0 ||1 ||0 ||1 ||−I |
| || |
FIGS. 7a-7 c show circuit diagrams of a preferred implementation of this particular embodiment, using the selected values from the previous example. All components except for a single reference resistor are implemented as MOS transistors. All transistors preferably have the same gate length, most preferably the minimum value supported in a given fabrication technology, but may have varying gate widths. According to one aspect of the invention, the entire output circuit is fabricated as part of a single integrated circuit where the current output circuits are simply scaled versions of each other. In a simple scaling scheme, the lengths of the transistor gates are kept constant, while the widths of the gates are adjusted to provide the desired relative resistance and/or current.
Turning now to FIG. 7a, representative current driver 56 consists of a current source formed by p-channel MOS transistor 150 biased to a gate voltage Pch, and a current sink formed by N-channel MOS transistor 152 biased to a gate voltage Nch, which together produce current I1. Transistors 150 and 152 have gate widths of 70 and 18 microns, respectively, and are connected to output nodes 154, 156 through a current switch 64 formed by N-channel transistors 160, 162, 164, and 166, having gate widths of 95, 50, 95, and 50 microns respectively. The gates of transistors 160 and 162 are connected to input voltage IN1 and together correspond to switch pair S1 shown in FIG. 6a. The gates of transistors 164 and 166 are connected to input voltage INN1 and together correspond to switch pair S2 shown in FIG. 6a. The input gate voltages IN1 and INN1 are set to either turn off the given transistors (i.e., zero volts), or bias them to a common voltage, Vcom, which places them in a resistive conducting state having the desired resistance.
FIG. 7b is one embodiment of a voltage reference circuit 168 for generating the bias voltages Pch and Nch for transistors 150 and 152. The circuit consists of a current source formed by P-channel transistor 170 and a current sink formed by N-channel transistor 172. The current driver formed by transistors 170 and 172 is connected to output nodes 174, 176 through a current switch formed by N-channel transistors 180, 182, 184, and 186, which are connected as shown. The gates of transistors 180 and 182 are driven by a bias voltage Vcom which places them in a resistive conducting state. The particular resistance of these switches is set by adjusting the value of Vcom as desired and will be discussed below. The gates of transistors 184 and 186 are grounded and therefore, these transistors are non-conducting.
A reference resistance RREF 188 is connected across nodes 174 and 176. Resistor 188 is chosen to have a resistance equal to the expected load resistance RL 26 which will be connected to the output buffer. The geometric dimensions of transistors 170, 172, 180, 182, 184, and 186 correspond symmetrically to transistors 150, 152, 160, 162, 164, and 166 respectively (FIG. 7a). Preferably, corresponding transistors have the same dimensions. Because the circuits of FIG. 7a and 7 b correspond geometrically, it can be appreciated that once the bias voltages for transistors 170 and 172 are set to provide a specific current through known resistance RREF 188, i.e., a current equal to I1, these bias voltages can then be applied to corresponding transistors in output buffer circuits 56 to produce currents with a predefined magnitude when driving a load having resistance equal to RREF.
The proper biasing voltages, Pch and Nch, are generated using feedback comparators 190 and 192. Comparator 190 is driven by input voltage V2 and the voltage at node 174 and its output is used to bias transistor 170. This circuit will adjust the biasing of transistor 170 until the sourced current places the voltage at node 174 equal to V2. Similarly, comparator 192 is driven by input voltage V1 and the voltage at node 176 and its output used to bias transistor 172. This circuit will adjust the bias of transistor 172 until the sunk current places the voltage at node 176 equal to V1. Since the reference resistance RREF 188 is known, the voltage drop across the resistor 188 can be set to provide the desired current, i.e., I=(V2−V1)/RREF. For example, with a resistance of 100 Ohms, a set voltage drop between 2.4 and 2 volts would result in a current of 4 milliamps.
Voltage reference circuits of this type are described in more detail in commonly assigned U.S. patent application Ser. No. 08/882,827 entitled “Low Voltage Differential Swing Interconnect Buffer Circuit,” filed on Jun. 26, 1997, the entire contents of which are hereby incorporated by reference.
FIG. 7c shows a complete output circuit implementing the specific example discussed above and using the circuits of FIGS. 7a and 7 b. As shown, reference circuit 168 generates bias voltages Pch and Nch based on reference resistor 188 and input voltages V1, V2, and Vcom. In this example, voltages V1 and V2 have a difference of several hundred millivolts, i.e., V1=1.0 volts and V2=1.4 volts. A plurality of output drivers are connected in parallel between output nodes 52 and 54. The output circuit 56 shown in FIG. 7b corresponds to the “1x” output circuit. Circuits 57 and 58 are scaled versions of circuit 56, where the transistors have gate widths of 3x and 6x, respectively, of the corresponding transistors in circuit 56. Accordingly, if circuit 56 is defined to produce output current I1=0.1I, circuit 57 will produce I2=3*I1=0.3I and circuit 58 will produce output current I3=6*I1=0.6I. The switch states of each output driver are controlled by a digital control circuit 200, which implements, for example, the state table shown above by providing a zero or ground voltage to a given transistor gate when it is to be non-conducting and providing an output voltage equal to Vcom when a switch is to be in a conducting state. Control circuits of this type are well known in the art.
As mentioned above, varying the biasing voltage for conducting transistors (i.e., Vcom) results in different output currents because the resistance of the transistors is varied. FIG. 8 is a graph of simulated output currents for the example circuit discussed above with Vcom varied between 2.0 volts and 3.8 volts. As can be seen, results generally in line with the ideal situation are achieved with a driving voltage Vcom of approximately 2.2 volts. Various methods of producing the appropriate driving voltage are known to those skilled in the art and include, for example, resistive divider circuits, feedback circuits, etc.
While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.