Abstract:
A line driver provides an output signal onto an output. The line driver includes a first current driver coupled to a first terminal of the output. The first current driver is capable of providing a first current to the first terminal that is sufficient to cause an output voltage having a magnitude Y to appear across the output. The first current driver includes a first plurality of elements to provide the first current to the first terminal of the output, each of the plurality of elements having a maximum voltage tolerance that is less than the magnitude Y.

Description:
PRIORITY APPLICATIONS 
     This application claims the benefit of priority to provisional U.S. Appl. No. 60/223,855, filed Aug. 8, 2000, naming Mark Summers and John Mullen as inventors. This priority application is incorporated by reference in this application. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to mechanisms for driving analog signals across transmission medium. In particular, the present invention relates to a low voltage supply line driver capable of providing relatively high voltage output with a low power supply voltage. 
     BACKGROUND OF THE INVENTION 
     Advances in semi-conductors have led to increased speed, lower power consumption, and higher levels of integration in digital circuits. In general, analog circuits have not benefited equally from process advancements due to the inability to utilize minimum size transistors and the reduction in power supply voltages. Successful mixed-signal design in state of the art semi-conductor processes requires efficient, low-voltage analog topologies. 
     Ethernet systems typically use line drivers as part of a process for converting digital data into an analog signal for a transmission line. One basic line driver topology is referred to as a 10Base-T Ethernet driver. The specification for the 10Base-T Ethernet line driver is defined by the IEEE 802.3 industry standard. One aspect of this specification is that the Ethernet line driver must provide sufficient current to enable an output voltage to be generated at a level of 2.5+/−0.3Vpk. 
     Typically, Ethernet line drivers use external voltage supplies that are large enough to enable the line drivers to provide the required output voltage levels. But with advancements in semiconductor processing, devices used in line drivers are increasingly becoming smaller. The smaller devices come with smaller voltage tolerances. These devices have limited use in Ethernet line drivers if their voltage tolerances is less than the supply voltage required for generating the required output voltage. 
     In addition, attempts to use smaller devices in Ethernet line drivers have had difficulty generating necessary output voltage swings required by the industry standard. The voltage swings within the line drivers cannot cause voltage drops to appear across devices that have a smaller voltage tolerance. The small tolerances may cause those devices to breakdown as a result of the voltage swings. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention include a line driver for driving an output signal onto an output having a first terminal and a second terminal. A first current driver is coupled to the first terminal of the output. A power supply having a voltage X is supplied to the first current driver. The first current driver is capable of providing a first current to the first terminal. The first current is sufficient to cause an output voltage having a magnitude Y to appear across the output, where the magnitude Y is greater than the voltage X. The first current driver includes a first plurality of elements to provide the first current. Each of the plurality of elements has a maximum voltage tolerance that is less than the magnitude Y. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
     FIG. 1 illustrates a low supply voltage line driver. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A component incorporating a low supply voltage line driver is described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
     FIG. 1 illustrates a low supply voltage line driver  100 . The line driver  100  is a component, such as a microprocessor, having an integrated line driver circuit. The line driver  100  includes components that amplify an input current from a current source. The amplified input current is signaled to one or more terminals of an output, in order to generate a voltage across the output. The output may be used to create a signal on a transmission line  188  using the voltage generated by the amplified current. 
     In one application, the output corresponds to a transformer  180  having one or more terminals. The line driver  100  signals an output current to terminals of the transformer  180  in order to generate a desired voltage across the transformer. The transformer  180  is coupled to the transmission line  188 . A low supply voltage may be provided to components of the line driver  100  from an external power supply. 
     According to an embodiment, the line driver  100  provides an output current to one or more terminals of transformer  180 . The output current is sufficiently large to create a desired voltage level across terminals of transformer  180 . The desired voltage level may be 2.5+/−, so as to create a 5 volt differential. The transformer  180  may apply the voltage to generate an analog signal on the transmission medium  188 . The line driver  100  uses components having tolerance voltage levels of about 1.8 volts to provide the output current. In one implementation, the line driver  100  conforms to an IEEE 802.3 10Base-T standard. 
     In an embodiment, line driver  100  includes a first current driver  110  and a second current driver  120 . The first current driver  110  may have a positive or negative polarity, and the second current driver  120  may have the opposite polarity. In this way, each current driver  110 ,  120  may provide an output current to generate about the same magnitude voltage across transformer  180 . The voltages caused by current drivers  110 ,  120  may have opposite polarities and he applied to different terminals  182 ,  184  of transformer  180 , so as to create a peak-to peak voltage differential on the transformer that is the sum of the two voltages caused by current drivers  110 ,  120 . For a 10Base-T configuration, first current driver  110  provides an output current to a first terminal  182  of transformer  180  that is sufficient to create a voltage having a magnitude of about 2.5 volts across transformer  180 . The second current driver  120  provides an output current to a second terminal  184  of transformer  180  that is sufficient to create an opposite voltage having a magnitude of about 2.5 volts across transformer  180 . The first terminal  182  and second terminal  184  may correspond to a positive and negative terminal respectively, so that the voltage swing across transformer  180  is about 5 volts. As used herein, the terms ‘about’ and ‘approximately’ refers to within 90% of the stated value. 
     Each current driver  110 ,  120  is connected to a ground  140 . The first current driver  110  may receive an input current from a first current source  115 . The second current driver  120  may receive an input current from a second current source  125 . The current sources  115 ,  125  may be external or internal to the line driver  100 . The input current from first current source  115  may be of a first polarity. The input current from second current source  125  may be of a second polarity. 
     The first current driver  110  and second current driver  120  each include a plurality of elements or devices that combine to distribute a voltage differential between the ground  140  and a corresponding one of the first terminal  182  or second terminal  184  of transformer  180 . For each current driver  110 ,  120 , the plurality of elements amplify the respective input current in order to supply the corresponding output current to first terminal  182  or second terminal  184 . The input currents are amplified so that the output current to the first terminal  182  and to the second terminal  184  have sufficient magnitude to create the required voltage differential across transformer  180 . 
     For example, first current source  115  may provide a small positive input current. The plurality of elements of first current driver  110  combine to amplify the input current for first terminal  182 . A positive output current signaled from first current driver  110  to first terminal  182  results in a positive voltage being created across transformer  180 . The magnitude of the positive voltage is sufficient for creating the output signal on the transmission medium. Furthermore, the voltage differential between the first terminal  182  and ground is distributed amongst the plurality of elements for the first current driver  110 . 
     The plurality of elements may correspond to transistors. In one implementation, the plurality of devices are metal oxide semiconductor field effect transistors (MOSFETs). 
     According to an embodiment, each of the plurality of devices in first current driver  110  and second current driver  120  have a voltage tolerance that is less than the maximum voltage differential across that current driver. For example, the maximum voltage tolerance of one or more of the plurality of devices may be about 1.8 volts. For a MOSFET element used in one implementation, the 1.8 volt tolerance is the maximum voltage differential that can exist between any terminal pairs of the drain, gate or source terminals, before that device breaks down. In a 10Base-T application, the maximum voltage differential between the first terminal  182  or second terminal  184  and ground  140  is about 2.5 volts. This voltage differential is distributed amongst the plurality of devices in each current driver  110 ,  120  so that none of the devices experience a voltage drop across that device exceeding 1.8 volts. 
     In an embodiment, the plurality of devices for first current driver  110  include a first transistor  104 , a second transistor  108 , and a third transistor  112 . The first transistor  104  may correspond to an input transistor that receives the input current from first current source  115 . The second transistor  108  may correspond to a current amplification transistor that amplifies the input current. The third transistor  112  may correspond to an output transistor that provides an output current to first terminal  182  of transformer  180 . 
     In one configuration, a drain terminal of first transistor  104  receives the input current from first current source  115 . A gate terminal for first transistor  104  is connected to receive the input current supplied to the drain terminal. The source terminal of first transistor  104  is grounded. The gate terminal for second transistor  108  receives the input current from first current source  115 . The second transistor  108  is aligned in series with a third transistor  112  to distribute the voltage differential between first terminal  182  and ground  140 . The second transistor  108  amplifies the input current. 
     In an embodiment such as shown by FIG. 1, second transistor  108  is the primary source of current amplification. The second transistor  108  may be larger than first transistor  104 , causing the input current passing through the second transistor  108  to be amplified in comparison to the current passing through first transistor  104 . In an embodiment, second transistor  108  amplifies the input current five-fold. 
     The first transistor  104  may combine with second transistor  108  to form a 1:5 current mirror. In this type of configuration, a source terminal of second transistor  108  is grounded. A drain terminal of second transistor  108  is connected to a source terminal of third transistor  112 . The third transistor  112  may be the same size as first transistor  104 . The drain terminal of third transistor  112  provides the output current to first terminal  182  of transformer  180 . A gate terminals of third transistor  112  is tied to an external voltage (Vext), shown in the example to be 1.8 volts. 
     Vext may be supplied to the center of transformer  180 . One or more line termination resistors  142  are provided between first terminal  182  and second terminal  184 . The line resistors  142  may be are external to the line driver  100 . 
     Vext is supplied to select components to ensure tolerance voltage levels of the transistors  104 ,  108  and  112  are not exceeded. By applying Vext to the center of transformer  180 , and to the gate terminal of third transistor  112 , the voltage drop between the third transistor  112  and the transformer  180  will be less than the tolerance voltage level for that transistor. Similarly, the voltage drop between the gate terminal of the third transistor  112  and the ground  140  will also be less than the tolerance voltage level of that transistor. This ensures the voltage drop across second transistor  108  will also be less than the tolerance voltage for that transistor. In one implementation, the external voltage is the same as the tolerance voltage (1.8 volts) for the third transistor  112 . The voltage tolerance for the first transistor  104  and second transistor  108  may also be about 1.8 volts. 
     In an embodiment, second current driver  120  has a similar configuration as the first current driver  110 . The second current driver  120  produces an output current for second terminal  184  of transformer  180 . The output current to second terminal  184  may have the opposite polarity as the output current to first terminal  182 . The voltage differential created across transformer  180  by the output current to second terminal  184  may also have an opposite polarity as the voltage differential created by the output current to first terminal  182 . The magnitude of the voltage differential created by the output currents to first terminal  182  and second terminal  184  may be about the same. The output to positive terminal  182  from first current driver  110 , and to negative terminal  184  from second current driver  120 , combine to create the total voltage differential needed for driving an analog signal on the transmission line  188 . 
     According to an embodiment, second current driver  120  includes a fourth transistor  122 , a fifth transistor  126 , and a sixth transistor  132 . The fourth transistor  122  may correspond to an input transistor. The fifth transistor  126  may correspond to a current amplification transistor for second current driver  120 . The sixth transistor  132  may correspond to an output transistor for the second current driver  120 . 
     The drain and one of the gate terminals of the fourth transistor  122  receive the input current from second current source  125 . As with first current driver  110 , fourth transistor  122  amplifies the input current five-fold. The fourth transistor combines with fifth transistor  126  to form a 1:5 mirror. 
     The gate terminal for fifth transistor  126  receives the input current from second current, source  125 . The source terminal for fifth transistor  126  is grounded. The fifth transistor  126  and sixth transistor  132  are positioned in series, with the drain terminal of fifth transistor  126  feeding the source terminal of sixth transistor  132 . The gate terminals for sixth transistor  132  is supplied Vext. The drain terminal of sixth transistor  132  is supplied to the negative terminal  184  of transformer  180 . 
     As with first current driver  110 , Vext is provided to select components to ensure the tolerance levels of the transistors is not exceeded. The gate terminal of sixth transistor  132  is supplied Vext to match the center line voltage of transformer  180 . Therefore, the voltage difference between second terminal  184  of transformer  180  and sixth transistor  132  is ensured to be less than the voltage tolerance of sixth transistor  122 . This ensures the voltage tolerance of fifth transistor  126  is also not exceeded. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.