1. Field of the Invention
The present invention relates to a local area networking device, especially to a driving circuit for a transceiver output port of local area networking device.
2. Description of the Related Art
The IEEE announced an IEEE 802.3 standard for Ethernet communication system. According to the IEEE 802.3 standard, the transceiver output port of each local area networking device is coupled to other local area networking devices via a unshielded twisted pair (UTP) cable. The data outputted from the transceiver output port is in the form of a differential output signal. In a 10 Mbps Ethernet communication system, for example, the peak-to-peak magnitude of the differential output signal VPP is from ±2.2V to ±2.8V as illustrated in
The advantage for the circuit as illustrated in
Although the power consumption of the conventional circuit as shown in
wherein I is a constant.
From the equations (2) and (3), equation (4) can be derived as follows:
Based on equation (2) and equation (4), equation (5) is generated as follows:
Based on equations (1) and (5), equation (6) is obtained to represent V+as follows:
Furthermore, based on equations (4) and (6), equation (7) is obtained to represent V−as follows:
The differential voltage (V+−V−) can then be represented by equation (8) according to equations (6) and (7) as follows:
In a 10 Mbps Ethernet communication system, for example, the peak-to-peak magnitude of the differential output signal VPP is from ±2.2V to ±2.8V. If the differential output signal VPP is positive (V+>V−), an inequality (9) can be derived from the equation (8):
and another inequality (10) can be derived from equation (7):
Based on inequalities (9) and (10), the following inequality can be obtained:
As a result, the supply voltage VDD has a minimum value of 3.3V when I2=0. Therefore, the supply voltage VDD cannot be further reduced if the same amplitude of the differential output signal is to be kept. In other words, the conventional circuit for a transceiver output port at least has the following disadvantages:
1. The power consumption to drive the transceiver output port is still considerably too large.
2. Magnitude of the supply voltage VDD must also be large. For example, in a 10 Mbps Ethernet communication system, the supply voltage VDD must be larger than 3.3V to maintain the magnitude of the differential output signal to be within a predetermined range.
Therefore, it is one of the objectives of the present invention to provide a circuit for a transceiver output port of a local networking device, wherein the power consumption can be further decreased.
In addition, it is another one of the objectives of the present invention to provide a circuit for a transceiver output port of a local networking device, wherein the circuit can operate particularly at a supply voltage with the magnitude lower than the conventional supply voltage.
According to the objectives of the present invention, a driving circuit of a transceiver output port of a local area networking device is disclosed. The transceiver output port is coupled to a cable, such as an unshielded twisted pair (UTP), shielded twisted pair (STP), or the like, and the driving circuit is for outputting a differential output signal to the cable. The transceiver output port driving circuit comprises a first output node and a second output node for outputting the differential output signal; a first current source coupled to the first output node for providing a first current; a second current source coupled to the second output node for providing a second current; a first resistor coupled to the first output node; a second resistor coupled to the second output node; a third current source coupled between the first output node and a supply voltage source for providing a third current; and a fourth current source coupled between the second output node and the supply voltage source for providing a fourth current.
The resistance of the first resistor and the second resistor each are equal to ½ of the equivalent resistance of the cable for the purpose of impedance matching to the cable.
The third and fourth current sources are for providing additional currents while the first and second current sources are operating such that the magnitude of the differential output signal is determined not only according to the first and second currents but also according to the third and fourth currents.
Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.
In this embodiment, the current sources I3 and I4 are connected to the resistors R1 and R2 in parallel respectively. In other words, the current source I3 is coupled between the current source I1 and the supply voltage source VDD while the current source I4 is coupled between the current source I2 and the supply voltage source VDD. One of the features of the present invention is that the current sources I3 and I4 are employed in order to provide additional current for the driving circuit so that the magnitude of the differential output signal outputted from the transceiver output port is determined not only by the current sources I1, and I2 but also by the current sources I3 and I4. In this way, the circuit can operate particularly with a supply voltage source VDD at a low level, even if the supply voltage VDD is lower than the minimum magnitude of the supply voltage required by the conventional driving circuit.
According to IEEE 802.3 standard, RL=100 Ω and R1=R2=50 Ω. In the following explanation, the resistance of the resistors R1 and R2 are both represented by R and the resistance of RL can be represented by 2R. Therefore, the resistors R1 and R2 can both be represented by R while RL can be represented by 2R. One of the advantages of the driving circuit shown in
I1+I2=I12 (11)
I3+I4=I34 (12)
In this embodiment, the sum of the first current I1 and the second current I2, denoted by I12, is a constant while the sum of the third current I3 and the fourth current I4, denoted by I34, is also a constant. Applying Kirchhoff's voltage law at the output nodes y and x respectively, we have:
Summing up equations (13) and (14), equation (15) is obtained as follows:
Based on equations (15) and (13), equation (16) is obtained as follows:
V+ be expressed by equation (17) which is derived from equations (11), (12), and (16), as follows:
Moreover, V− can be represented by equation (18) which is derived from equations (17) and (15), as follows:
According to equations (17) and (18), equation (19) is obtained to represent V+−V− as follows:
In a 10 Mbps Ethernet communication system, for example, the peak-to-peak magnitude of the differential output signal VPP is from ±2.2V to ±2.8V. When the differential signal is positive (V+>V−), according to equation (19), inequality (20) is obtained:
According to equation (18), inequality (21) is generated:
Based on inequalities (20) and (21), inequality (22) is obtained as follows:
VDD≧3.3+R(I2−I4) (22)
According to inequality (22), if I2=0, and
VDD can be an arbitrary positive value. However, in practical applications, the supply voltage VDD is maintained within a range which enables the current sources I1 to I4 to operate properly. According to practical experiments, the driving circuit of the transceiver output port can operate normally when the magnitude of the supply voltage VDD is larger than 1V, which is much lower than the supply voltage (3.3V) required by the conventional driving circuit. Therefore, the magnitude of the differential signal outputted from the driving circuit as illustrated in
In addition, the currents required to drive the transceiver output port to output the differential output signal are provided not only by the first and second current sources I1, and I2, but also by the third and fourth current sources I3 and I4. In other words, the third and fourth current sources I3 and I4 provide additional currents while the first and second current sources I1 and I2 are operating. In this manner, the power consumption of the driving circuit of the transceiver output port can be further reduced.
In the embodiments of the present invention, the magnitude of the differential output signal is determined not only by the first and second current sources I1 and I2, but also by the third and fourth current sources I3 and I4. In the above-described embodiment, the sum of the first current I1 and the second current I2 (I12) is a constant and the sum of the third current I3 and the fourth current I4 (I34) is also a constant. In addition, the magnitudes of I12 and I34 are substantially the same. Under these conditions, the magnitudes of the first current I1 and the fourth current I4 are substantially the same and the magnitudes of the second current I2 and the third current I3 are substantially the same.
Referring to
There is another implementation to determine the magnitude of the differential output signal, wherein neither the sum of the first current I1 and the second current I2 (I12) nor the sum of the third current I3 and the fourth current I4 (I34) are constant. Referring to
It should be noted that the configuration of the impedance matching resistors, i.e. R1 and R2, for matching the impedance of the UTP, for example, the configuration shown in
There are various ways to implement the current sources of the embodiments of the present invention. For example, each of the current sources can be implemented as a current mirror, as shown in
While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.
This is a continuation-in-part of application Ser. No. 09/447,709, filed on Nov. 23, 1999, now U.S. Pat. No. 6,795,493, the subject matter of which is incorporated herein by reference.
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Number | Date | Country | |
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20050053161 A1 | Mar 2005 | US |
Number | Date | Country | |
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Parent | 09447709 | Nov 1999 | US |
Child | 10937189 | US |