Abstract:
A low voltage different signaling (LVDS) includes an LVDS transmitter and an LVDS receiver. The LVDS transmitter includes a feedback compensation circuit, which adjusts and stabilizes the analog image signal to be transmitted to the LVDS receiver according to the voltage difference of the analog image signal and a base signal. The feedback compensation circuit includes a voltage-to-current converting circuit and a pair of current mirror circuits.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a low voltage differential signaling device, and more particularly to a low voltage differential signaling device with feedback compensation.  
         BACKGROUND OF THE INVENTION  
         [0002]    With the highly development of the internet, various kinds of data transmission on the internet are required. Therefore, a diversity of communication and transmission means are developed and used to facilitate the work on the internet. Particularly for the means requiring large bandwidth for supporting serial data flow, e.g. digital display, high resolution frame or color graphics, the transmission of the large amount of data relies much on the analog technique to design circuit system and support data transmission. Low voltage differential signaling (LVDS) is one of the analog techniques for transmission. This technique can be utilized to design hybrid signaling systems. The LVDS technique is a high-speed analog circuiting technique, which supports data transmission on copper wires in a multi-gigabit order.  
           [0003]    Since LVDS transmission technique is a universal interface standard for high-speed, low-power and low noise data transmission, it can be well applied to the system structure requiring modulating digital image signals into analog image signals and transmitting the analog image signals to a digital display at a high rate.  
           [0004]    Please refer to FIG. 1, which is a functional block diagram schematically showing a digital display system using LVDS transmission technique. The system comprises a computer host  10  and a display host  20  such as a liquid crystal display (LCD). An image control chip  101  inside the computer host  10  outputs a digital image signal S 1  to an LVDS transmitter  102  to be modulated into an analog image signal S 10  having a swing as low as 300 mV˜350 mV. Then, via an LVDS driving circuit  1021 , the analog image signal S 10  is outputted to an LVDS receiver  201 .  
           [0005]    In order to modulate and recover the analog image signal S 10  into the digital image signal S 1  capable of being processed by the downstream devices such as a timing control device  202  and a display driving device  203 , at least two resistors (FIG. 2, R 1  and R 2 ) are arranged at the input of the LVDS receiver  201 .  
           [0006]    The circuit of a conventional LVDS driving circuit  1021  is shown in FIG. 2. The skeleton of the circuit is a differential amplifier comprising of four transistors Q 1 ˜Q 4 , from which the analog image signal S 10  is generated and outputted to the resistors R 1  and R 2  of the LVDS receiver  201 . On the other hand, the transistor Q 5  and the parallelly interconnected transistors Q 6  and Q 7  constitute a first and a second current sources, respectively, wherein the first current source is electrically connected to a power source +Vcc and the differential amplifier, and the second current source is electrically connected to the differential amplifier and ground. By way of the first and the second current sources, a constant source current Io is provided for the differential amplifier. The voltage state of the constant current source can be stabilized with the presence of the transistor Q 8 .  
           [0007]    As is understood by those skilled in the art, the properties of the transistors Q 5 , Q 6  and Q 7 , which serve as the first and the second current sources, likely vary with some certain or uncertain factors during the manufacturing process. Once this happens, the constant source current Io provided for the differential amplifier comprising of the transistors Q 1 ˜Q 4  will deviate from the originally designed specification so as to make the voltage swing Va of the analog image signal S 10  outputted by the differential amplifier beyond the predetermined range. Then, the erroneous operation of the downstream LVDS receiver  201  may be rendered. Unfortunately, the signal drift problem resulting from the manufacturing factors of the above-mentioned LVDS driving circuit  1021  cannot be solved in the conventional LVDS transmitter  102 .  
         SUMMARY OF THE INVENTION  
         [0008]    Therefore, it is an object of the present invention to provide a low voltage differential signaling device, the output signal of which is capable of exempting from the effect of manufacturing factors.  
           [0009]    A first aspect of the present invention relates to a low voltage differential signaling (LVDS) transmitting device, which comprises a signal driving circuit for generating and outputting an analog image signal to an LVDS receiving device; and a signal compensation circuit in communication with the signal driving circuit, generating a compensation signal in response to the analog image signal and a base signal, and feeding the compensation signal back to the signal driving circuit to adjust the analog image signal.  
           [0010]    In an embodiment, the signal compensation circuit comprises a voltage-to-current converting circuit for outputting the compensation signal as a current type in response to the analog image signal and the base signal; and a first and a second current mirror circuits for feeding the compensation signal back to the signal driving circuit, wherein the first current mirror circuit is coupled to the voltage-to-current converting circuit and ground, and the second current mirror circuit is coupled to the first current mirror circuit, a power source and a differential signaling circuit of the signal driving circuit.  
           [0011]    Preferably, the voltage-to-current converting circuit outputs the compensation signal according to a voltage difference between the analog image signal and base signal.  
           [0012]    Preferably, the signal driving circuit comprises a first and a second current sources and a differential signaling circuit, wherein the first current source comprises a transistor coupled between a power source and the differential signaling device, the second current source comprises at least two transistors coupled between the differential signaling device and ground, and the differential signaling circuit comprises at least four transistors.  
           [0013]    In an embodiment, the compensation signal fed back to the signal driving circuit compensates the current signal variation of the first current source so as to stabilize the analog image signal generated by the signal driving circuit.  
           [0014]    Each of the first and the second current mirror circuits, for example, includes two transistors.  
           [0015]    In an embodiment, the signal compensation circuit further comprises a first and a second shunting resistors electrically connected between the signal driving circuit and the voltage-to-current converting circuit for shunting the analog image signal before the analog image signal is transmitted to the voltage-to-current converting circuit.  
           [0016]    In an example, the signal driving circuit is electrically connected to two serially connected resistors of the LVDS receiving device for transmitting the analog image signal with a swing of 300 mV 350 mV to the LVDS receiving device.  
           [0017]    Preferably, the base signal is a band-gap voltage signal generated by a band-gap circuit.  
           [0018]    A second aspect of the present invention relates to a compensation circuit for use with a signal driving circuit of a low voltage differential signaling (LVDS) transmitting device, which comprises a signal converting circuit receiving an analog image signal from the signal driving circuit, and generating a compensation signal according to a voltage difference between the analog image signal and a base signal; and a current mirror circuit in communication with the signal driving circuit and the signal converting circuit, feeding the compensation signal back to the signal driving circuit for compensating signal variation of the signal driving circuit.  
           [0019]    A third aspect of the present invention relates to a method for stabilizing an analog image signal outputted from a signal driving circuit of an low voltage differential signaling (LVDS) transmitter to an LVDS receiver. The method comprises steps of: shunting the analog image signal; generating a compensation signal according to a voltage difference between the shunted analog image signal and a base signal; and feeding the compensation signal to the signal driving circuit to compensate the analog image signal via a mirroring operation between a current source and the compensation signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The present invention may best be understood through the following description with reference to the accompanying drawings, in which:  
         [0021]    [0021]FIG. 1 is a functional block diagram schematically showing a conventional digital display system using LVDS transmission technique;  
         [0022]    [0022]FIG. 2 is a circuit diagram showing the LVDS driving circuit of the digital display system of FIG. 1;  
         [0023]    [0023]FIG. 3 is a functional block diagram schematically showing a digital display system using an LVDS transmitting device according to a preferred embodiment of the present invention;  
         [0024]    [0024]FIG. 4 is a circuit diagram showing the LVDS transmitting device of the digital display system of FIG. 3; and  
         [0025]    [0025]FIG. 5 is a circuit diagram showing an example of a band-gap circuit for generating a base signal to be provided for the LVDS transmitting device of the present invention for compensation. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only; it is not intended to be exhaustive or to be limited to the precise form disclosed.  
         [0027]    Please refer to FIG. 3, an embodiment of the LVDS transmitting device according to the present invention is shown. The LVDS transmitting device  103  is applied to a system comprising a computer host  10  and a display host  20  such as a liquid crystal display (LCD). An image control chip  101  inside the computer host  10  outputs a digital image signal S 1  to the LVDS transmitting device  103  to be modulated into an analog image signal S 10  having a swing as low as 300 mV˜350 mV. Then, via an LVDS driving circuit  1031 , the analog image signal S 10  is outputted to an LVDS receiver  201 .  
         [0028]    The LVDS transmitting device  103  according to the present invention further comprises a signal compensation device  1032  coupled to the LVDS driving circuit  1031  for stabilizing the analog image signal S 10  outputted by the LVDS driving circuit  1031  by way of feedback compensation. In order to achieve this purpose, a constant base signal VBG is introduced as a reference for the analog image signal S 10  to be compared with. The voltage difference of the analog image signal S 10  with the base signal VBG is fed back to the LVDS driving circuit  1031  to be referred for adjusting the analog image signal S 10 . By this way, the signal drift resulting from the physical property variation of the elements in the LVDS driving circuit  1031  due to manufacturing differentiations can be minimized. The constant base signal VBG can be provided by for example a band-gap circuit shown in FIG. 5. The base signal VBG is thus a band-gap voltage signal. As shown, due to the voltage difference between the base-emitter voltage of the transistor QA and the base-emitter voltage of the transistor QB, an electric current I is generated and a voltage is rendered at the resistor R. Via the voltage comparator C, the base signal VBG is generated and outputted from the transistor QC.  
         [0029]    Please refer to FIG. 4, which is a circuit diagram showing an embodiment of the LVDS transmitting device  103 . As mentioned above, the LVDS transmitting device  103  comprises the LVDS driving circuit  1031  and the signal compensation circuit  1032 . The LVDS driving circuit  1031 , similar to the one  1021  shown in FIG. 2, comprises a differential signaling circuit comprising of four transistors Q 1 ˜Q 4 , a first current source comprising a transistor Q 5  and a second current source comprising transistors Q 6  and Q 7  interconnected in parallel. The first current source is electrically connected to a power source +Vcc and the differential signaling circuit, and the second current source is electrically connected to the differential signaling circuit and ground.  
         [0030]    On the other hand, the signal compensation circuit  1032  comprises a first and a second shunting resistors R 3  and R 4 , a voltage-to-current converter  10321 , a first current mirror circuit comprising of transistors Q 12  and Q 13 , and a second current mirror circuit comprising of transistors Q 14  and Q 15 . The first and the second shunting resistors R 3  and R 4  are electrically connected between the LVDS driving circuit  1031  and the voltage-to-current converter  10321  for shunting the analog image signal S 10  to be transmitted to the LVDS receiving device  201 , and transmitting the shunted signal S 10  to the voltage-to-current converter  10321 . The voltage-to-current converter  10321 , which comprises of transistors Q 9 ˜Q 11 , generates a current-type compensation signal Io 1  according to the voltage difference between the analog image signal S 10  and the base signal VBG. The first current mirror circuit comprising of transistors Q 12  and Q 13  is electrically connected between the voltage-to-current converter  10321  and ground. The second current mirror circuit comprising of transistors Q 14  and Q 15  is electrically connected to the first current mirror circuit, the power source +Vcc and the differential signaling circuit (transistors Q 1 ˜Q 4 ) of the LVDS driving circuit  1031 . The first and the second current mirror circuits feed the compensation signal lol back to the LVDS driving circuit  1031  so as to compensate the current signal variation of the first current source (i.e. the transistor Q 5 ). By this way, the analog image signal S 10  generated by the LVDS driving circuit  1031  can be stabilized.  
         [0031]    For example, when the constant source current Io outputted by the transistor Q 5  has a density higher than expected due to varying manufacturing factors, the intensity of the signal shunted by the resistors R 3  and R 4  and inputted into the voltage-to-current converter  10321  will become increased accordingly. Since the base signal VBG is kept at a constant level, the voltage difference between the analog image signal S 10  and the base signal VBG will become decreased. Thus, the density of the current-type compensation signal Io 1  is also reduced. By way of the mirroring functions of the first and the second current mirror circuits, the current density at the transistor Q 15  is reduced as well. In other words, once the current density of the constant source current Io is enhanced, the compensation signal Io 1  will function to decrease the current density so as to maintain the source current  11  consisting of the constant source current Io and the current-type compensation signal Io 1  at a constant value. Accordingly, the differential signaling circuit can output the analog image signal S 10  with substantially constant swing in response to the compensated source current  11 . The signal variation problem resulting from varying manufacturing factors can thus be solved.  
         [0032]    In brief, the present invention provides feedback compensation means to adjust and stabilize the analog image signal so as to make use of the LVDS technique in the digital display system.  
         [0033]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.