Patent Publication Number: US-8988009-B2

Title: LED driver apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s).102102161, filed in Taiwan, Republic of China on Jan. 21, 2013, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to the LED driving circuits, and in particular, related to the LED driving circuits that suppress brightness error. 
     2. Description of the Related Art 
     In a LED display, brightness errors often occur among different modules because of the driving current variations thereof. For a full-color display, when the driving current is inaccurate, the screen is prone to color blocks, and the display quality is negatively affected. 
     Brightness errors usually occur due to inter-channel current errors or inter-chip current errors. The inter-chip current errors are caused due to process drift between different ICs which are manufactured in different batches. Though it is difficult to prevent process drifts, there are various manners in the prior art to deal with the inter-chip current errors. The contemporary approaches have limited effect on obliterating the inter-chip current errors. 
     In general, the human eyes can discern the brightness difference of 6% difference or above, the human eyes can even discern the brightness difference of 1% for low-brightness image frames. Thus, merely obviating the inter-chip current errors is insufficient to meet the requirements of today&#39;s high-definition displays. In view of this deficiency, the present invention provides new LED drivers that suppress brightness errors of the LED display by reducing the inter-channel current errors. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a LED driving apparatus. The apparatus comprises: an output transistor, having a drain coupled to the LED, a source, and a gate; a node, coupled to the source of the output transistor; a ground transistor, having a drain coupled to the node, and a source coupled to the ground; an operational amplifier, comprising: a first input end and a second input end, for respectively receiving a driving signal and a feedback signal; and an output end, for outputting an output signal to the gate of the output transistor; a compensating capacitor, comprising a first end and a second end; and a switching unit, for switching between a first connection mode and a second connection mode. Under the first connection mode, the compensating capacitor stores a bias difference between the first input end and the second input end of the operational amplifier, and under the second connection mode, the compensating capacitor compensates the bias difference by offsetting the stored bias difference to the node. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a LED driving apparatus according to the prior art. 
         FIG. 2A  is a schematic diagram of the LED driving apparatus according to an embodiment of the present invention. 
         FIG. 2B  shows the LED driving apparatus  200  of  FIG. 2A  operating under the first connection mode. 
         FIG. 2C  shows a LED driving apparatus  200  of  FIG. 2A  operating under the second connection mode. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 1  is a schematic diagram of a LED driving apparatus according to the prior art. In  FIG. 1 , the LED driving apparatus  100  comprises an output NMOS transistor  110 , a ground NMOS transistor  120 , and an operational amplifier  130 . The output NMOS transistor  110  has a drain and a source, where the drain is connected to the output end Out, and the source is connected in series to the drain of the ground NMOS transistor  120 . The output end Out is further connected to a LED (not shown). The ground NMOS transistor  120  has a gate for receiving a constant voltage V_G and a source connected to the ground. The operational amplifier  130  can receive a driving signal S, and output a voltage to the gate of the NMOS transistor  110 . With this negative feedback configuration, the operational amplifier  130  provides a voltage to the gate of the NMOS transistor  110 , so that the source of the NMOS transistor  120  keeps the same voltage with the node S. The operational amplifier  130  drives the ground NMOS transistor  120  to operate in a linear region, and steers the driving current on the LED to the ground through the output end OUT. 
     It is noteworthy that the inter-channel current errors are caused by: (1) the NMOS transistor  120 ; and (2) the bias difference of the operational amplifier  130 . To reduce the he inter-channel current errors caused by the NMOS transistor  120 , the transistor area usually has to be enlarged, thereby increasing costs. The LED driving apparatus of the present invention is aimed to lower the influences as a result of the bias difference of the operational amplifier. 
       FIG. 2A  is a schematic diagram of the LED driving apparatus according to an embodiment of the present invention. In this embodiment, the LED driving apparatus  200  comprises: an output transistor  210 , a ground transistor  220 , an operational amplifier  230 , a compensating capacitor  240 , a switching unit  250  and a controller  260 . These components will be described in the following in accordance with  FIG. 2A . 
     In the embodiment, the output transistor  210  and the ground transistor  220  are both NMOS transistors. The output transistor  210  has a drain and a source, where the drain is coupled to the output end Out and further coupled to the LED (not shown), and the source is coupled to a node P. The ground transistor has a drain, a source and a gate, where the drain is coupled to the node P, the source is grounded, and the gate is coupled to a power supply V_G, as shown in  FIG. 2 . 
     The operational amplifier  230  has two input ends (labeled as “+” and “−”, respectively), for respectively receiving a driving signal S and a negative feedback signal from the node P. In addition, the operational amplifier  230  further comprises an output end for providing an output voltage to the gate of the output transistor  210 . Due to the process drift, it is difficult for the voltages on the two input ends of the operational amplifier  230  to be identical to each other, thereby causing the bias difference on the node P and affecting the current accuracy. 
     To suppress the bias difference described above, the present invention provides a compensating capacitor  240  and a switching unit  250 . In the present invention, the switching unit  250  is switched between a first connection mode and a second connection mode in order to change the connection among the compensating capacitor  240  and other components of the LED driving apparatus  200 . Under the first connection mode, the compensating capacitor  240  stores a bias difference between the first input end (“+”) and the second input end (“−”) of the operational amplifier  230 . Under the second connection mode, the compensating capacitor  240  offsets the bias difference stored in the first connection mode to the node P. As such, the bias difference which causes the unstable current can be compensated through the switching of the switching unit  250 . In an embodiment of the present invention, the switching unit  250  is composed of three switches  251 ,  252  and  253 , which will be further described in the following embodiment by illustrating the first connection mode and the second connection mode of the present invention. However, it is to be noted that the switching unit  250  of the present invention should not be limited thereto, and those skilled in the art can implement the switching unit  250  in various manners. 
       FIG. 2B  shows the LED driving apparatus  200  of  FIG. 2A  operating under the first connection mode. Please refer to  FIGS. 2A and 2B . Under the first connection mode, the switches  251  and  252  of the switching unit  250  are closed, and the switch  253  is open. Meanwhile, the first end (the positive end) of the compensating capacitor  240  is coupled to the driving signal S and the first input end (“+”) of the operational amplifier  230 , and the second end (the negative end) of the compensating capacitor  240  is coupled to the second input end (“−”) of the operational amplifier  230  and the node P. The first connection mode is set to store the bias difference between the first input end (“+”) and the second input end (“−”) of the operational amplifier  230 . 
       FIG. 2C  shows an LED driving apparatus  200  of  FIG. 2A  operating under the second connection mode. Please refer to  FIGS. 2A and 2C . In contrast to the first connection mode, the switches  251  and  252  are open, and the switch  253  is closed under the second connection mode. Meanwhile, the first end (the positive end) of the compensating capacitor  240  is coupled to the node P, and the second end (the negative end) of the compensating capacitor  240  is coupled to the second input end (“−”) of the operational amplifier  230 . The second connection mode is set to offset the bias difference which is stored in the first connection mode to the node P. By switching the operation mode of the driving apparatus between the two modes, the bias difference caused by the operational amplifier  230  can be compensated. 
     To make sure that each switch of the switching unit  250  operates correctly, the LED driving apparatus of the present invention further comprises a controller  260 . The controller  260  of the present invention can not only coordinate the switching of the switches of the switching unit  250 , but also controls the switching frequency and switching period of the switching unit  250 . Those skilled in the art can set an appropriate switching frequency based on specifications of the components of the LED driving apparatus  200  (for example, the capacitance of the compensating capacitor  24 ), and thus the details in connection with the setting of the switching frequency will not be further discussed. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.