Abstract:
A regulated voltage protection circuit includes a bias-voltage generating circuit and a clamping circuit. The bias-voltage generating circuit provides a bias-voltage. The clamping circuit is coupled to a plurality of light-emitting diode (LED) strings and a driving circuit, and generates a plurality of clamped voltages that are respectively transmitted to a plurality of input pads of the driving circuit according to the bias-voltage. The regulation voltage protection circuit protects against electrical overstress (EOS) by protecting the driving circuit during low-voltage fabrication process chip manufacturing.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This patent application is based on Taiwan, R.O.C. patent application No. 099130937 filed on Sep. 13, 2010. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a regulated voltage protection circuit, and more particularly, to a regulated voltage protection circuit of a light-emitting diode (LED) driving module. 
       BACKGROUND OF THE INVENTION 
       [0003]    Since an LED has numerous advantages, e.g., small size, short reaction time, low power consumption, high reliability, and high feasibility of mass production, the LED is widely applied to electronic apparatuses as light sources. For example, an LED serves as a backlight source of a liquid crystal display (LCD) to replace a conventional fluorescent tube. 
         [0004]      FIG. 1  is a schematic diagram of a part of a backlight module structured by LEDs. The backlight module comprises a plurality of LED strings  10  and a driving circuit  12 . Each of the LED strings  10  comprise a plurality of serially-connected LEDs  100 , and an outermost LED  100  of the LED strings  10  has an anode coupled to a high voltage source V DC  and a cathode coupled to an input pad  14  of the driving circuit  12 . 
         [0005]    For the LED strings  10  shown in  FIG. 1 , when one or numerous LEDs  100  short-circuit due to invalidation, a voltage at the input pad  14  is increased. When the voltage is dramatically increased and beyond a rated voltage of the driving circuit  12 , damage and invalidation are created in the driving circuit  12 . Such an abnormal input voltage is so-called electrical overstress (EOS). The conventional driving circuit  12  (e.g., a driving circuit chip) is manufactured via a high-voltage fabrication process, and a chip manufactured via the high-voltage fabrication process may burden a relatively high input voltage. 
         [0006]    However, a circuit manufactured via the high-voltage fabrication process has a larger area than that circuit manufactured via a low-voltage fabrication process, and requires higher cost during manufacture. In addition, since the high-voltage fabrication process is incompatible with the low-voltage fabrication process, the driving circuit  12  is uneasily integrated with other system circuits of an LCD. Therefore, when the driving chip manufactured via the low-voltage fabrication process is applied to an LED string of a high-voltage source, damage created in the driving circuit due to EOS needs to be prevented. 
         [0007]    Therefore, a new regulated voltage protection mechanism is needed to protect the driving circuit  12  manufactured via the low-voltage fabrication process so as to prevent internal circuit components of the driving circuit  12  from being undesirably affected by EOS caused by a voltage from the LED strings. 
       SUMMARY OF THE INVENTION 
       [0008]    In view of the foregoing issues, according to the present invention, a regulated voltage protection circuit, applied to a LED driving module, in a driving circuit or outside a circuit chip, controls a voltage of an input pad to avoid EOS. Accordingly, the driving circuit is manufactured using a low-voltage manufacture process to integrate with other system circuits, to reduce circuit area and cost of the overall system in addition to increasing efficiency. 
         [0009]    According to an embodiment of the present invention, the regulated voltage protection circuit provides regulated voltage protection to a driving module, which is coupled to a plurality of LED strings, and comprises a bias-voltage generating circuit and a clamping circuit. The bias-voltage generating circuit provides a bias voltage. The clamping circuit is simultaneously coupled to the plurality of LED strings and the driving module. According to the bias voltage, the clamping circuit generates a plurality of clamping voltages that are respectively transmitted to a plurality of input pads of a driving circuit. 
         [0010]    According to another embodiment of the present invention, a display controller comprises an LED driving module comprising a plurality of LED strings, a driving circuit, a bias-voltage generating circuit, and a clamping circuit. Each of the LED strings comprises a plurality of LEDs in serial, and has one end coupled to a voltage source. The driving voltage drives the plurality of LED strings. The bias-voltage generating circuit provides a bias voltage. The clamping circuit is coupled to the plurality of LED strings and the driving circuit, and the clamping circuit generates a plurality of clamping voltages to a plurality of input pads of the driving circuit according to the bias voltage. 
         [0011]    According to yet another embodiment of the present invention, a method for driving an LED comprises generating a bias voltage; clamping via the bias voltage a plurality of voltages from a plurality of LED strings to generate a plurality of clamped voltages; and transmitting the clamped voltages to an LED driving circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic diagram of a backlight module structured by LEDs shown in part. 
           [0013]      FIG. 2A  is a schematic diagram of a regulated voltage protection voltage in accordance with an embodiment of the present invention. 
           [0014]      FIG. 2B  is a flow chart of a method for driving LED strings in accordance with an embodiment of the present invention. 
           [0015]      FIG. 3  is a detailed schematic diagram of the regulated protection circuit illustrated in  FIG. 2A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]      FIG. 2A  shows a schematic diagram of a regulated voltage protection circuit in accordance with an embodiment of the present invention. The regulated voltage protection circuit applied to an LED driving module of a display controller protects a driving circuit of LEDs, so that internal circuit components of the driving circuit are not undesirably affected by EOS. In this embodiment, the LED driving module is a backlight module of an LCD; however, it shall not be construed as limiting the present invention.  FIG. 2B  is a flow chart of a method for driving an LED in accordance with an embodiment of the present invention. 
         [0017]    In this embodiment, the regulated voltage protection circuit mainly comprises a bias-voltage generating circuit  20  and a clamping circuit  22 . In Step  31 , the bias-voltage generating circuit  20  provides a bias voltage V bias  to the clamping circuit  22 , which is coupled to a plurality of LED strings  10 . Each LED string includes a plurality of LEDs  100  connected in series. The outermost LED  100  of each LED string  10  has an anode coupled to a high-voltage source V DC , and a cathode coupled to the clamping circuit  22 . The clamping circuit  22  is further coupled to a plurality of input pads  26  of a driving circuit  24 . In Step  32 , the clamping circuit  22  clamps voltages at the cathodes of the outermost LEDs of the LED strings  100  to generate a plurality of clamping voltages. In Step  33 , the clamping voltages are used as feedback to the input pads  26  of the LED driving circuit  24 , so that a voltage at each input pad  26  is kept below a predetermined voltage or an arbitrary rated voltage, so that the driving circuit  24  is protected from being undesirably affected or damaged by electrical overstress (EOS). The predetermined (or rated) voltage is dependent on the manufacturing process of the driving circuit  24 . For example, when a 5 Volt (V) manufacture process is implemented, the predetermined voltage is accordingly determined as 5V. In this embodiment, the driving circuit  24  comprises a plurality of current sources respectively controlling luminance of the LED strings  10 . The driving circuit  24  is a semiconductor integrated circuit (IC) of a low-voltage manufacture process and is integrated with other system circuits of an LCD to form a system-on-chip (SOC), e.g., a display control chip. 
         [0018]      FIG. 3  shows a schematic of detailed circuits of the regulated voltage protection voltage illustrated in  FIG. 2A . In this embodiment, the clamping circuit  22  comprises a plurality of parallel N-type metal-oxide-semiconductor (NMOS) clamping transistors M 0 , M 1 , . . . and Mn, which are respectively connected to LED strings  10  and input pads  16 . More specifically, each clamping transistor M 0 /M 1 /Mn has a source S coupled to a corresponding input pad  26 , and a drain D coupled to a cathode of the outermost LED  100  of each corresponding LED string  10 . All clamping transistors M 0  to Mn have gates G coupled to the bias voltage V bias  provided by the bias-voltage generating circuit  20 . An object of the present invention is to limit the voltage at the source end S of each clamping transistor to be below a rated voltage (e.g., 5V). Thus, it is necessary to have the bias voltage generator regulate the gate voltage of the clamping transistor; also, the source voltage of the clamping transistor is controlled without being undesirably affected by variations of the drain voltage by utilizing the characteristics of a regulated voltage between a source and a gate of a clamping transistor. An approach for achieving such object is described below. 
         [0019]    In this embodiment, the bias-voltage generating circuit  20  mainly comprises an NMOS bias-voltage transistor Ma and a regulated voltage circuit  200 . The bias-voltage transistor Ma has a gate G coupled to the gates G of the clamping transistors M 0  to Mn of the clamping circuit  22 , a drain D electrically coupled to a voltage source V, and a source S coupled to ground via voltage-divider resistors R 2  and R 3 . It is to be noted that, in this embodiment, the bias-voltage transistor Ma and the clamping transistors M 0  to Mn share the same manufacture process, which means the bias-voltage transistor Ma and the clamping transistors M 0  to Mn have the same threshold voltage. Accordingly, source voltages of the clamping transistors M 0  to Mn are regulated by controlling a gate voltage and a source voltage of the bias-voltage transistor Ma. The regulated voltage circuit  200  is a programmable shunt regulator, e.g., a programmable shunt regulator TL431, which has three terminals—an anode A, a cathode K, and a reference voltage terminal V REF . More specifically, the anode A is coupled to ground, the cathode K is coupled to both the gate G of the bias-voltage transistor Ma and the voltage source V via a current limiting resistor R 1 , and the reference voltage terminal V REF  is coupled to a central node of the voltage dividing resistors R 2  and R 3 . The voltage at the gate G of the bias-voltage transistor Ma is adjusted to a desired voltage by matching the voltage source with the resistors. 
         [0020]    Through the foregoing circuit connection configurations, a regulated voltage is generated between the gate G and the source S of the bias-voltage transistor Ma. For example, according to the design of the regulated voltage circuit, in the event that the voltage at the source S of the bias-voltage transistor Ma is 5V, the voltage at the gate G is (5+Vth)V, wherein Vth is the threshold voltage of the bias-voltage transistor Ma. Generally, the regulated voltage circuit  200  adjusts the voltage at the source S of the bias-voltage transistor Ma to be identical to a predetermined voltage of the input pads  26 , and adjusts the voltage at the gate G to be equal to sum of the predetermined voltage and the threshold voltage Vth. 
         [0021]    As mentioned above, the bias-voltage transistor Ma and the clamping transistor M 0  to Mn share the same manufacture process, so that each of the clamping transistors M 0 , M 1 , and Mn has a bias voltage identical to that of the bias-voltage transistor Ma. For example, when the voltage at the gate G of the clamping transistor M 0 /M 1 /Mn is (5+Vth)V (i.e., Vth is its threshold voltage), the voltage at the source S maintains as 5V. Therefore, the configuration of the embodiment of the present invention avoids electrical overstress (EOS) of the driving circuit  24  because each voltage of the input pads  26  does not exceed the predetermined voltage (e.g., the foregoing 5V). For example, when one or several LEDs  100  of the LED strings  10  short-circuit due to component failure, the drain-source D-S voltage of the corresponding clamping transistor M 0 /M 1 /Mn is increased; however, the voltage at the source S of the clamping transistor M 0 /M 1 /Mn maintains as the predetermined voltage, protecting the driving circuit from EOS. 
         [0022]    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 to be limited to the above 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.