Abstract:
A feedback regulating circuit provides a regulated voltage to a multi-output circuit that outputs a plurality of output voltages. The feedback regulating circuit includes a voltage control unit, coupled to the plurality of output voltages, for generating a first voltage according to the plurality of output voltages and outputting a voltage control signal according to the first voltage; and a reference voltage generator, coupled to the voltage control unit, for receiving the voltage control signal and generating a reference voltage according to the voltage control signal, with the reference voltage being fed back to the multi-output circuit to regulate voltage to a high degree of accuracy.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This patent application is based on Taiwan, R.O.C. patent application No. 099136632 filed on Oct. 27, 2010. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a monitor circuit, and more particularly, to efficiency optimization of a light emitting diode (LED) driving system and a fault detection circuit. 
       BACKGROUND OF THE INVENTION 
       [0003]    Since an LED has numerous advantages, e.g., small volume, short response time, low power consumption, high reliability, and high mass-production feasibility, the LED is widely applied to electronic apparatuses as a light source. For example, the 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 conventional LED driving system comprising a plurality of LED strings  10 , a boost controller  14 , and a boost power stage circuit  16 . 
         [0005]    For the conventional LED driving system illustrated in  FIG. 1 , even if each LED string  10  implements a same voltage source V DC  and the same number of LEDs  100 , since the LEDs  100  do not match with each other, voltages at input pads  11  are different. In order to reduce power consumption of the LED strings  10 , a predetermined voltage is fed back to the boost controller  14  in the circuit design, e.g., a 1V voltage serves as a basis of control of the voltage source V DC  that is controlled via a feedback circuit design between the boost controller  14  and the boost power stage circuit  16 , so that voltages at the input pads  11  are regulated to the predetermined voltage of 1V. 
         [0006]    However, a demanded precision in accuracy cannot be achieved via the foregoing design, and thus efficiency optimization of the LED driving system cannot be achieved. In addition, the conventional LED driving system is lacking of a mechanism for monitoring a fault of the LED strings  10 , e.g., a short-circuit fault or an open-circuit fault. Accordingly, when a serious fault of the LEDs  100  occurs, efficiency optimization of the LEDs  100  cannot be achieved via the boost controller  14  and the boost power stage circuit  16  of the conventional LED driving system. 
         [0007]    Therefore, a novel mechanism of efficiency optimization and fault detection needed to effectively monitor an LED driving system as well as increasing an emitting efficiency. 
       SUMMARY OF THE INVENTION 
       [0008]    In view of the foregoing issues, according to an embodiment of the present invention, a feedback regulating circuit applied to an LED driving system monitors fault abnormality on top of reducing power consumption of the LED driving system. 
         [0009]    According to an embodiment of the present invention, a feedback regulating circuit provides a regulated voltage to a multi-output circuit that outputs a plurality of output voltages. The feedback regulating circuit comprises a voltage control unit and a reference voltage generator. The voltage control unit coupled to the plurality of output voltages generates a first voltage according to the plurality of output voltages and outputs a voltage control signal according to the first voltage. The reference voltage generator coupled to the voltage control unit receives the voltage control signal and generates a reference voltage according to the voltage control signal, and the reference voltage is fed back to the multi-output circuit to regulate voltage to a given accuracy. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram of a conventional LED driving system. 
           [0011]      FIG. 2  is a schematic diagram of a feedback regulating circuit in accordance with an embodiment of the present invention. 
           [0012]      FIG. 3  is a schematic diagram of detailed circuits of an analog-to-digital converter (ADC) in accordance with an embodiment of the present invention. 
           [0013]      FIG. 4  is a schematic diagram of detailed circuits of a low-pass filter (LPF) in accordance with an embodiment of the present invention. 
           [0014]      FIG. 5  is a schematic diagram of detailed circuits of a boost controller and a boost power stage circuit in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]      FIG. 2  shows a schematic diagram of a feedback regulating circuit in accordance with an embodiment of the present invention. The feedback regulating circuit provides a regulated voltage to a multi-output circuit  20  that outputs a plurality of output voltages. In this embodiment, the multi-output circuit  20  comprises a plurality of LED strings  10  whose luminances are controlled by a plurality of current sources I S . Each LED string  10  comprises a plurality of LEDs  100  connected in serial. An outermost LED  100  of each LED string  10  has an anode coupled to a voltage source V DC , and each LED string  100  has an inner-most LED  100  having a cathode coupled to an input pad of an integrated circuit (IC)  2 . The multi-output circuit  20  is applicable to an LED driving system, e.g., a backlight module of a LCD. 
         [0016]    In this embodiment, the feedback regulating circuit comprises a voltage control unit  22  and a reference voltage generator  23 . The voltage control unit  22  is coupled to the output voltages so that the voltage control unit  22  generates a first voltage according to the output voltages and outputs a voltage control signal according to the first voltage, controlled by logic unit  220 . In addition, the reference voltage generator  23 , which is coupled to the voltage control unit  22 , receives the voltage control signal and generates a reference voltage accordingly. The reference voltage feeds back to the multi-output circuit  20 . Typically, the foregoing voltage control unit  22  is integrated in the IC  2 , and the reference voltage generator  23  is typically disposed outside the IC  2 . However, it is dependent on different design choices whether to integrate various modules and components into an IC circuit, and should not be limiting to the invention. 
         [0017]    In addition, the feedback regulating circuit in this embodiment comprises an analog to digital converter ADC  21 , coupled to the multi-output circuit  20 , to convert the plurality of analog output voltages to a plurality of digital voltages that are fed back to the voltage control unit  22 . The voltage control unit  22  selects one of the digital voltages as the first voltage.  FIG. 3  shows a schematic diagram of details of the ADC  21  in accordance with an embodiment of the present invention. In this embodiment, the ADC  21  comprises a multiplexer  210  and an ADC unit  212 . The multiplexer  210  receives the output voltages of the plurality of input pads  11 , and selects one of the output voltages at a time to feed back to the ADC unit  212 . The ADC unit  212  converts the output voltage selected by the multiplexer  210  to a digital voltage. 
         [0018]    In this embodiment, the voltage control unit  22  comprises a logic unit  220 , which selects one of the digital voltages according to a predetermined rule as the first voltage, and outputs a corresponding voltage control signal. In different embodiments, the predetermined rule is different according to different requirements. For example, in this embodiment, in order to increase an efficiency of the multi-output circuit  20 , it is designed that a minimum output voltage is selected to minimize power consumption of the multi-output circuit  20 . Therefore, the logic unit  220  selects the minimum voltage among the digital voltages as the first voltage. The logic unit  220  can be realized as hardware, as well as software, or any combinations thereof. 
         [0019]    The reference voltage generator  23  is connected to the logic unit  220 , and receives voltage control signals from the voltage control unit ( 22 ). The reference voltage generator  23  comprises a signal generator for generating a waveform according to the voltage control signal generated according to the first voltage. In this embodiment, the signal generator is a pulse width modulation (PWM) signal generator for generating different pulse widths that reflect the voltage control signal. In one embodiment, the logic unit  220  compares the first voltage with a target voltage to generate a difference to generate the voltage control signal accordingly, so that the signal generator  232  can adjust the output pulse width. More specifically, the signal generator  232  adjusts the pulse width in proportion to a ratio of the determined difference and the target voltage. When the feedback system is regulated, the target voltage is adjusted to approximate the first voltage. In an embodiment, the voltage control unit  22  adjusts the first voltage, and the reference voltage generator  23  generates a reference voltage, so that the target voltage is adjusted with the change of the first voltage. Accordingly, the voltage source connected to the multi-output circuit is adjusted to achieve a predetermined value or an ideal value of the system. 
         [0020]    The reference voltage generator  23  further comprises a low-pass filter (LPF)  234  for retrieving direct-current (DC) components of the modulated waveform outputted from the signal generator  232  as the reference voltage.  FIG. 4  shows a schematic diagram of details of the LPF  234  in accordance with an embodiment of the present invention. The LPF  234  comprises a resistor R LPF  and a capacitor C LPF . The resistor R LPF  is connected to an output end of the signal generator  232 , and the capacitor C LPF  is connected between the resistor R LPF  and ground. 
         [0021]    The reference voltage from the reference voltage generator  23  is under the control of the voltage control signal. By adjusting its duty cycle, the output voltage is adjusted accordingly, so that the output voltage source of the multi-output circuit is modified; as a result, the minimum voltage of the output voltages can be accurately regulated and maintained at the target voltage. 
         [0022]    In another embodiment, the feedback regulating circuit further comprises a boost controller  24  and a boost power stage circuit  26 , as shown in  FIG. 5 , showing a schematic diagram of detail circuits of the boost controller  24  and the boost power stage circuit  26 . The boost controller  24  compares the reference voltage with a voltage at a voltage-divided node of the adjustable voltage source V DC  of the multi-output circuit, and accordingly outputs a feedback control signal to the boost power stage circuit  26 . According to the feedback control signal, the boost power stage circuit  26  controls the voltage at the voltage-divided node of the voltage source V DC  to approximate the reference voltage so as to adjust the voltage of the adjustable voltage source V DC . The desired voltage (e.g. the reference voltage) at voltage divided node is decided by using appropriate resistors R 1  and R 2 . In this embodiment, for example, the boost power stage circuit  26  comprises a switching power supply, which comprises an N-type metal-oxide-semiconductor (NMOS) switch transistor (SW), and an energy storing circuit formed by an inductor L and/or a capacitor C, as shown in  FIG. 5 . The NMOS SW under the control of the voltage control signal performs voltage source switching. 
         [0023]    In addition to feedback voltage regulation, the embodiments of the present invention also achieves error detection of the LEDs. For example, when a short circuit (or similar fault) occurs in one or more of the LEDs  100  of a given LED string  10 , an abnormal rise of output voltages at a corresponding input pad  11  can be detected by the logic unit  220  by monitoring the voltage at the input pad  11  corresponding to the LED strings  10 . In another embodiment, when an open circuit occurs in one or more LEDs  100  of a certain LED string  10 , an abnormal decrease of output voltage at a corresponding input pad  11  is detected by the logic unit  220  by monitoring the voltage at the input pad  11 . The result of detected errors is provided to other blocks of the system for further processing. 
         [0024]    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.