Abstract:
Disclosure has current-controlling stages, backlight systems and control methods for driving LEDs. A disclosed current-controlling stage has a current controller and a feedback apparatus. The current controller is coupled to a light-emitting device, for making the driving current through the light-emitting device substantially a predetermined value. The current controller has a control node, at which a control voltage substantially controls the driving current. The feedback apparatus influences a compensation voltage based on the control voltage to keep the control voltage substantially around a first predetermined value. The compensation voltage substantially determines an output power of a voltage-controlling stage.

Description:
BACKGROUND 
       [0001]    The present disclosure relates generally to backlight modules and more particularly to power supplies for driving the light-emitting devices in a backlight module. 
         [0002]    For hand-held or portable devices, such as smart phones and notebook computers, power efficiency, referring to how electric power is used for designed purposes, is always a concern in the art. Only if the electric power of the hand-held or portable devices is plenty and effectively used, then they can last long enough to operate when disconnected from power cords. Backlight consumes considerable electric power that the hand-held or portable devices carry. To save electric power and have higher power efficiency, lots of backlight systems employ light-emitting diodes, LEDs in short, to be their light sources. It has become an issue how to effectively drive LEDs, nevertheless. 
         [0003]      FIG. 1  illustrates backlight system  8  in the art. In general, backlight system  8  has three portions: voltage-controlling stage  4 , current-controlling stage  6 , and a light source with LED chains L 1 ˜L N , each LED chain having LEDs connected in series. Voltage-controlling stage  4  builds output voltage V OUT  at output node OUT, based on both the feedback mechanism provided through feedback node FB and compensation node COM, and the power transferring provided by inductor PRM. Control unit  20  of current-controlling stage  6  controls gate nodes of NMOS transistors N 1 ˜N N  to make currents flowing through LED chains substantially the same, such that each diode in LED L 1 ˜L N  illuminates with substantially the same light intensity. The diode numbers of LED L 1 ˜L N  might be the same or differ from one another. 
         [0004]    It seems that each of NMOS transistors N 1 ˜N N  acts as a voltage-controlled current source, the current flowing through which is determined by the control voltage at its gate node. Taking NMOS transistor N 1  as an example, drain-to-source voltage V DS  of NMOS transistor N 1  must exceed a minimum value V DS-MIN  for NMOS transistor N 1  to perform as a voltage-controlled current source. If drain-to-source voltage V DS  exceeds minimum value V DS-MIN  too much, nevertheless, NMOS transistor N 1  itself will consume much electric power, lowering the power efficiency for lighting. Accordingly, in order the optimize the power efficiency of backlight system  8 , it is better to keep each of drain-to-source voltages of NMOS transistors N 1 ˜N N  higher than and close to minimum value V DS-MIN . In current-controlling stage  6 , diode array  12  forwards the minimum one among the drain voltages of NMOS transistors N 1 ˜N N  to control unit  20 , which accordingly adjusts, via node CRT, feedback voltage V FB  at feedback node FB, such that compensation voltage V COM  at compensation node COM and the output power provided by voltage-controlling stage  4  are adjusted. 
         [0005]    It is supposed that control unit  20  finds the minimum one among the drain voltages of NMOS transistors N 1 ˜N N  is 0.6V, exceeding a target value of 0.5V. Control unit  20  then pours current through node CRT to raise feedback voltage V FB . To keep feedback voltage V FB  substantially the same, all compensation voltage V COM , output power of voltage-controlling stage  4 , and output voltage V OUT  decrease, such that the minimum one among the drain voltages of NMOS transistors N 1 ˜N N  decreases and approaches to the target value of 0.5V, and the power efficiency of backlight system  8  increases. 
         [0006]    If backlight system  8  desires dimming control to periodically turn on and off LED chains L 1 ˜L N , diode array  12  must endure high voltage when LED chains L 1 ˜L N  are off, and might not be integrated into a monolithic chip with control unit  20 . Generally speaking, the more discrete devices the high manufacture cost. Backlight system  8  might need some improvement in view of manufacture cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0008]      FIG. 1  illustrates a backlight system in the art; 
           [0009]      FIG. 2  demonstrates a backlight system according to one embodiment of the invention; and 
           [0010]      FIG. 3  demonstrates a backlight system according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In this specification, the devices with the same symbol refer to the devices with substantially the same or similar function, structure, compound or application, but are not necessarily all the same. After reading this specification, persons skilled in the art can replace or alter some devices in the embodiments without departing the essence of the invention. Accordingly, the embodiments herein are not used for limiting the scope of the invention. 
         [0012]      FIG. 2  demonstrates backlight system  60  according to one embodiment of the invention, having voltage-controlling stage  4 , current-controlling stage  62 , and a light source with LED chains L 1 ˜L N . 
         [0013]    As exemplified in  FIG. 1 , voltage-controlling stage  4  could be a booster. Shown in  FIG. 1 , power manager  18  controls the ON and OFF of power switch  15 , to determine the output power of voltage-controlling stage  4  and build output voltage V OUT  at output node OUT. 
         [0014]    Current-controlling stage  62  of  FIG. 2  has N current controller C 1 ˜C N , corresponding to LED chains L 1 ˜L N , respectively. Anodes of LED chains L 1 ˜L N  are commonly connected to output node OUT. Each cathode of LED chains L 1 ˜L N  is connected to a drain of a corresponding NMOS transistor. It is preferred for current controller C 1 ˜C N  to have a common circuit structure. Based on one current controller disclosed, persons skilled in the art can acknowledge or derive other current controllers without further explanation. 
         [0015]    Current controller C 1 , for example, has operational amplifier  64   1 , NMOS transistor N 1 , and detection resistor RS 1 . Of operational amplifier  64   1 , the non-inverted input is coupled to predetermined voltage V ref , the inverted input to detection resistor RS 1 , and the output to gate node GATE 1  of NMOS transistor N 1 . Detection resistor RS 1  is coupled between the source of NMOS transistor N 1  and a ground line. NMOS transistor N 1  is a power transistor as large current flows through it when turned on. The control voltage at gate node GATE 1  substantially controls the current through NMOS transistor N 1 , which equals to the current through LED chain L 1 . Thus, NMOS transistor N 1  is a voltage-controlled current source. It can be derived from the circuit shown in  FIG. 2  that current controller C 1  will keep the current through NMOS transistor N 1  substantially a preset constant, equal to V ref /R RS1 , where R RS1  refers to the resistance of detection resistor RS 1 . 
         [0016]    Further included in current-controlling stage  62  is feedback apparatus  66 , which has operational amplifier  68  and maximum value provider  70 . Maximum value provider  70  has a diode array with diodes, each having a cathode commonly coupled to the inverted input of operational amplifier  68  and an anode coupled to a corresponding gate node GATE, of a current controller C n . Supposed that each diode in maximum value provider  70  is ideal, the voltage at the inverted input of operational amplifier  68  will equal to the maximum voltage V GATE-MAX  among the control voltages at gate nodes GATE 1 ˜GATE N . The non-inverted input of operational amplifier  68  is coupled to target voltage V trgt . The output of operational amplifier  68  is coupled to node CRT, which is deemed to be an adjusting node. If the voltage at node CRT is lowered, then the target value that output voltage V OUT  approaches is adjusted to increase. 
         [0017]    As can be derived from both the circuits of current-controlling stage  62  and voltage-controlling stage  4 , at an equilibrium state, the voltage at the inverted input of operational amplifier  68  has substantially the same value as target voltage V trgt . In other words, the maximum voltage V GATE-MAX  will be maintained around a value corresponding to target voltage V trgt . For example, it is supposed that target voltage V trgt  is 4V and the voltage at the inverted input of operational amplifier  68  is 4.3V in an instant. Accordingly, operational amplifier  68  drains current from node CRT, feedback voltage V FB  is decreased, compensation voltage V COM  is increased, and output power of voltage-controlling stage  4  is increased, such that output voltage V OUT  is increased. The increment of output voltage V OUT  implies that control voltages at gate nodes GATE 1 ˜GATE N  should decrease to keep the currents through NMOS transistors N 1 ˜N N  substantially a preset constant. Thus, current controller C 1 ˜C N  decrease the control voltages at gate nodes GATE 1 ˜GATE N , and the maximum voltage V GATE-MAX  is reduced as a result. Correlating to the maximum voltage V GATE-MAX , the voltage at the inverted input of operational amplifier  68  decreases and approaches to target voltage V trgt . 
         [0018]    As the maximum voltage V GATE-MAX  is kept around a constant value corresponding to target voltage V trgt  the minimum channel resistance of NMOS transistor N 1 ˜N N  is kept as a constant, to effectively control the overall power efficiency. 
         [0019]    In comparison with backlight system  8  of  FIG. 1 , backlight system  60  of  FIG. 2  does not need diode array  12  of  FIG. 1 , and feedback apparatus  66  confronts no high voltages occurring at the drains of NMOS transistor N 1 ˜N N . Thus, it is possible for feedback apparatus  66  to be integrated into a single monolithic chip with current controllers C 1 ˜C N . Furthermore, if control unit  20  in  FIG. 1  is formed in an integrated-circuit chip, then that chip requires a specific pin dedicated to connect externally to diode array  12 . To the opposite, if current-controlling stage  62  is formed in an integrated-circuit chip, that chip does not require such a specific pin because what feedback apparatus  66  detects are control voltages at gate nodes GATE 1 ˜GATE N  inside that chip. Thus, current-controlling stage  62 , if formed in an integrated-circuit chip, might have a less pin-count. 
         [0020]    Even though the embodiment shown in  FIG. 2  drives LED chains L 1 ˜L N , the invention can be applied to embodiments driving a single LED chain or a single LED. One embodiment of the invention, for example, has the same circuit as backlight system  60  in  FIG. 2 , but the current-controlling stage  62  therein has only one current controller C 1  and the light source therein has only one LED chain L 1 . 
         [0021]    Even though  FIG. 2  exemplifies an embodiment with voltage-controlling stage  4  being a booster, the invention is not limited to. Persons skilled in the art could employ other power converter topologies in the art, such as flyback converters, buck converters, buck-boosters, or the like, to replace voltage-controlling stage  4  of  FIG. 2  while embodying the invention. 
         [0022]      FIG. 3  demonstrates backlight system  80  according to another embodiment of the invention. Voltage-controlling stage  44  could be any kind of power converters, such as booster, flyback converter, buck converter, buck-booster, and the like, to provide output voltage V OUT  at output node OUT. Voltage-controlling stage  44  has compensation node COM, the compensation voltage V COM  at which substantially determines the output power output from output node OUT from voltage-controlling stage  44 . For example, the higher the compensation voltage V COM , the more the output power output provided from voltage-controlling stage  44 . Current-controlling stage  72  has current controller C and feedback apparatus  76 . The connection and operation of current controller C can be derived or understood by persons skilled in the art based on a previous embodiment, such that the relevant explanation is omitted herein for brevity. Feedback apparatus  76  is an operational amplifier, whose inverted input is coupled to target voltage V trgt2 , non-inverted input to gate node GATE, and output to compensation node COM. 
         [0023]    Similar to the analysis of  FIG. 2 , if gate node GATE in  FIG. 3  is lower than target voltage V trgt2 , compensation voltage V COM  at compensation node COM will be decreased by feedback apparatus  76 , and output voltage V OUT  at output node OUT decreases. Accordingly, current controller C will raise control voltage V GATE  at gate node GATE, approaching target voltage V trgt2 . Thus, in an equilibrium state, control voltage V GATE  is kept to be about target voltage V trgt2 , and the current through diode chain L is substantially a preset constant. 
         [0024]    While the invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. 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.