Abstract:
Short protection control circuits and related control methods are disclosed. A disclosed short protection control circuit is adapted for controlling a short protection mechanism providing short protection to several LED chains. The disclosed short protection control circuit has a detection circuit, a first logic circuit and a timer. Coupled to the LED chains, the detection circuit asserts an indication signal when one of the node voltages of the LED chains is lower than an under-current reference. When the indication signal is enabled, the first logic circuit starts blocking the short protection mechanism. The timer times to provide a result when the short protection mechanism is blocked. When the result indicates that the short protection mechanism has been blocked for at least a predetermined time period, the first logic circuit resumes the short protection mechanism.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to control methods and circuits for LED chains, and more particularly to control methods for short protection in LED chains. 
         [0003]    2. Description of the Prior Art 
         [0004]    In an age concerned with energy conservation and carbon reduction, light-emitting diodes (LEDs) are already a widely adopted light source due to their superior lighting efficiency and miniature component size. For example, LEDs have already replaced cold-cathode fluorescent lamps (CCFLs) as a backlight in current liquid crystal display (LCD) panels. 
         [0005]      FIG. 1  is a diagram illustrating an LED power supply  18  used in a backlight module of an LCD panel, which is primarily used to control lighting of LED chains L 1 -L N . Each LED chain has a plurality of series-connected LEDs. Backlight controller  20  controls a power switch of booster  19  to cause an inductive element to draw energy from input node IN, and release energy into output node OUT, so as to establish an appropriate output voltage V OUT  on output node OUT to drive the LED chains. Backlight controller  20  detects output voltage V OUT  through over-voltage protection node OVP and voltage divider resistors RD 1 , RD 2 . 
         [0006]    Driving nodes LED 1 -LED 4  of backlight controller  20  are connected to LED chains L 1 -L 4 , respectively, for draining driving current of LED chains L 1 -L 4 , and controlling current flowing through each LED chain to be approximately equal to achieve the goal of uniform brightness. 
         [0007]    Backlight controller  20  may also determine whether any LED encounters a fault condition from driving nodes LED 1 -LED 4 , so as to trigger related protection. For example, if LED detection voltage V LED-1  on driving node LED 1  is continually 0V, LED chain L 1  may be an open-circuited LED chain, where at least one LED thereof is open-circuited, in which case backlight controller  20  turns off driving of LED chain L 1 . This type of protection is typically called open circuit protection. In another example, if LED detection voltage V LED-1  on driving node LED 1  is much greater than LED detection voltage V LED-2  on driving node LED 2 , it can roughly be ascertained that LED chain L 1  has a few LEDs that are short-circuited, and driving of LED chain L 1  can be turned off. This type of protection is typically called short circuit protection. 
         [0008]    However, open circuit protection and short circuit protection may interfere with each other. Thus, an appropriate process is needed to activate or stop open and short circuit protections, so as to achieve the desired protection effect. 
       SUMMARY OF THE INVENTION 
       [0009]    According to an embodiment, a control method is used in controlling a short protection mechanism providing short protection to a plurality of light-emitting diode (LED) chains. A plurality of driving currents flow through the LED chains. The control method comprises detecting whether at least one of the driving currents encounters an under-current event; blocking a short protection mechanism when the under-current event is encountered; and resuming the short protection mechanism after the short protection mechanism is blocked for at least a predetermined time period. The short protection mechanism provides short protection to the LED chains. 
         [0010]    According to an embodiment, a short protection control circuit is for controlling a short protection mechanism. The short protection mechanism provides short protection applied to a plurality of light-emitting diode (LED) chains. The short protection control circuit comprises a detection circuit coupled to the LED chains for generating an indication signal whenever any terminal voltage of the LED chains is lower than an under-current reference value; a first logic circuit for starting blocking of the short protection mechanism when the indication signal is enabled; and a timer for counting time when the short protection mechanism is blocked to generate a timing result. The first logic circuit resumes the short protection mechanism after the timing result indicates that the short protection mechanism has been blocked for at least a predetermined time period. 
         [0011]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a diagram illustrating an LED power supply used in a backlight module of an LCD panel. 
           [0013]      FIG. 2  is a diagram of backlight controller according to an embodiment. 
           [0014]      FIG. 3  shows one type of protection circuit. 
           [0015]      FIG. 4  shows some signal waveforms of  FIG. 2  and  FIG. 3  when an LED chain becomes open-circuited. 
           [0016]      FIG. 5  shows another protection circuit. 
           [0017]      FIG. 6  shows  FIG. 2  and  FIG. 5  some signal waveforms when an LED chain becomes open-circuited. 
           [0018]      FIG. 7  shows another detection circuit. 
           [0019]      FIG. 8  shows an analog timer. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 2  is a diagram of backlight controller  20  according to an embodiment, which can be used in LED power supply  18  of  FIG. 1 . In some embodiments, backlight controller  20  is a monolithic integrated circuit (IC). In the present disclosure, backlight controller  20  drives four LED chains L 1 -L 4 . In other embodiments, backlight controller  20  may drive different numbers of LED chains, and is not limited to four. 
         [0021]    In backlight controller  20 , fixed current drivers  22   1 - 22   4  are connected to driving nodes LED 1 -LED 4 , respectively, to cause driving currents I LED-1 -I LED-4  flowing through LED chains L 1 -L 4  to be roughly equal, so that LED chains L 1 -L 4  have uniform brightness. For example, fixed current driver  22   1  has error amplifier  24   1 , NMOS transistor N 1 , and current sense resistor RS 1 . From the circuit diagram of fixed current driver  22   1  it can be seen that, in normal operation, driving current I LED-1  is approximately equal to set voltage V C-SET  divided by R RS1 , where R RS1  is resistance value of current sense resistor RS 1 . Fixed current drivers  22   2 - 22   4  may be known from the description of fixed current driver  22   1 . 
         [0022]    In some embodiments, feedback selector  26  takes the smallest of LED detection voltages V LED-1 -V LED-4  to be minimum detection voltage V LED-MIN  for transmission to inverting terminal of error amplifier  27 . Non-inverting terminal of error amplifier  27  receives preset target voltage V TAR . Pulse width adjuster  30  generates driving signal V DRV  according to output of error amplifier  27  to control power switch of booster  19 . Under stable conditions, minimum detection voltage V LED-MIN  is roughly equal to target voltage V TAR , which roughly causes LED power supply  18  of  FIG. 1  to operate in a relatively more efficient state. 
         [0023]    Protection circuit  28  determines whether any of LED chains L 1 -L 4  encounters a fault condition according to detection voltage V OVP  on over-voltage protection node OVP and LED detection voltages V LED-1 -V LED-4  on driving nodes LED 1 -LED 4  to generate selection signals S EN-1 -S EN-4 . For example, fault conditions comprise LED open circuit events (at least one LED chain has an open circuit), LED short circuit events (at least one LED chain has a short circuit), etc. In other embodiments, protection circuit may determine whether any of LED chains L 1 -L 4  encounters a fault condition further (or only) according to input or output of error amplifier  24   1 - 24   4 . For example, if LED chain L 1  is identified as encountering a fault condition, protection circuit  28  disables selection signal S EN-1 . Disabled selection signal S EN-1  causes fixed current driver  22   1  to not drive LED chain L 1 , meaning driving current I LED-1  becomes 0A. Disabled selection signal S EN-1  also causes minimum detection voltage V LED-MIN  to be independent of LED detection voltage V LED-1 , causing feedback selector  26  to not select LED detection voltage V LED-1  as minimum detection voltage V LED-MIN . 
         [0024]    LED open circuit events may cause mistaken triggering of short circuit protection. For example, LED chain L 1  may become open-circuited for some reason at a point in time, while LED chains L 2 -L 4  are normal. LED detection voltage V LED-1  may roughly equal 0V, causing minimum detection voltage V LED-MIN  to also be roughly 0V, which is lower than target voltage V TAR . At this time, output voltage of error amplifier  27  is pulled up continuously, and pulse width adjuster  30  causes booster  19  to increase output energy, pulling up output voltage V OUT  and LED detection voltages V LED-2 -V LED-4 . If careful circuit design is not employed, at this time, LED chains L 2 -L 4  are likely to be mistakenly determined as encountering LED short circuit events, which will mistakenly trigger short circuit protection, because LED detection voltages V LED-2 -V LED-4  are much greater than LED detection voltage V LED-1  or target voltage V TAR  at this time. 
         [0025]      FIG. 3  shows one type of protection circuit  28   a , which when used in  FIG. 2 , can prevent LED open circuit events from erroneously triggering short circuit protection. Protection circuit  28   a  has detection circuit  50 , timer  57 , protection determination circuit  64 , AND gate  54 , and SR flip-flop  62 . 
         [0026]    In detection circuit  50 , comparators  60   1 - 60   4  respectively couple to LED chains L 1 -L 4 . Outputs of comparators  60   1 - 60   4  couple to pulse generators  70   1 - 70   4  having rising and falling edge triggering. Outputs of pulse generators  70   1 - 70   4  are all connected to OR gate  52 . For the most part, any time any one LED detection voltage V LED-n  (where n is an integer from 1-4) is lower than under-current reference value V OVER-LOW , comparator  60   n  changes state to logic “1,” thereby triggering pulse generator  70   n  to send out a pulse as an indicator signal. This pulse passes through OR gate  52  to appear in reset signal S RESET . Taking LED chain L 1  as an example, when LED detection voltage V LED-1  is lower than under-current reference value V OVER-LOW , this indicates that driving current I LED-1  is also too low, so that LED chain L 1  is determined to have encountered an under-current event. As long as any one LED chain encounters an under-current event, a pulse will appear on reset signal S RESET  outputted by detection circuit  50 . 
         [0027]    Pulse on reset signal S RESET  may set SR flip-flop  62 , enabling SR flip-flop  62  outputted by short circuit blocking signal S SP-BLOCK . Pulse on reset signal S RESET  also resets timer  57 , causing timer  57  to return to a starting point thereof, to prepare to start counting. 
         [0028]    Protection determination circuit  64  has LED open circuit protection circuit  66  and LED short circuit protection circuit  68 , which respectively provide open circuit protection mechanisms and short circuit protection mechanisms. In an embodiment, when one LED chain is determined to have encountered an under-current event, and detection voltage V OVP  exceeds over-voltage reference value V OVP-REF , open circuit protection mechanism provided by LED open circuit protection circuit  66  will determine that the LED chain encountered an LED open circuit event, and disable a corresponding selection signal, which is one of selection signals S EN-1 -S ED-4 . In some embodiments, when LED detection voltage V LED-n  exceeds a short circuit reference value V SP-REF , short circuit protection mechanism of LED short circuit protection circuit  68  will determine that LED chain L n  encountered an LED open circuit event, and disable selection signal S EN-n . When short circuit protection blocking signal S SP-BLOCK  is disabled, LED short circuit protection circuit  68  operates normally to provide short circuit protection mechanisms. When short circuit protection blocking signal S SP-BLOCK  is enabled, short circuit protection mechanisms provided by LED short circuit protection circuit  68  are blocked, meaning selection signals S EN-1 -S ED-4  are not affected by detection result of LED short circuit protection circuit  68 , or LED short circuit protection circuit  68  completely ignores LED detection voltages V LED-1 -V LED-4 . 
         [0029]    Timer  57  has counter  56  and digital comparator  58 . Counter  56  counts according to a signal inputted by a clock. When timer results D 1 -D 10  of counter  56  reach a certain condition, e.g. timer results D 1 -D 10  are the same as reference values D S-1 -D S-10 , digital comparator  58  generates a pulse, resetting SR flip-flop  62 , disabling short circuit blocking signal S SP-BLOCK , and restoring short circuit protection mechanisms provided by LED short circuit protection circuit  68 . 
         [0030]    AND gate  54  controls clock input to counter  56 . Only when dimming signal S DIM  and short circuit blocking signal S SP-BLOCK  are both enabled is clock signal CLK able to be sent to clock input of counter  56  by AND gate  54 . Dimming signal S DIM  being enabled represents normal LED chains (LED chains that have not been discovered to have encountered fault conditions) need to be lit. Conversely, when dimming signal S DIM  is disabled, all LED chains are unlit. 
         [0031]    Simply stated, if anyone LED chain encounters an under-current event, short circuit protection blocking signal S SP-BLOCK  will be enabled, blocking short circuit protection mechanisms, and resetting counter  56 . Counter  56  counts a paused time that passes while normal LED chains are lit, and short circuit protection blocking signal S SP-BLOCK  is enabled. After this paused time reaches a preset time corresponding to reference value D S-1 -D S-10 , short circuit protection blocking signal S SP-BLOCK  is disabled, restoring short circuit protection mechanisms. 
         [0032]    In the embodiment of  FIG. 3 , when short circuit protection mechanisms are blocked, if another under-current event is encountered, counter  56  will be reset again, and prepare to count again. In some embodiments, when short circuit protection mechanisms are blocked, if another under-current event occurs, counter does not necessarily restart counting. 
         [0033]      FIG. 4  shows some signal waveforms of  FIG. 2  and  FIG. 3  when LED chain L 1  becomes open-circuited. From top to bottom, signals represented include detection voltage V OVP  , driving signal V DRV  , LED detection voltage V LED-2 , LED detection voltage V LED-1 , selection signal S EN-1 , minimum detection voltage V LED-MIN , reset signal S RESET , dimming signal S DIM , timer results D 1 -D 10 , and short circuit protection blocking signal S SP-BLOCK . 
         [0034]    Please refer to  FIG. 2 ,  FIG. 3 , and  FIG. 4 . In  FIG. 4 , prior to time t OP , LED chains L 1 -L 4  are approximately the same, and are all normal, with LED detection voltages V LED-1 V LED-4  and minimum detection voltage V LED-MIN  all roughly equal to target voltage V TAR . 
         [0035]    Assume LED chain L 1  suddenly becomes open-circuited at time t OP , and LED chains L 2 -L 4  are normal. Thus, at time t OP , LED detection voltage V LED-1  and minimum detection voltage V LED-MIN  both suddenly change to 0V. Because LED detection voltage V LED-1  is lower than under-current reference value V OVER-LOW , at time t OP , a pulse appears in reset signal S RESET , and short circuit protection blocking signal S SP-BLOCK  is enabled. Starting from time t OP , short circuit protection mechanisms provided by LED short circuit protection circuit  68  are blocked, and short circuit protection is no longer provided. 
         [0036]    In order to cause minimum detection voltage V LED-MIN  to approach target voltage V TAR  , error amplifier  27  and pulse width adjuster  30  cause output voltage V OUT  and detection voltage V OVP  to rise together. Voltage drop across a normal LED chain is roughly fixed, so LED detection voltage V LED-2  rises with rising output voltage V OUT . However, because LED chain L 1  is open-circuited, LED detection voltage V LED-1  and minimum detection voltage V LED-MIN  stay at 0V, and do not change with varying output voltage V OUT . 
         [0037]    At time t OVP , detection voltage V OVP  exceeds over-voltage reference value V OVP-REF , thus LED open circuit protection circuit  66  determines that LED chain L 1  corresponding to relatively low LED detection voltage V LED-1  (currently 0V) encounters an LED open circuit event, and disables selection signals S EN-1 . Disabled selection signal S EN-1  causes minimum detection voltage V LED-MIN  to break away from control of LED detection voltage V LED-1 , so that minimum detection voltage V LEL-MIN  suddenly jumps up, and starts to follow the minimum of other normal LED detection voltages, as shown. 
         [0038]    After time t OVP , and in order to cause minimum detection voltage V LED-MIN  to approach target voltage V TAR , output voltage V OUT  and detection voltage V OVP  slowly drop with consumed energy. 
         [0039]    Counter  56  is reset at time t OP . Then, in a dimming ON period, i.e. when dimming signal S DIM  is enabled, counter  56  counts with clock signal CLK. In a dimming OFF period, i.e. when dimming signal S DIM  is disabled, counter  56  cannot receive clock signal CLK, and pauses counting. At time t RCV , timer results D 1 -D 10  of counter  56  equal reference values D S-1 -D S-10 , and short circuit protection blocking signal S SP-BLOCK  is disabled, restoring short circuit protection mechanisms provided by LED short circuit protection circuit  68 . 
         [0040]    It can be seen from  FIG. 4  that between time t OP  and time t RCV , short circuit protection blocking signal S SP-BLOCK  is enabled, so that short circuit protection of all LED chains L 1 -L N  is blocked and has no effect. It can be understood from  FIG. 4  that time t OP  and time t RCV , i.e. paused time in which short circuit protection mechanisms are blocked, is approximately equal to the sum of preset time corresponding to reference value D S-1 -D S-10  and dimming OFF period. Thus, as long as preset time is designed to be sufficiently long, even though LED detection voltage V LED-2  may be relatively high due to open circuiting of LED chain L 1 , LED chain L 2  will not be erroneously determined to have encountered a short circuit event. 
         [0041]      FIG. 5  shows another protection circuit  28   b , which when used in  FIG. 2 , may prevent LED open circuit events from erroneously triggering short circuit protection. Similar or the same features of protection circuit  28   b  of  FIG. 5  and protection circuit  28   a  of FIG.  3  can be understood according to the above description of  FIG. 3 , and are not repeated. 
         [0042]    Different from protection circuit  28   a  of  FIG. 3 , protection circuit  28   b  of  FIG. 5  additionally includes SR flip-flop  82 , comparator  80 , and AND gate  54   a.  Stated simply, after detection voltage V OVP  exceeds over-voltage reference value V OVP-REF , comparator  80  sets SR flip-flop  82 , so that clock signal CLK can reach clock input of counter  56 , and counter  56  can begin counting. While comparator  58  disables short circuit protection blocking signal S SP-BLOCK , SR flip-flop  82  is also reset, and outputs logic “0.” 
         [0043]      FIG. 6  shows  FIG. 2  and  FIG. 5  some signal waveforms when LED chain L 1  becomes open-circuited. From top to bottom, signals represented include detection voltage V OVP , driving signal V DRV , LED detection voltage V LED-2 , LED detection voltage V LED-1 , selection signal S EN-1 , minimum detection voltage V LED-MIN , reset signal S RESET , dimming signal S DIM , timer results D 1 -D 10 , and short circuit protection blocking signal S SP-BLOCK . 
         [0044]    Please refer to  FIG. 5  and  FIG. 6 . Even though timer results D 1 -D 10  become 0 when counter  56  is reset at time t OP , output of SR flip-flop  82  becomes logic 0, and counter  56  does not receive clock signal CLK, so that counter  56  does not start counting. Clock signal CLK must wait until after detection voltage V OVP  exceeds over-voltage reference value V OVP-REF  at time t OVP  before being able to reach clock input of counter  56 , so that counter  56  begins to count. Thus, it can be seen from  FIG. 6  that time t OP  and time t RCV , i.e. paused time during which short circuit protection mechanisms are blocked, is roughly equal to the sum of time from time t OP  to time t OVP , preset time corresponding to reference value D S-1 -D S-10 , and dimming OFF period. If reference values D S-1 -D S-10  are all the same, compared to paused time in  FIG. 4 , paused time in  FIG. 6  is longer due to additional time from time t OP  to time t OVP . 
         [0045]      FIG. 7  shows another detection circuit  50   a,  which replaces detection circuit  50  of  FIG. 3  and  FIG. 5  in some embodiments. Minimum detection voltage V LED-MIN  roughly corresponds to minimum LED detection voltage corresponding to normal LED chains. So, minimum detection voltage V LED-MIN  dropping below under-current reference value V OVER-LOW  represents one lit LED chain already encountered an under-current event, so that comparator  90  causes rising-edge-triggered pulse generator  92  to emit a pulse. 
         [0046]      FIG. 8  shows an analog timer  57   a,  which replaces digital timer  57  of  FIG. 3  and  FIG. 5  in some embodiments. A pulse on clock input CLK-IN can cause stepwise increase of voltage drop across capacitor  96 . When voltage drop across capacitor  96  reaches time reference voltage V TIME-REF , comparator  94  causes rising-edge-triggered pulse generator  98  to emit a pulse. Enable signal on reset node R may cause voltage drop across capacitor  96  to become 0V, causing timer  57   a  to count again with pulses on clock input CLK-IN. 
         [0047]    In the above embodiments, protection circuit has the following features: 
         [0048]    Short circuit protection mechanisms are blocked after under-current events occur. 
         [0049]    Short circuit protection mechanisms are blocked for paused time at least as long as a preset time corresponding to reference values D S-1 -D S-10 . 
         [0050]    After each under-current event occurs, counter starts counting again. 
         [0051]    In dimming OFF period, counter pauses counting. 
         [0052]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.