Abstract:
Integrated circuits, control methods, and related lighting systems are provided. One integrated circuit controls the currents flowing through light-emitting-diode chains, each having several light emitting diodes forward-connected between a main cathode and a main anode while all the main anodes are connected to a power node. The integrated circuit has a short detection node, a constant current source, a voltage clamping circuit, and a short-circuit comparator. The short detection node detects the highest cathode voltage of the main cathodes. The constant current source provides a constant current to the short detection node. While the light-emitting-diode chains are unlit, the voltage clamping circuit clamps the short detection node at a predetermined voltage. When the voltage of the short detection node exceeds a threshold voltage, the short-circuit comparator asserts a fault signal, indicating a short circuit of a light emitting diode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to integrated circuits, control methods, and related light-emitting diode (LED) lighting systems. 
     2. Description of the Prior Art 
     Light-emitting diodes (LEDs) have a very good electro-optical conversion rate, which is higher than fluorescent lamps, cold-cathode fluorescent lamps, and light bulbs. Thus, the current trend is to replace these types of lamps with LEDs. For example, LEDs have already gradually replaced CCFLs as a backlight source in liquid crystal display (LCD) panels. 
     When using LEDs as a backlight source for an LCD panel, due to the LCD panel&#39;s large area, a very large number of LEDs must be used, and these LEDs are normally arranged in chains, each chain driven by a controllable current source. Current flowing through each LED chain is controlled to be the same, so that brightness of all LEDs is approximately the same. If light emitted by each LED is given appropriate propagation, brightness of the LCD panel will be reasonably even. 
     However, if even one LED out of all LEDs in the LCD panel is short-circuited or open-circuited, the LCD panel brightness will be uneven. Thus, a good LED chain driving circuit should have appropriate sensing circuitry to sense whether any LEDs are open- or short-circuited, and take appropriate preventative measures. 
     SUMMARY OF THE INVENTION 
     According to an embodiment, an integrated circuit is for controlling current of a plurality of light-emitting diode (LED) chains. Each LED chain has a plurality of LEDs forward-connected between a main anode and a main cathode. Each main anode is coupled to a power node. The integrated circuit comprises a short circuit detection node for detecting maximum cathode voltage of the main cathodes, a constant current source for providing a constant current to the short circuit detection node, a voltage clamping circuit for clamping the short circuit detection node at a predetermined voltage when the LED chains are unlit, and a short circuit comparator for comparing a sense voltage of the short circuit detection node and a threshold voltage, thereby asserting a short-circuit signal when the sense voltage exceeds the threshold voltage. 
     According to an embodiment, a control method is for using in an integrated circuit. The integrated circuit controls current of a plurality of light-emitting diode (LED) chains. Each LED chain has a plurality of LEDs forward-connected between a main anode and a main cathode. Each main anode is coupled to a power node. The integrated circuit has a short circuit detection node for detecting maximum cathode voltage of the main cathodes. The control method comprises lighting the LED chains, providing a constant current to the short circuit detection node of the integrated circuit when the LED chains are lit, comparing a sense voltage with a threshold voltage, asserting a short-circuit signal when the sense voltage exceeds the threshold voltage, turning off the LED chains, and clamping the sense voltage of the short circuit detection node at a predetermined voltage when the LED chains are unlit. 
     According to an embodiment, a lighting system comprises a power supply for providing a power node and a ground node, a plurality of light-emitting diode (LED) chains, and an integrated circuit. Each LED chain has a plurality of LEDs forward-connected between a main anode and a main cathode, each main anode coupled to the power node. The integrated circuit comprises a feedback node for detecting minimum cathode voltage of the main cathodes, and a short circuit detection node for detecting maximum cathode voltage of the main cathodes. The short circuit detection node has no signal paths to the ground node other than signal paths through the main cathodes or through internal signal paths of the integrated circuit. 
     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 
         FIG. 1  and  FIG. 2  are diagrams of LED lighting systems. 
         FIG. 3  is a partial circuit diagram of integrated circuit of  FIG. 2 . 
         FIG. 4  illustrates signal waveforms of LED lighting systems of  FIG. 1  and  FIG. 2  during normal operation (no LEDs short-circuited) and during abnormal operation (one or more LEDs short-circuited). 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram of an LED lighting system  10 , which may act as a backlight source for an LCD panel. 
     Power supply  12  provides a power node OUT and a ground. Voltage of power node OUT may be as high as 100V. LEDs acting as a light source are grouped into N LED chains CLED 1  . . . CLEDN. As shown, each LED chain has a plurality of LEDs electrically connected in a forward-connected series. From top to bottom, the anode of the first LED of each LED chain is defined as a main anode, and the cathode of the last LED of each LED chain is defined as a main cathode. In the following, all LED chains have the same number of LEDs for purposes of illustration, but LED chains may also have different numbers of LEDs. 
     All main anodes are tied to power node OUT. Main cathodes A 1  . . . AN are individually electrically connected to power transistors M 1  . . . MN. Integrated circuit IC 1  individually controls corresponding power transistors M 1  . . . MN from gates GATE 1  . . . GATEN to control current flowing through LED chains CLED 1  . . . CLEDN. For example, if integrated circuit IC 1  has a dimming signal S DIMMING , assertion of dimming signal S DIMMING  indicates LEDs are emitting light, and current flowing through each LED chain is 100 mA. Disassertion of dimming signal S DIMMING  indicates LEDs are not emitting light, and current flowing through each LED chain is 0 mA. 
     Integrated circuit IC 1  uses feedback node FB to detect minimum cathode voltage V A-MIN  of main cathodes A 1  . . . AN. From circuit connections between feedback node FB and main cathodes A 1  . . . AN, it can be seen that voltage V FB  of feedback node FB is described by the following equation:
 
 V   FB   =V   A-MIN   +V   TH-DIODE   (1)
 
where V TH-DIODE  is one diode forward voltage drop. According to voltage V FB , integrated circuit IC 1  may provide compensation signal to power supply  12  through compensation node COMP to adjust voltage at power node OUT, so that minimum cathode voltage V A-MIN  can be held at approximately target value V TAR , e.g. 1V.
 
     Integrated circuit IC 1  uses LED short-circuit protection node LEDSP to detect maximum cathode voltage V A-MAX  of main cathodes A 1  . . . AN. Cathodes of N LEDs are coupled to common node MAX, and anodes thereof are individually coupled to main cathodes A . . . AN. Zener diode ZD 1  and resistor  14  are electrically connected in series from LED short-circuit protection node LEDSP and common node MAX. It can be seen from interconnections between LED short-circuit protection node LEDSP and main cathodes A 1  . . . AN that sense voltage V LEDSP  at LED short-circuit protection node LEDSP is roughly described by the following equations:
 
 V   MAX   =V   A-MAX   −V   TH-DIODE ,  (2)
 
 V   LEDSP =( V   MAX   −V   BD-ZD1 )* R   16 /( R   14   +R   16 )  (3)
 
where V MAX  is voltage at common node MAX, V BD-ZD1  is breakdown voltage of Zener diode ZD 1 , and R 14  and R 16  are resistance values of resistors  14  and  16 .
 
     If operating voltage V ON-LED  of each LED is approximately the same, and number of LEDs in each LED chain is the same, then it can be seen that minimum cathode voltage V A-MIN  and maximum cathode voltage V A-MAX  are both approximately equal to target value V TAR  when all LEDs emit light normally. Equations (2) and (3) describe sense voltage V LEDSP  of LED short-circuit protection node LEDSP corresponding to no LEDs being short-circuited. 
     If one LED chain has k short-circuited LEDs, maximum cathode voltage V A-MAX  will be higher than minimum cathode voltage V A-MIN  by approximately k*V ON-LED . This difference is reflected in a change in sense voltage V LEDSP . For example, if integrated circuit IC 1  discovers that sense voltage V LEDSP  is higher by one threshold voltage V TH-SH  when LED chains CLED 1  . . . CLEDN are lit, it can be determined that one or more LEDs have short-circuited, and short-circuit signal S SHT  can be asserted to take corresponding short-circuit protection measures. For example, protection measures may include forced shutdown of power transistors M 1  . . . MN to stop LED chains CLED 1  . . . CLEDN from emitting light. 
     Zener diode ZD 2  limits maximum value of sense voltage V LEDSP . Zener diode ZD 2  can prevent maximum cathode voltage V A-MAX  from approaching power node OUT voltage (up to 100V) when LED chains are not lit, which would damage integrated circuit IC 1  if sense voltage V LEDSP  goes too high. 
       FIG. 2  is a diagram of an LED lighting system  20 , which can also act as an LCD panel backlight source.  FIG. 2  is different from  FIG. 1  in that circuit design of integrated circuit IC 2  is different from integrated circuit IC 1 , and  FIG. 2  does not require Zener diode ZD 2 , resistor  16 , and capacitor  18  of  FIG. 1 . As shown in  FIG. 2 , no additional external discrete components need be electrically connected between LED short-circuit protection node LEDSP of integrated circuit IC 2  and ground node of power supply  12  beyond signal path through main cathodes A 1  . . . AN and power transistors M 1  . . . MN, and internal signal paths of integrated circuit IC 2 . Or, no additional signal paths connect LED short-circuit protection node LEDSP to power supply  12  ground node other than signal path through main cathodes A 1  . . . AN, and internal signal paths of integrated circuit IC 2 . It can be seen by comparing  FIG. 1  and  FIG. 2  that LED lighting system  20  uses a lower number of external discrete components, which makes LED lighting system  20  more cost competitive than LED lighting system  10 . 
       FIG. 3  is a partial circuit diagram of integrated circuit IC 2  of  FIG. 2 . When dimming signal S DIMMING  is asserted, driving circuits D 1  . . . DN individually drive power transistors M 1  . . . MN to cause LED chains CLED 1  . . . CLEDN to emit light. When driving signal S DIMMING  is disasserted, driving circuits D 1  . . . DN are disabled to shut off power transistors M 1  . . . MN, and cause LED chains CLED 1  . . . CLEDN not to emit light. 
     At about the time dimming signal S DIMMING  is disasserted, switch  32  acting as a voltage clamping circuit becomes a short circuit to keep LED short-circuit protection node LEDSP fixed at 0V. Current flowing through switch  32  can be externally limited appropriately through Zener diode ZD 1  and resistor  14 . In this way, even if maximum cathode voltage V A-MAX  equals power node OUT voltage, integrated circuit IC 2  will not be damaged by high voltage. Switch  32  does not necessarily need to fix LED short-circuit protection node LEDSP at 0V, but may also fix LED short-circuit protection node LEDSP at another voltage, e.g. operation voltage VCC of integrated circuit IC 2 . 
     At about the time dimming signal S DIMMING  is asserted, switch  32  becomes an open circuit, and constant current source  30  draws a constant current I SET  from LED short-circuit protection node LEDSP. At this time, sense voltage V LEDSP  can be approximated by the following equation:
 
 V   LEDSP   =V   MAX   −V   BD-ZD1   −I   SET   *R   14 .  (4)
 
     From equations (4) and (2), it can be seen that sense voltage V LEDSP  can correspond to maximum cathode voltage V A-MAX  of main cathodes A 1  . . . AN. Comparator CM compares sense voltage V LEDSP  and threshold voltage V TH-SH . When sense voltage V LEDSP  is higher than threshold voltage V TH-SH , it can be determined that at least one LED has short-circuited, so short-circuit signal S SHT  is asserted to engage corresponding short-circuit protection measures. 
     Clamping circuit  26  is used for limiting maximum value of sense voltage V LEDSP  to prevent any problems that may occur if sense voltage V LEDSP  goes too high due to too many LEDs shorting. In  FIG. 3 , clamping circuit  26  is formed of operational amplifier OP and NMOS transistor MX as an example, and can limit sense voltage V LEDSP  to be lower than clamp voltage V TH-CLP . 
     Delay circuit  28  provides delay. For example, delay circuit  28  provides delay period T B1  to rising edge of dimming signal S DIMMING , and delay period T B2  to falling edge of dimming signal S DIMMING . It can be seen from the circuit of  FIG. 3  that after rising edge of dimming signal S DIMMING , switch  32  can only be opened after delay period T B1  Thus, within delay period T B1  after LED chain starts emitting light, because sense voltage V LEDSP  is still fixed at 0V by short-circuited switch  32 , which is lower than threshold voltage V TH-SH , short-circuit signal S SHT  is not asserted. 
       FIG. 4  illustrates signal waveforms of LED lighting systems  10 ,  20  of  FIG. 1  and  FIG. 2  during normal operation (no LEDs short-circuited) and during abnormal operation (one or more LEDs short-circuited). From top to bottom, signal waveforms of  FIG. 4  represent dimming signal S DIMMING , current I CLEDn  flowing through LED chain CLEDn, voltage V MAX  of common terminal MAX, sense voltage V LEDSP  of  FIG. 1 , sense voltage V LEDSP  of  FIG. 2 , and short-circuit signal S SHT , respectively. The left half of  FIG. 4  shows signal waveforms under normal operation with no LEDs short-circuited, and the right half of  FIG. 4  shows signal waveforms when one or more LEDs is short-circuited. 
     Please refer to  FIG. 4  and  FIG. 1 . When dimming signal S DIMMING  is disasserted, current I CLEDn  is approximately 0 A, and voltage V MAX  is very high, so sense voltage V LEDSP  shown by waveform  36  is also very high. At this time, although sense voltage V LEDSP  is higher than threshold voltage V TH-SH , integrated circuit IC 1  forces disassertion of short-circuit signal S SHT . When dimming signal S DIMMING  is asserted, current I CLEDn  is controlled approximately to a predetermined value. If no LEDs are short-circuited, as shown in the left half of  FIG. 4 , voltage V MAX  falls approximately to an abnormally low value, so that sense voltage V LEDSP  is lower than threshold voltage V TH-SH , so short-circuit signal S SHT  remains disasserted. If one or more LEDs is short-circuited, as shown in the right half of  FIG. 4 , voltage V MAX  will drop to a relatively high value, so that sense voltage V LEDSP  exceeds threshold voltage V TH-SH , so short-circuit signal S SHT  is asserted. 
     Please refer to  FIG. 4 ,  FIG. 2 , and  FIG. 3 . When dimming signal S DIMMING  is disasserted, voltage V MAX  is very high, but sense voltage V LEDSP  shown by waveform  38  is fixed to 0V due to switch  32  being open-circuited, so short-circuit signal S SHT  is disasserted. After dimming signal S DIMMING  is asserted for delay period T B1 , current I CLEDn  is approximately controlled to a predetermined value. If no LEDs are short-circuited, as shown in the left half of  FIG. 4 , sense voltage V LEDSP  will rise to a relatively low voltage value, which is lower than threshold voltage V TH-SH , so short-circuit signal S SHT  remains disasserted. If one or more LEDs is short-circuited, as shown in the right half of  FIG. 4 , sense voltage V LEDSP  will rise to a relatively high voltage value, so that sense voltage V LEDSP  exceeds threshold voltage V TH-SH , so short-circuit signal S SHT  will be asserted. 
       FIG. 1  and  FIG. 2  both use Zener diode ZD 1 . However, in other embodiments, Zener diode ZD 1  may be omitted, such that resistor  14  is directly coupled to common node MAX. 
     Power supply  12  may use any power conversion architecture, including, but not limited to, flyback, boost, and buck architectures. 
     Power transistors M 1  . . . MN shown in  FIG. 1  and  FIG. 2  may be MOS transistors or BJT transistors. In some embodiments, integrated circuit IC 1  or IC 2  and power transistors M 1  . . . MN are integrated into a single chip or a single IC package. 
     LED lighting systems  10 ,  20  can both detect whether or not any LED is short-circuited. Compared to LED lighting system  10 , LED lighting system  20  is more cost competitive. 
     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.