Abstract:
The present invention discloses a backlight control circuit capable of distinguishing an under current condition, comprising: at least one light emission device path having a voltage node; at least one current source for controlling the current amount on the light emission device path; and at least one under current detection circuit for generating a first control signal according to the voltage at the voltage node, wherein when the first control signal changes its state, the under current detection circuit generates a second control signal to change the voltage on the voltage node if the light emission device path is normally connected.

Description:
RELATED APPLICATIONS 
       [0001]    The present invention is a continuation-in-part application of U.S. Ser. No. 11/906,477, filed on Oct. 2, 2007. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a backlight control circuit, more particularly, to a backlight control circuit capable of distinguishing under current condition even when the brightness of the light emitting devices is very low. 
       DESCRIPTION OF RELATED ART 
       [0003]    In a liquid crystal display (LCD), a backlight control circuit is used which controls light emitting diodes (LEDs) to illuminate from the back side of an LCD screen, so that a user can observe an image from the front side of the LCD screen. 
         [0004]    In early days, LED backlight is used only in a small size screen, which does not require high backlight brightness. Therefore, the LEDs can be connected all in series or all in parallel.  FIG. 1  shows a conventional backlight control circuit with LEDs all connected in parallel. As shown in the figure, in a backlight control circuit  20 , the currents passing through LEDs L 1 -LN are respectively controlled by the current sources CS 1 -CSN. The backlight control circuit  20  comprises a lowest voltage selection circuit  21  which chooses a lowest voltage value among all voltages at cathode ends of the LEDs L 1 -LN, and the error amplifier circuit  13  compares the lowest voltage value with a reference voltage Vref to generate a signal controlling the voltage supply circuit  11 . Thus, the output voltage Vout is under control so that all current source circuits are provided with sufficient operating voltage for normal operation, and all LEDs can illuminate normally thereby. 
         [0005]    The backlight control circuit  20  can further comprise an over voltage protection circuit to prevent the output voltage Vout from unlimitedly increasing.  FIG. 2  shows a typical structure of an over voltage protection circuit  12 , wherein the output voltage Vout is monitored by comparing the voltage at the node Vsense 2  with a reference voltage Vovp. The result of comparison determines a signal for controlling the voltage supply circuit  11 . 
         [0006]    Because the backlight control circuit  20  is an integrated circuit, the number of its pins (shown by hollow squares in  FIG. 1 ) is fixed. When the number of pins is larger than the number of LED strings to be connected with, prior art suggests connecting the excess pins to the output voltage Vout. An excess pin can not be left floating or grounded; otherwise the lowest voltage selection circuit  21  will select the input corresponding to it and keep increasing the output voltage Vout. By connecting the excess pin to the output voltage Vout, it can be sure that the lowest voltage selection circuit  21  will not select the input corresponding to the excess pin. 
         [0007]    As the size of an LCD screen increases, the requirement for backlight brightness increases, and the number of LEDs correspondingly increases. Under such circumstance, it is impossible to connect all the LEDs in parallel; they have to be connected partially in series and partially in parallel, as shown in  FIG. 3 . In this case, the required output voltage Vout is much higher than that in  FIG. 1 ; for example, the output voltage Vout in  FIG. 1  may be around 5V, while the output voltage Vout in  FIG. 3  may be as high as 60V. Accordingly, if any pin becomes an excess pin that has to be connected to the output voltage out, the device inside the integrated circuit in connection with the pin has to be a costly high voltage device. In addition, the electro-static damage issue will become worse, and the internal circuit will unnecessarily consume huge power and generate heat. Moreover, in either the prior art of  FIG. 1  or  FIG. 3 , if any LED functions abnormally such as causing a corresponding path to be open, or a corresponding pin is caused to short to ground, the lowest voltage selection circuit  21  will select the input corresponding to it, and the error amplifier circuit  13  will keep asking the voltage supply circuit  11  to increase the output voltage Vout; the voltage supply circuit  11  can not adjust its output according to normal LEDs. In the case where an over voltage protection circuit is provided, the output voltage Vout will be kept at its upper limit, consuming huge power unnecessarily, while in the case where an over voltage protection circuit is not provided, the integrated circuit may be damaged due to continuously providing high power, and the LEDs may be burned out. To the above drawbacks and concerns, none of the prior art provides any solution. 
         [0008]    In view of the foregoing, the U.S. Ser. No. 11/906,477, filed on Oct. 2, 2007 and assigned to the same assignee as that of the present invention, has proposed a solution wherein excess pins or corresponding LED paths can be shorted to ground or left floating. The application Ser. No. 11/906,477 discloses a circuit structure as shown in  FIG. 4 , wherein a backlight control circuit  30  comprises, in addition to a voltage supply circuit  11 , an error amplifier circuit  13 , and current sources CS 1 -CSN (illustrated by functional blocks), under current detection (UCD) circuits  31 - 3 N. An example of the UCD circuit is shown in  FIG. 5  (on the same page of  FIG. 2 ). The UCD circuits  31 - 3 N detect whether an “under current condition”, i.e., an “abnormally low current” or “no current” condition occurs in a corresponding LED path  101 - 10 N. (An LED path  101 - 10 N is a path from the node of the output voltage Vout to ground.) When there is no “low current” or “no current” condition, the UCD circuits  31 - 3 N will forward the voltage signals on the LED paths  101 - 10 N to the corresponding voltage comparison paths  111 - 11 N, so that the lowest voltage selection circuit  21  can receive these signals. When an under current condition occurs in one or more LED paths  101 - 10 N, the corresponding UCD circuits  31 - 3 N exclude corresponding voltage comparison paths  111 - 11 N paths from valid inputs of the lowest voltage selection circuit  21 , that is, the lowest voltage selection circuit  21  will not accept any voltage signal from such voltage comparison paths  111 - 11 N. 
         [0009]    Although the solution provided by U.S. Ser. No. 11/906,477 has properly solved the problems in prior art, certain product applications requires adjusting the backlight brightness of an LCD. In this case, when the brightness of LEDs is lower than a certain limit, i.e., when the current amount on a corresponding LED path is below a certain threshold, an UCD circuit  31 - 3 N may fail to distinguish between the under current condition and the normally low current condition. 
       SUMMARY 
       [0010]    In view of the foregoing, it is therefore an objective of the present invention to provide a backlight control circuit capable of distinguishing under current condition even when the brightness of the light emitting devices is very low, to solve the problems in prior art. The backlight control circuit of the present invention is compatible with dimming control for the light emitting devices. 
         [0011]    It is another objective of the present invention to provide a light emitting device path status detection method. 
         [0012]    It is a further objective of the present invention to provide an under current detection circuit. 
         [0013]    In accordance with the foregoing and other objectives, and from one aspect of the present invention, a backlight control circuit comprises: at least one light emission device path having a voltage node; at least one current source for controlling the current amount on the light emission device path; and at least one under current detection circuit for generating a first control signal according to the voltage at the voltage node, wherein when the first control signal changes its state, the under current detection circuit generates a second control signal to change the voltage on the voltage node if the light emission device path is normally connected. 
         [0014]    In another aspect of the present invention, a light emitting device path status detection method comprises: A light emitting device path status detection method, comprising: providing at least one light emission device path having a voltage node; generating a first control signal according to the voltage on the voltage node; and when the first control signal changes its state, changing the voltage at the voltage node if the light emission device path is normally connected. 
         [0015]    In yet another aspect of the present invention, an under current detection circuit comprising: a comparator for generating a control signal by comparing a node voltage with a reference voltage; a pulse generator for generating a pulse according to the control signal; and a node voltage adjustment circuit for adjusting the node voltage according to the pulse. 
         [0016]    In this invention, preferably, the node voltage may be changed by dropping the current on the light emission device path, so that the node voltage bounces up. 
         [0017]    These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description of preferred embodiments and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a schematic circuit diagram showing a prior art circuit including LEDs which are all connected in parallel and a backlight control circuit thereof. 
           [0019]      FIG. 2  is a schematic circuit diagram showing a conventional over voltage protection circuit. 
           [0020]      FIG. 3  is a schematic circuit diagram showing a prior art circuit including LEDs which are connected partially in series and partially in parallel, and a backlight control circuit thereof. 
           [0021]      FIG. 4  is a schematic circuit diagram showing a backlight control circuit including UCD circuits, which has been assigned to the same assignee as that of the present invention. 
           [0022]      FIG. 5  is a schematic circuit diagram showing an example of the UCD circuit of  FIG. 4 . 
           [0023]      FIG. 6  is a schematic circuit diagram showing a backlight control circuit according to an embodiment of the present invention. 
           [0024]      FIG. 7  is a schematic circuit diagram showing a backlight control circuit according to another embodiment of the present invention. 
           [0025]      FIG. 8  is a schematic circuit diagram showing yet another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]      FIG. 6  is a schematic circuit diagram showing a backlight control circuit according to an embodiment of the present invention. For simplicity, only one LED path  101  is illustrated in the figure; however, in a real case, the number of the LED paths may be more than one (denoted by N, N being a positive integer). In the backlight control circuit  40  of this embodiment, the reference voltage Vref of the error amplifier circuit  13  is controlled by a dimming circuit  50 , for adjusting the brightness of the LEDs. The backlight control circuit  40  also includes UCD circuit  41 - 4 N (but only UCD circuit  41  is shown in this figure). 
         [0027]    As shown in the figure, the UCD circuit  41  includes a comparator  411 , a latch  412 , a pulse generator  413 , and a voltage drop circuit  414 . These devices operate as below. The comparator  411  compares the voltage at the node VD 1  with the reference voltage Vuc, to determine whether the switch SW 1  should be closed or opened. During normal operation, the voltage at the node VD 1  is higher than the reference voltage Vuc, so the output of the comparator  411  is at low level. The comparator  411  may be a general comparator or a hysteresis comparator (as shown) for better signal judgment. The output of the comparator  411 , which is preferably stored in the latch  412 , controls the switch SW 1 , to close it in normal operation. Of course, depending on how the switch SW 1  is designed, the output of the comparator  411  may have to be inversed. 
         [0028]    On the other hand, if an LED path is open due to malfunction, not in use, or other reasons, the voltage at the node VD 1  would be lower than the reference voltage Vuc, and the output of the comparator  411  becomes high, to open the switch SW 1 . 
         [0029]    When the output of the comparator  411  maintains at either the low level or the high level, it does not affect the pulse generator  413 . However, when the output of the comparator  411  changes state, either from low to high or from high to low, the state switching will cause the pulse generator  413  to generate a pulse. The output level switching of the comparator  411  means that the interrelationship between the voltage at the node VD 1  and the reference voltage Vuc changes. This may happen in several occasions: in the initialization stage; due to state change in the connection of the corresponding LED path (because of malfunction or manually changing the connection state); in a transient state due to manually adjusting the LED brightness too low; or simply by a transient misoperation of the circuit. If the reference voltage Vref is set at a low value, the voltage at the node VD 1  is very close to the reference voltage Vuc, and therefore a transient signal in any part of the circuit may very possibly cause the output of the comparator  411  to change state. No prior art has proposed any solution to this issue; here the present invention provides the solution, which is to verify the accuracy of the state change by the circuit shown in the figure. According to the present invention, in one embodiment, verification can be made every time when a state change occurs in any LED path. 
         [0030]    As an example, the pulse generator  413  may be embodied as shown in the figure. When the output of the comparator  411  changes state, because of the operation of a delay circuit  4131 , an XOR gate  4132  generates a positive pulse. The positive pulse temporarily turns ON the switch Q 1  in a voltage drop circuit  414 , forming a parallel-connection circuit of resistors R 1  and R 2  to decrease the total resistance. Hence, the voltage at the node VB drops (temporarily). In normal operation, the decrease of the voltage at the node VB causes the current Id 1  on the path  101  to decrease. Correspondingly, the voltage drop of the LEDs L 11 -L 1 N decreases; however, the output voltage Vout does not change at this instant period, so the voltage at the node VD 1  (equal to the output voltage Vout minus the total voltage drop of the LEDs L 11 -L 1 N) will bounce up at this instant period. On the contrary, if the LED path  101  is open due to malfunction, not in use, or other reasons, the voltage at the node VD 1  will keep unchanged, i.e., still lower than the reference voltage Vuc. Thus, by the pulse from the pulse generator  413 , the voltage at the node VD 1  will have two distinctly different states in normal and abnormal operations, and more distinguishable. 
         [0031]    After the pulse ends, the output of the comparator  411  will be kept in the latch  412  with the correct level, to ensure that the switch SW 1  receives the correct signal. In one embodiment, the output of the pulse generator  413  is sent to the latch  412  as its clock signal so that the latch  412  updates its data according to the clock and stores the final data at the end of the clock. In this way, the latch  412  stores the correct data for controlling the switch SW 1 . 
         [0032]    Note that the reference voltages Vref and Vuc are illustrated to be connected in series, and a resistor RA is provided therebetween. This is to imply the functional relationship Vref&gt;Vuc between the reference voltages Vref and Vuc. However, it does not mean that these two reference voltages have to be connected in the way shown in the figure. For example, the resistor RA may be replaced by another voltage source, or the reference voltages Vref and Vuc may be set individually. 
         [0033]    Similarly, the reference voltages Vref and VB are illustrated to be connected in series, and resistors RA and RB are provided therebetween. This is to imply the functional relationship between the reference voltages Vref and VB, so that the dimming control (adjusting the brightness of the LEDs by adjusting the current on the LED path) may be achieved by adjusting the reference voltage Vref. However, it does not mean that these two reference voltages have to be connected in the way shown in the figure. The resistors RA and RB may be replaced by other voltage sources, or the reference voltages Vref and VB may be set individually. Moreover, the relationship Vuc&gt;VB shown in the figure is not always true; in fact, the reference voltages Vuc and VB are independent from each other. 
         [0034]    In the case where the latch  412  is employed, its content may be uncertain during power ON or power recovery stage. To be prudential, in one embodiment, the latch  412  may optionally be reset by a power ON reset signal POR or a power recovery reset signal PRR. 
         [0035]    Referring to  FIG. 7 , in a more prudential embodiment, it can be arranged so that when any node VD 1 -VDN in any of the LED paths changes its relative position with respect to the reference voltage Vuc (i.e., when any one of the pulse generators generates a pulse), the conditions of all of the LED paths are verified. As shown in the figure, the outputs of the pulse generator  413  and the other pulse generators  423 - 4 N 3  (the UCD circuits  42 - 4 N are not shown in the figure; the pulse generator  423  is the pulse generator in the UCD circuit  42 , the pulse generator  4 N 3  is the pulse generator in the UCD circuit  4 N, and so on) are subject to logic operation in a logic circuit  60 , whose output controls the switch Q 1  in the voltage drop circuit  414 . In this embodiment, the logic circuit  60  is an OR gate, meaning that as long as one of the pulse generators  413 - 4 N 3  generates a pulse, the voltage drop circuit  414  will be enabled and the current source CS 1  will decrease the current on the path  101 , so that the comparator  411  is more capable of distinguishing the difference between its two inputs. The output of the logic circuit  60  is not only provided to the voltage drop circuit  414  but also provided to the voltage drop circuits and latches in the other UCD circuits  42 - 4 N (not shown). 
         [0036]    The embodiments of  FIGS. 6 and 7  are only two of the many possible arrangements. Those skilled in this art can think of many variations within the spirit of the present invention. For example, the voltage drop circuit  414  in  FIGS. 6 and 7  may be replaced by the voltage drop circuit  415  in  FIG. 8 , in which the transistor switch Q 2  is ON during normal operation, but when the pulse generator  413  generates a pulse, the transistor switch Q 2  turns OFF in the short period of the pulse, so that the resistance of the parallel-connection circuit composed of the resistors R and R 2  increases. Thus, the current Id 1  drops, and the voltage at the node VD 1  bounces up (in normal condition), to provide two distinctly different states between normal and abnormal conditions. 
         [0037]    By the arrangement of the present invention, the circuit can accurately identify whether each path is operating normally or is inoperative. Therefore, the over voltage protection circuit  12  is not absolutely required; however, it can still be provided for safety. 
         [0038]    Although the present invention has been described in considerable detail with reference to certain preferred embodiments, these embodiments are for illustrative purpose and not for limiting the scope of the present invention. Other variations and modifications are possible. For example, in all of the embodiments, one can insert a circuit which does not affect the primary function, such as a delay circuit, between any two devices which are shown to be directly connected. The input level and output level of the digital devices may be arranged in a way different from that shown in the figures; as an example, the XOR gate  4132  in  FIG. 7  may be replaced by an XNOR gate, and the logic circuit  60  correspondingly be replaced by a NAND gate. The backlight control circuit is shown to be one integrated circuit, but it can be divided into several integrated circuits, or integrated with other circuit functions. The present invention is not only applicable to series-parallel connection circuits, but also to all-in-parallel and all-in-series circuits. The light emitting devices, although shown as LEDs in the above, are not limited thereto but can be other light emitting devices such as organic light emitting diodes. And the word “backlight” in the term “backlight control circuit” is not to be taken in a narrow sense that the circuit has to control the backlight of a screen; the present invention can be applied to “active light emission display”, or “LED illuminator”, or other apparatuses that employ light emitting devices. Therefore, all modifications and variations based on the spirit of the present invention should be interpreted to fall within the scope of the following claims and their equivalents.