Abstract:
An exemplary backlight control circuit includes a load ( 250 ), an inverter circuit ( 230 ), a pulse width modulation integrated circuit (PWM IC) ( 210 ), a protecting circuit ( 270 ), and a feedback circuit ( 240 ). The load ( 250 ) includes two backlight lamps ( 251, 252 ) with first terminals ( 241 ). The PWM IC with a protecting output ( 215 ) is connected to the load via the inverter circuit. The protecting circuit haves a reference voltage. The first feedback circuit is capable of outputting a voltage to the protecting circuit corresponding to the voltage detected from the first terminals. The protecting circuit is configured to control the PWM IC to stop outputting a backlight adjusting signal to the inverter circuit such that the inverter circuit stops driving the load when the output voltage is higher than the reference voltage of the protecting circuit.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a backlight control circuit with a protecting circuit, the backlight control circuit typically being used in a liquid crystal display (LCD). 
       GENERAL BACKGROUND 
       [0002]    LCDs are widely used in various modem information products, such as notebooks, personal digital assistants (PDAs), video cameras and the like. Because liquid crystal in an LCD does not emit any light itself, a backlight system is usually needed to enable the LCD to display images. 
         [0003]    A typical backlight system includes a plurality of backlight lamps, and a backlight control circuit. The backlight control circuit is used for feeding back currents of the backlight lamps, and protecting the backlight system when an open circuit occurs in any of the backlight lamps. 
         [0004]    Referring to  FIG. 3 , one such backlight control circuit  100  includes a pulse width modulation integrated circuit (PWM IC)  110 , an inverter circuit  130 , a backlight lamp unit  150 , a first feedback circuit  140 , a second feedback circuit  160 , and a protecting circuit  170 . 
         [0005]    The backlight lamp unit  150  includes a first lamp  151  and a second lamp  152 . The first lamp  151  and the second lamp  152  both have a positive end and a negative end. The PWM IC  110  includes a signal output terminal  111 , a current feedback terminal  113 , a protecting output terminal  115 , and a voltage feedback terminal  116 . The signal output terminal  111  is connected to the inverter circuit  130 . The voltage feedback terminal  116  is connected to the first feedback circuit  140 . The current feedback terminal  115  is connected to the second feedback circuit  160 . The protecting output terminal  115  is connected to the protecting circuit  170 . 
         [0006]    The inverter circuit  130  includes a signal input  131 , a first driving terminal  132 , and a second driving terminal  133 . The first driving terminal  132  and the second driving terminal  133  output an AC voltage to the positive ends of the lamps  151 ,  152  respectively. A value of the AC voltage can be 1500V. The AC voltage at the first driving terminal  132  has a phase opposite to that at the second driving terminal  133 . 
         [0007]    The first feedback circuit  140  includes two high voltage feedback inputs  141  and a high voltage feedback output  142 . The two high voltage feedback inputs  141  are connected to the positive ends of the lamps  151 ,  152  respectively. The high voltage feedback output  142  is connected to the voltage feedback terminal  116  of the PWM IC  110 . The first feedback circuit  140  outputs a first feedback signal to the voltage feedback terminal  116 . 
         [0008]    The second feedback circuit  160  includes a current input  161  and a low-voltage feedback output  162 . The current input  161  is connected to the negative ends of the lamps  151 ,  152 . The low-voltage feedback output  162  is connected to the current feedback terminal  113  of the PWM IC  110 . The second feedback circuit  160  outputs a second feedback signal to the PWM IC  110  corresponding to the current at the negative ends of the lamps  151 ,  152 . 
         [0009]    The protecting circuit  170  includes a first resistor  171  and a capacitor  172 . One end of the first resistor  171  is coupled with the protecting output terminal  115  of the PWM IC  110 , and the other end of the first resistor  171  is grounded via the capacitor  172 . The first resistor  171  is used for controlling the charging time of the capacitor  172 . 
         [0010]    When an open circuit occurs in any of the lamps  151 ,  152 , the current input  161  feeds back the current of the lamps  151 ,  152 , and the second feedback circuit  160  outputs a lower second signal to the PWM IC  110 . When the second signal is lower than a first reference voltage, the PWM IC  110  outputs a pulse-time ratio signal to increase the working voltage of the backlight unit  150  through the inverter circuit  130 . At the same time, the first feedback circuit  140  outputs the first signal to the PWM IC  110 . The PWM IC  110  compares the first signal with a second reference voltage. When the first signal is higher than the second reference voltage, the PWM IC  110  outputs a signal to charge the capacitor  172  via the protecting output terminal  115 . When the voltage of the capacitor  172  reaches a predetermined potential, for example  3 V, the PWM IC  110  stops the inverter circuit  130  from driving the backlight lamp unit  150 , so as to protect the backlight lamp unit  150 . 
         [0011]    As described above, the inverter circuit  130  stops the backlight lamp unit  150  after a period of time has elapsed from the time when the PWM IC  110  outputs the signal to charge the capacitor  172 . During this period, the PWM IC  110  continuously increases the voltage difference between the lamps  151 ,  152 . The voltage difference between the lamps  151 ,  152  may increase and induce a spark discharge. The spark discharge is liable to destroy the backlight lamp unit  150 . Thus, the backlight control circuit  100  has low reliability. 
         [0012]    It is, therefore, desired to provide a backlight control circuit that can overcome the above-described deficiencies. 
       SUMMARY 
       [0013]    In an exemplary embodiment, a backlight control circuit includes a load, an inverter circuit, a pulse width modulation integrated circuit (PWM IC), a protecting circuit, and a feedback circuit. The load includes a plurality of backlight lamps. Each lamp includes a first terminal. The inverter circuit is configured to drive the load. The PWM IC is connected to the load via the inverter circuit. The PWM IC includes a protecting output. The protecting circuit is connected to the protecting output of the PWM IC. The protecting circuit has a reference voltage. The first feedback circuit is connected to the first terminals of the lamps, the PWM IC, and the protecting circuit. The first feedback circuit is capable of detecting voltage from the first terminals of the lamps. The first feedback circuit is capable of outputting a voltage to the protecting circuit. The output voltage is corresponded to the voltage detected from the first terminals. The protecting circuit is configured to control the PWM IC to stop outputting a backlight adjusting signal to the inverter circuit such that the inverter circuit stops driving the load when the output voltage is higher than the reference voltage of the protecting circuit. 
         [0014]    Other novel features and advantages of the present backlight control circuit will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a diagram of a backlight control circuit according to an exemplary embodiment of the present invention. 
           [0016]      FIG. 2  is a diagram of a backlight control circuit according to another exemplary embodiment of the present invention. 
           [0017]      FIG. 3  is a diagram of a conventional backlight control circuit. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0018]    Reference will now be made to the drawings to describe preferred and exemplary embodiments of the present invention in detail. 
         [0019]      FIG. 1  is an abbreviated circuit diagram of a backlight control circuit according to an exemplary embodiment of the present invention. The backlight control circuit  200  is typically installed in a backlight system (not shown). The backlight system can be used together with an LCD, both being installed in a product such as a notebook, a PDA, a video camera, etc. The backlight control circuit  200  includes a PWM IC  210 , an inverter circuit  230 , a load  250 , a first feedback circuit  240 , a second feedback circuit  260 , and a protecting circuit  270 . The PWM IC  210  outputs a backlight correction signal to the inverter circuit  230 , and the inverter circuit  230  drives the load  250  to function. The first feedback circuit  240  is connected to the load  250  to feed back signals to the PWM IC  210  and the protecting circuit  270 . The second feedback circuit  260  outputs a signal to the PWM IC  210  corresponding to the current of the load  250 . 
         [0020]    The load  250  includes a first lamp  251  and a second lamp  252 . Each of the lamps  251 ,  252  has a first terminal and a second terminal. The PWM IC  210  includes a signal output end  211 , a current feedback end  213 , a protecting output  215 , and a voltage feedback end  216 . The signal output end  211  is connected to the inverter circuit  230 . The current feedback end  213  is connected to the second feedback circuit  260 . The voltage feedback end  216  is connected to the first feedback circuit  240 . The protecting output  215  is connected to the protecting circuit  270 . 
         [0021]    The inverter circuit  230  includes a signal input end  231 , a first driving end  232 , and a second driving end  233 . The signal input end  231  is connected to the signal output end  211  of the PWM IC  230 . The first driving end  232  is connected to the first terminal of the first lamp  251  to supply a high alternating current (AC) voltage, and the second driving end  233  is connected to the first terminal of the second lamp  252  to supply another high AC voltage. A value of each high AC voltage can be 1500V. The two AC voltages have an opposite phase from each other. 
         [0022]    The first feedback circuit  240  includes two high voltage detecting terminals  241 , and a feedback signal output terminal  242 . The two high voltage detecting terminals  241  are connected to the first terminals of the first and second lamps  251 ,  252 , respectively. The feedback signal output terminal  242  is connected to the voltage feedback end  216  of the PWM IC  210 . 
         [0023]    The second feedback circuit  260  includes a current voltage detecting terminal  261 . The current voltage detecting terminal  261  is connected to the second terminals of the lamps  251 ,  252 . 
         [0024]    The protecting circuit  270  includes a charging branch  1 , and a comparison circuit  272 . The comparison circuit  272  controls the charging branch  1  to be charged or discharged. The charging branch  1  includes a current-limiting resistor  271 , a switch element  278 , and a charging capacitor  279 . One end of the current-limiting resistor  271  is connected to the protecting output  215  of the PWM IC  210 . The other end of the current-limiting resistor  271  is grounded via the switch element  278  and the charging capacitor  279  in sequence. The switch element  278  is typically a diode, which includes a positive terminal connected to the current-limiting resistor  271 , and a negative terminal connected to the charging capacitor  279 . 
         [0025]    The comparison circuit  272  includes a comparator  273 , a first resistance divider R 11 , a second resistance divider R 12 , and a reference voltage input terminal  277 . The comparator  273  has a positive input  274 , a negative input  275 , and an output end  276 . The reference voltage input terminal  277  is grounded via the first resistance divider R 11  and the second resistance divider R 12 . The positive input  274  is connected to a node between the first resistance divider R 11  and the second resistance divider R 12 , and is configured to set a first reference voltage which is greater than or equal to a second reference voltage of the PWM IC  210 . The negative input  275  is connected to the feedback signal output terminal  242  of the first feedback circuit  240  to receive the first feedback signal. The output end  276  is connected to a node between the switch element  278  and the charging capacitor  279 . 
         [0026]    When an open circuit occurs in any of the lamps  251 ,  252  of the load  250 , the current of the load  250  decreases. The second feedback circuit  260  sends a signal to the PWM IC  210  corresponding to the current. Then the PWM IC  210  provides a correction signal to the inverter circuit  230 . The inverter circuit  230  outputs a higher voltage to the load  250 . At the same time, the first feedback circuit  240  feeds back the voltage of the first terminals of the lamps  251 ,  252 , and outputs voltage feedback signals to the PWM IC  210  and the protecting circuit  270 . While the voltage feedback signal is higher than the second reference voltage of the PWM IC  210 , the PWM IC  210  turns on its over voltage protection function. That is, the protecting output  215  of the PWM IC  210  outputs a charging signal to the charging capacitor  279  via the current-limiting resistor  271  and the switch element  278 . While the voltage feedback signal is higher than the first reference voltage of the comparison circuit  272 , the comparator  273  turns off the switch element  278  to cut off the charging branch  1 . Therefore the protecting output  215  reaches a predetermined potential, for example, 3V, immediately. The PWM IC  210  stops outputting the charging signal to the charging capacitor  279  and stops outputting a backlight adjusting signal to the inverter circuit  230 . The inverter circuit  230  turns off the load  250  to protect the load  250  from spark discharge. 
         [0027]    The backlight control circuit  200  includes the comparison circuit  272  and the switch element  278 . The comparison circuit  272  receives the voltage feedback signal. When the voltage feedback signal is higher than the first reference voltage, the PWM IC  210  consequently stops outputting the backlight adjusting signal to the inverter circuit  230 . The inverter circuit  230  turns off the load  250  according to the dormant PWM IC  210 , so as to protect the load  250  from spark discharge. Therefore the backlight control circuit  200  has high reliability. 
         [0028]      FIG. 2  is a diagram of a backlight control circuit  300  according to another exemplary embodiment of the present invention. The backlight control circuit  300  is similar to the above-described backlight control circuit  200 , only differing in that a charging branch  2  of a protecting circuit  370  includes a current-limiting resistor  371 , a switch element  378 , and a charging capacitor  379 . One terminal of the current-limiting resistor  371  is connected to a protecting output  315  of a PWM IC  310 . The other terminal of the current-limiting resistor  371  is grounded via the switch element  378  and the charging capacitor  379  in sequence. The switch element  378  is typically a negative-channel metal-oxide semiconductor (NMOS) transistor, which includes a source electrode, a gate electrode, and a drain electrode. The source electrode is connected to the comparison circuit  372 , the drain electrode is grounded, and the gate electrode is connected to the charging capacitor  379 . The NMOS transistor performs substantially the same function as the diode (switch element  278 ). Compared with the backlight control circuit  200 , the backlight control circuit  300  can achieve substantially the same function and advantages. 
         [0029]    It is to be understood, however, that even though numerous characteristics and advantages of preferred and exemplary embodiments have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only; and that changes may be made in detail within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.