Abstract:
An exemplary backlight control circuit ( 200 ) includes two load circuits ( 210 ), a pulse width modulation integrated circuit (PWM IC) ( 250 ) having a current sampling pin ( 251 ), a first transistor ( 271 ), a first input circuit ( 230 ) and a second input circuit ( 240 ) including an input resistor. Each load circuit includes an output end ( 212, 222 ). The first input circuit includes a second transistor ( 234 ). The drain electrode of the first transistor is connected to the current sampling pin. The gate electrode of the first transistor is connected to the drain electrode of the second transistor and is connected to a power supply. The gate electrode of the second transistor is connected to one of the output ends of the load circuits via the diode. The other one of the output ends of the load circuits is connected to the current sampling pin via the input resistor.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a backlight control circuit typically used in a liquid crystal display (LCD).  
       GENERAL BACKGROUND  
       [0002]     An LCD has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the LCD is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.  
         [0003]     A typical LCD includes an LCD panel, a plurality of backlights for illuminating the LCD panel, an inverter circuit for driving the backlights, and a backlight control circuit. The baclight control circuit generally includes a pulse width modulation integrated circuit (PWM IC) for driving the inverter circuit, and a backlight protecting circuit for shutting down the PWM IC when any one of the backlights has an open circuit or a short circuit connecting to ground.  
         [0004]      FIG. 3  is an abbreviated diagram of a typical backlight control circuit used in an LCD. The backlight control circuit  100  includes load circuits  110 , a pulse width modulation integrated circuit (PWM IC)  150 , and a backlight protection circuit (not labeled). The backlight protection circuit includes a first transistor  171 , a current limiting resistor  172 , and an input circuit  130 .  
         [0005]     Each load circuit  110  includes a backlight  111  and a backlight inspecting circuit  123  connected in series between a backlight power supply (not shown) and ground. The backlight inspecting circuit  123  includes an output end  112 . The output end  112  provides a high voltage when the corresponding backlight  111  works. The output end  112  provides a low voltage when the corresponding backlight  111  has an open circuit or a short circuit connecting to ground.  
         [0006]     The PWM IC  150  includes a current sampling pin  151 . The PWM IC  150  stops working if the current sampling pin  151  has a low voltage.  
         [0007]     The first transistor  171  includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The source electrode “S” is connected to ground. The drain electrode “D” is connected to the current sampling pin  151  of the PWM IC  150 . The gate electrode “G” is connected to a power supply via the current limiting resistor  172 . The power supply is provided by a power pin (not labeled) of the PWM IC  150 .  
         [0008]     The input circuit  130  includes four diodes  131 , four resistors  132 , four capacitors  135 , a second transistor  1332 , a third transistor  1333 , a fourth transistor  1334 , and a fifth transistor  1335 . Each transistor  1332 ,  1333 ,  1334 ,  1335  includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The drain electrode “D” of the second transistor  1332  is connected to the gate electrode “G” of the first transistor  171 . The drain electrode “D” of the third transistor  1333  is connected to the source electrode “S” of the second transistor  1332 . The drain electrode “D” of the fourth transistor  1334  is connected to the source electrode “S” of the third transistor  1333 . The drain electrode “D” of the fifth transistor  1335  is connected to the source electrode “S” of the fourth transistor  1334 . The source electrode “S” of the fifth transistor  1335  is connected to ground. The gate electrodes “G” of the second, third, fourth, and fifth transistors  1332 ,  1333 ,  1334 ,  1335  are connected to the negative terminals of the four diodes  131 , respectively. The positive terminals of the four diodes  131  are respectively connected to the output ends  112  of the backlight inspecting circuits  123 . Each of the gate electrodes “G” of the second, third, fourth, and fifth transistors  1332 ,  1333 ,  1334 ,  1335  is connected to ground via the corresponding resistor  132 , and is connected to ground via the corresponding capacitor  135 .  
         [0009]     The first transistor  171 , the second transistor  1332 , the third transistor  1333 , the fourth transistor  1334  and the fifth transistor  1335  are negative-channel metal oxide semiconductor (NMOS) type transistors.  
         [0010]     Operation of the backlight control circuit  100  is as follows. When all the backlights  111  work normally, each of the output ends  112  provides a high voltage to the gate electrode “G” of the corresponding one of the second, third, fourth, and fifth transistors  1332 ,  1333 ,  1334 ,  1335  via the corresponding diode  131 . Then the second, third, fourth, and fifth transistors  1332 ,  1333 ,  1334 ,  1335  are switched to an activated state, and the gate electrode “G” of the first transistor  171  is connected to ground via the activated second, third, fourth, and fifth transistors  1332 ,  1333 ,  1334 ,  1335 . Thus the first transistor  171  is turned off, and the current sampling pin  151  of the PWM IC  150  maintains an original working voltage.  
         [0011]     When any one of the backlights  111  has an open circuit or has a short circuit connecting to ground, the corresponding output end  112  of the backlight inspecting circuit  123  provides a low voltage to the gate electrode “G” of the corresponding one of the second, third, fourth, and fifth transistors  1332 ,  1333 ,  1334 ,  1335  via the corresponding diode  131 . Then the corresponding second, third, fourth, or fifth transistor  1332 ,  1333 ,  1334 ,  1335  is turned off, so that the gate electrode “G” of the first transistor  171  is charged to a high voltage via the current limiting resistor  172 . Thus the first transistor  171  is switched to an activated state, and the current sampling pin  151  of the PWM IC  150  is connected to ground via the activated first transistor  171 . Consequently, the current sampling pin  151  of the PWM IC  150  is charged to a low voltage, and the PWM IC  150  stops working.  
         [0012]     The backlight control circuit  100  includes the five transistors  171 ,  1332 ,  1333 ,  1334 ,  1335  needed to carry out the function of protecting the backlights  111 . Further, the number of transistors needed increases with the number of backlights  111  used in the LCD. Consequently, the cost of the backlight control circuit  100  is high, particularly in the case where the number of backlights  111  is large.  
         [0013]     It is desired to provide a backlight control circuit that can be used in an LCD, which backlight control circuit overcomes the above-described deficiencies.  
       SUMMARY  
       [0014]     In a preferred embodiment, a backlight control circuit includes a first backlight inspecting circuit, a second backlight inspecting circuit, a PWM IC, a first transistor, a first input circuit, and a second input circuit having an input resistor. Each backlight inspecting circuit includes a backlight and a output end. The PWM IC includes a current sampling pin. The first transistor includes a source electrode connected to ground, a drain electrode connected to the current sampling pin of the PWM IC, and a gate electrode connected to a power supply. The first input circuit includes a diode, a resistor, a capacitor, and a second transistor. The second transistor includes a drain electrode connected to the gate electrode of the first transistor, a source electrode connected to ground, and a gate electrode connected to ground via the resistor and the capacitor respectively. The gate electrode of the second transistor is connected to the negative terminal of the diode. The positive terminal of the diode is connected to one of the output ends of the backlight inspecting circuits. The other one of the output ends of the backlight inspecting circuits is connected to the current sampling pin of the PWM IC via the input resistor.  
         [0015]     Advantages and novel features of the above-described circuit will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is an abbreviated diagram of a backlight control circuit according to a first embodiment of the present invention, the backlight control circuit being typically used in an LCD.  
         [0017]      FIG. 2  is an abbreviated diagram of a backlight control circuit according to a second embodiment of the present invention, the backlight control circuit being typically used in an LCD.  
         [0018]      FIG. 3  is an abbreviated diagram of a conventional backlight control circuit used in an LCD. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]     Reference will now be made to the drawings to describe the present invention in detail.  
         [0020]      FIG. 1  is an abbreviated diagram of a backlight control circuit according to a first embodiment of the present invention, the backlight control circuit being typically used in an LCD. The backlight control circuit  200  includes two load circuits  210 ,  220 , a PWM IC  250 , and a backlight protection circuit (not labeled). The backlight protection circuit includes a first transistor  271 , a first input circuit  230 , and a second input circuit  240 .  
         [0021]     The load circuit  210  includes a backlight  211  and a backlight inspecting circuit  213  connected in series between a backlight power supply (not shown) and ground. The backlight inspecting circuit  213  includes an output end  212 . The load circuit  220  includes a backlight  221  and a backlight inspecting circuit  223  connected in series between the backlight power supply (not shown) and ground. The backlight inspecting circuit  223  includes an output end  222 . Each of the output ends  212 ,  222  provides a low voltage when the corresponding backlight  211  or backlight  221  has an open circuit or a short circuit connecting to ground.  
         [0022]     The PWM IC  250  includes a current sampling pin  251 . The PWM IC  250  stops working if the current sampling pin  251  has a low voltage.  
         [0023]     The first transistor  271  includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The source electrode “S” of the first transistor  271  is connected to ground. The drain electrode “D” of the first transistor  271  is connected to the current sampling pin  251  of the PWM IC  250 . The gate electrode “G” of the first transistor  271  is connected to a power supply via a current limiting resistor  272 . The power supply is provided by a power pin (not labeled) of the PWM IC  250 , and is typically a five volt direct current power supply.  
         [0024]     The first input circuit  230  includes a second transistor  234 , a diode  231 , a resistor  232 , and a capacitor  233 . The second transistor  234  includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The drain electrode “D” of the second transistor  234  is connected to the gate electrode “G” of the first transistor  271 . The source electrode “S” of the second transistor  234  is connected to ground. The gate electrode “G” of the second transistor  234  is connected to the negative terminal of the diode  231 , and is connected to ground respectively via the resistor  232  and via the capacitor  233 . The positive terminal of the diode  231  is connected to the output end  222  of the backlight inspecting circuit  223 .  
         [0025]     The second input circuit  240  includes an input resistor  241 . The output end  212  of the backlight inspecting circuit  213  is connected to the current sampling pin  251  of the PWM IC  250  via the input resistor  241 .  
         [0026]     The diode  231  can for example be an SN4148 type diode. A resistance of the current limiting resistor  272  is preferably 100 KΩ. A resistance of the input resistor  241  is preferably 3.9 KΩ. Alternatively, the resistance of the input resistor  241  can be 1.0MΩ. The PWM IC  250  can for example be an OZ9910G type PWM IC. The first transistor  271  and the second transistor  234  can be negative-channel metal oxide semiconductor (NMOS) type transistors or negative-positive-negative (NPN) type transistors.  
         [0027]     Generally, operation of the backlight control circuit  200  is as follows. When the backlight  211  of the load circuit  210  works, the output end  212  of the backlight inspecting circuit  213  provides a high voltage to the current sampling pin  251  of the PWC IC  250  via the input resistor  241 . When the backlight  221  of the load circuit  220  works, the output end  222  of the backlight inspecting circuit  223  provides a high voltage to the gate electrode “G” of second transistor  234  via the diode  231 . Then the second transistor  234  is switched to be in an activated state, and the gate electrode “G” of the first transistor  271  is connected to ground via the activated second transistor  234 . Thus the first transistor  271  is turned off, the current sampling pin  251  of the PWM IC  250  maintains the high voltage, and the PWM IC  250  continues working.  
         [0028]     When the backlight  221  has an open circuit or a short circuit connecting to ground, the output end  222  of the backlight inspecting circuit  223  provides a low voltage to the gate electrode “G” of the second transistor  234  via the diode  231 . Then the second transistor  234  is turned off, so that the gate electrode “G” of the first transistor  271  is charged to a high voltage by the power supply. Thus the first transistor  271  is switched to be in an activated state, so that the current sampling pin  251  of the PWM IC  250  is connected to ground via the activated first transistor  271 . Then the current sampling pin  251  of the PWM IC  250  is discharged to a low voltage, and the PWM IC  250  stops working.  
         [0029]     When the backlight  211  has an open circuit or a short circuit connecting to ground, the output end  212  of the backlight inspecting circuit  213  directly provides a low voltage to the current sampling pin  251  of the PWM IC  250  via the input resistor  241 . Thus, the PWM IC  250  stops working.  
         [0030]     The backlight control circuit  200  needs only the two transistors  271 ,  234  to carry out the function of protecting the backlights  211 ,  221 . Therefore, the backlight control circuit  200  has low cost.  
         [0031]      FIG. 2  is an abbreviated diagram of a backlight control circuit according to a second embodiment of the present invention, the backlight control circuit being typically used in an LCD. The backlight control circuit  300  includes load circuits  310 ,  320 ,  380 ,  390 , a PWM IC  350 , and a backlight protection circuit (not labeled). The backlight protection circuit includes a first transistor  371 , a first input circuit  330 , and a second input circuit  340 .  
         [0032]     The load circuit  310  includes a backlight  311  and a backlight inspecting circuit  313  connected in series between a backlight power supply (not shown) and ground. The load circuit  320  includes a backlight  321  and a backlight inspecting circuit  323  connected in series between the backlight power supply (not shown) and ground. The load circuit  380  includes a backlight  381  and a backlight inspecting circuit  383  connected in series between the backlight power supply (not shown) and ground. The load circuit  390  includes a backlight  391  and a backlight inspecting circuit  393  connected in series between the backlight power supply (not shown) and ground.  
         [0033]     Each backlight inspecting circuit  313 ,  323 ,  383 ,  393  includes an output end  312 ,  322 ,  382 ,  392  respectively. Each of the output ends  312 ,  322 ,  382 ,  392  provides a high voltage when the corresponding backlight  311 ,  321 ,  381 ,  391  works. Each of the output ends  312 ,  322 ,  382 ,  392  provides a low voltage when the corresponding backlight  311 ,  321 ,  381 ,  391  has an open circuit or a short circuit connecting to ground.  
         [0034]     The PWM IC  350  includes a current sampling pin  351 . The PWM IC  350  stops working if the current sampling pin  351  has a low voltage.  
         [0035]     The first transistor  371  includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The source electrode “S” of the first transistor  371  is connected to ground. The drain electrode “D” of the first transistor  371  is connected to the current sampling pin  351  of the PWM IC  350 . The gate electrode “G” of the first transistor  371  is connected to a power supply via a current limiting resistor  372 . The power supply is provided by a power pin (not labeled) of the PWM IC  350 , and is typically a five volt direct current power supply.  
         [0036]     The first input circuit  330  includes a second transistor  334 , a third transistor  336 , two diodes  331 , two resistors  332 , and two capacitors  333 . Each of the second and third transistors  334 ,  336  includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The drain electrode “D” of the second transistor  334  is connected to the gate electrode “G” of the first transistor  371 . The source electrode “S” of the second transistor  334  is connected to drain electrode “D” of the third transistor  336 . The source electrode “S” of the third transistor  336  is connected to ground. The gate electrode “G” of the second transistor  334  is connected to the negative terminal of one of the diodes  331 , and is connected to ground respectively via one of the resistors  332  and via one of the capacitors  333 . The gate electrode “G” of the third transistor  336  is connected to the negative terminal of the other diode  331 , and is connected to ground respectively via the other resistor  332  and via the other capacitor  333 . The positive terminals of the two diodes  331  are respectively connected to the output ends  382 ,  392  of the corresponding backlight inspecting circuits  383 ,  393 .  
         [0037]     The second input circuit  340  includes two input resistors  341 ,  342 . The output end  312  of the backlight inspecting circuit  313  is connected to the current sampling pin  351  of the PWM IC  350  via the input resistor  341 . The output end  322  of the backlight inspecting circuit  323  is connected to the current sampling pin  351  of the PWM IC  350  via the input resistor  342 .  
         [0038]     The diodes  331  can for example be SN4148 type diodes. A resistance of the current limiting resistor  372  is preferably 100 KΩ. A resistance of each of the input resistors  341 ,  342  is preferably 3.9 KΩ. Alternatively, the resistance of each of the input resistors  341 ,  342  can be 1.0MΩ. The PWM IC  350  can for example be an OZ9910G type PWM IC. The first transistor  371 , the second transistor  334 , and the third transistor  336  can be negative-channel metal oxide semiconductor (NMOS) type transistors or negative-positive-negative (NPN) type transistors.  
         [0039]     Generally, operation of the backlight control circuit  300  is as follows. When the backlights  311 ,  321  of the backlight inspecting circuits  310 ,  320  work, the corresponding output ends  312 ,  322  of the backlight inspecting circuits  313 ,  323  each provide a high voltage to the current sampling pin  351  of the PWC IC  350  via the input resistors  341 ,  342  respectively. When the backlights  381 ,  391  of the backlight inspecting circuits  380 ,  390  work, the corresponding output ends  382 ,  392  of the backlight inspecting circuits  383 ,  393  each provide a high voltage to the gate electrodes “G” of the second and third transistors  334 ,  336  respectively via the corresponding diodes  331 . Then the second and third transistors  334 ,  336  are switched to be in an activated state, and the gate electrode “G” of the first transistor  371  is connected to ground via the activated second and third transistors  334 ,  336 . Thus the first transistor  371  is turned off, the current sampling pin  351  of the PWM IC  350  maintains the high voltage, and the PWM IC  350  continues working.  
         [0040]     When the backlight  381  or the backlight  391  has an open circuit or a short circuit connecting to ground, the output end  382  or the output end  392  provides a low voltage to the gate electrode “G” of the second transistor  334  or the gate electrode “G” of the third transistor  336  via the corresponding diode  331 . Then the second transistor  334  or the third transistor  336  is turned off, so that the gate electrode “G” of the first transistor  371  is charged to a high voltage by the power supply. Thus the first transistor  371  is switched to be in an activated state, so that the current sampling pin  351  of the PWM IC  350  is connected to ground via the activated first transistor  371 . Then the current sampling pin  351  of the PWM IC  350  is discharged to a low voltage, and the PWM IC  350  stops working.  
         [0041]     When the backlight  311  or the backlight  321  has an open circuit or a short circuit connecting to ground, the output end  312  or the output end  322  directly provides a low voltage to the current sampling pin  351  of the PWM IC  350  via the input resistor  341  or the input resistor  342 . Thus, the PWM IC  350  stops working.  
         [0042]     The backlight control circuit  300  needs only the three transistors  371 ,  334 ,  336  to carry out the function of protecting the backlights  311 ,  321 ,  381 ,  391 . Therefore, the backlight control circuit  300  has low cost.  
         [0043]     In alternative embodiments, when the number of load circuits increases, the circuit configuration of the first input circuit and the circuit configuration of the second input circuit can be adjusted according to the principles on which the backlight control circuits  200 ,  300  are configured. For example, the number of the load circuits can be n+m, where n, m are equal to each other, and n, m are each &gt;=1. The first input circuit includes m second transistors, m diodes, m resistors, and m capacitors. The second input circuit includes n input resistors. The second transistors are connected in series with one another though the source electrode of one of the second transistors connecting with the drain electrode of an adjacent next one of the second transistors. This chain of second transistors is connected between the gate electrode of a first transistor and ground as follows. The drain electrode of a first one of the second transistors is connected to the gate electrode of the first transistor, and the source electrode of a last one of the second transistors is connected to ground. Further, the gate electrode of each of the second transistors is connected to ground respectively via one of the resistors and via one of the capacitors. The gate electrode of each of the second transistors is also connected to the negative terminal of one of the diodes. The positive terminal of each diode is connected to the output end of a respective one of the m load circuits. Each output end of each of the n load circuits is connected to the current sampling pin of the PWM IC via one of the input resistors.  
         [0044]     It is to be understood, however, that even though numerous characteristics and advantages of preferred 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. Changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.