Abstract:
A control circuit to vary the intensity of an electro luminescent display. The circuit is connected between a power source and a display and comprises a pair of conductors to be connected to the display for applying a voltage from a voltage generator. A switch controls application of the voltage from the generator to the conductors. A gating circuit connects selectively one or other of the conductors to the voltage source. A controller operates upon the switch to vary the duty cycle and upon the gating circuit to alternate periodically the relative polarity of the conductors. The voltage source includes an inductor and the switch controls current flow through said inductor.

Description:
This application claims priority from U.S. application Ser. No. 60/488,794 filed on Jul. 22, 2003, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a dimmer function for use with a display device such as an EL (electroluminescence) display. 
     BACKGROUND OF THE INVENTION 
     EL displays are frequently used to display images such as graphics, text and other types of visual information under ambient light conditions which can vary greatly. In high intensity ambient light conditions, it may become difficult to properly view the images displayed on the EL display. As well, under low ambient light conditions, the images displayed on the EL display may be overly bright. 
     Accordingly, it is helpful if the EL display is controlled by a dimmer so that image brightness may be increased when the intensity of the ambient light is high. When the intensity of ambient light is low, the brightness of the image may be decreased. The intensity of the display is a function of the maximum voltage applied and in the art the voltage is controlled by a silicon controlled rectifier (SCR). Control of SCR is difficult and has a significant power consumption. Moreover, with portable devices the battery voltage used as a power source will also vary over time and accordingly the intensity of the display tends to fluctuate with the battery voltage. 
     Accordingly, it is an object of the present application to obviate or mitigate the above disadvantages. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a solution to the problem of maintaining and adjusting the intensity of a display as a power source varies. 
     In one aspect, the present invention provides a control circuit to vary the intensity of an electro luminescent display. The circuit is connected between a power source and a display and comprises a pair of conductors to be connected to said display for applying a voltage thereto, an inductor connected to the power source, a switch to control current flow through the inductor from the power source to the conductors, a gating circuit to connect selectively one or other of said conductors to power source, and a controller operating upon the switch to vary the duty cycle thereof and operating upon the gating circuit to alternate periodically the relative polarity of conductors, wherein the gating circuit includes a selectively operable discharge path for each of the conductors, the discharge path including a current limiting element; and the inductor supplies the current flow directly to the conductors through the gating circuit. 
    
    
     
       An embodiment of the invention will now be described by way of example only with reference to the following detailed description in which reference is made to the following appended drawings, in which: 
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a hand held scanner, 
         FIG. 2  is a schematic diagram of a dimmer circuit utilized in the scanner of  FIG. 1 . 
         FIG. 3  is a simplified line diagram showing a period of an example of a pulse line used by the dimmer circuit. 
         FIG. 4  is a simplified line diagram showing an example of the charging and discharging cycles in relation to a pulse line used by the dimmer circuit. 
         FIG. 5  illustrates a generalized flow chart of an algorithm to produce the charging/discharging pulse. 
         FIG. 6  illustrates a generalized flow chart of an algorithm to produce the charging/discharging pulse and the synchronization of the H-bridge operations. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , there is shown a hand held scanner  2  having a body  4  and a display  14 . The scanner may include an input device, such as keypad  6 , and is used to read and store information from barcodes or the like through a scanner window  8 . The body  4  contains control and data acquisition components as well as a voltage source  9  ( FIG. 2 ) to supply power to the device. The scanner  2  may be used in a variety of environments in which different levels of intensity for the display  14  are desirable. 
     The keypad  6  includes a manual dimmer control  7  for use in setting a dimmer circuit  10  controlling the amplitude of voltage applied to an EL display  14  through a pair of conductors or lines  16 ,  18  shown in  FIG. 2 . The dimmer circuitry  10  includes an inductor  114 , that is supplied with power from the voltage source  9  at a nominal voltage. A N-chanel FET  110  controls current flow through the inductor  114 , which is connected in parallel to an H-bridge  19  having four legs  20 ,  22 ,  24 ,  26 . One pair of legs  20 ,  22  controls the voltage to line  16  connected to display  14  and the other pair  24 ,  26  controls the voltage applied to line  18  connected to display  14 . The voltage applied across the lines  16 ,  18  determines the intensity of the display  14 . The voltage in lines  16 ,  18  is also controlled by FETs  170 ,  150  respectively. 
     Each of the legs  20 ,  22 ,  24 ,  26  includes an N-chanel FET  120 ,  140 ,  130 ,  160  respectively. Current flow between legs  20 ,  22  and  24 ,  26  is controlled by diodes  142 ,  162  and within each leg by resistors  124 ,  134 ,  144 ,  164 . 
     The operation of the H-bridge  19  is controlled through a microprocessor  12 , which interfaces with the main processor  30  of the scanner  2 . The microprocessor  12  is grounded by line  280  and connected through line  220  to voltage regulator  50 . The N-chanel FETs  140 ,  150 ,  160 ,  170  are switched by control lines  240 ,  250 ,  260 ,  270  respectively from the microprocessor  12  to direct the inductor flyback energy into the EL display  14  through lines  16 ,  18 . The microprocessor  12  also controls the state of N-channel FET  110  through control line  210  to regulate the charging and discharging of the inductor  114 . 
     The H-bridge  19  capacitors  126 ,  136  are used to store a charge so that N-channel FETs  120 ,  130  may be turned ON. These capacitors  126 ,  136  are required to ensure that N-channel FETs  120 ,  130  maintain a positive voltage from gate to source. This is due to the fact that the EL display  14  is charged to a high voltage and that the gate voltage must be greater than the source voltage, but there are no voltage sources that are higher in voltage than that impressed on the EL display  14 . 
     Capacitor  126  is charged through H-bridge  19  diode  122  whenever N-channel FETs  140 ,  150  are ON. Capacitor  136  is charged through H-bridge  19  diode  132  whenever N-channel FETs  160 ,  170  are ON. Capacitors  126 ,  136  are initially charged up using the N-channel FETs  140 ,  160  which are connected in series with H-bridge  19  resistors  144 ,  164  just before starting the EL display  14  lighting sequence. The presence of resistors  144 ,  164  ensures that the current draw is not excessive when initially charging the uncharged capacitors  126 ,  136 . 
     The operation of the circuit of  FIG. 2  will be described assuming that capacitors  126 ,  136  are fully charged. The microcontroller  12  then selects a high leg and a low leg of H-bridge  19  by applying control signals so that N-channel FET  120  is turned ON by turning OFF N-channel FET  140 , N-channel FET  130  is turned OFF by turning ON N-channel FET  160 , N-channel FET  150  is turned OFF and that N-channel FET  170  is turned ON. The microprocessor  12  turns ON N-channel FET  110  and current flows through the inductor  114  to ground. 
     After a time determined by the microprocessor  12 , N-channel FET  110  is turned OFF and current flow through inductor  114  to ground is interrupted. The flyback energy from the inductor  114  causes current to flow through diode  112 , N-channel FET  120 , line  18 , the EL display  14  and N-channel FET  170 . This causes the EL display  14  to become charged to the voltage induced by the inductor  114 . The N-channel FET  110  is cycled by the microprocessor  12  so that several inductor  114  flyback pulses are sent to the EL display  14  while the H-bridge  19  is held in this state to increase the voltage applied across the lines  16 ,  18  until it attains the level required for the selected brightness. 
     After the required number of pulses, the EL display  14  is discharged by the microprocessor  12  turning ON the N-channel FET  140 , which turns OFF N-channel FET  120 , and leaving N-channel FET  170  ON. N-channel FET  140  is used instead of N-channel FET  150  because it has a series resistor  144  to limit the intensity of current flow. This helps minimize the presence of high current pulses so that Electro-Magnetic Interference (EMI) may be reduced. 
     The H-bridge  19  is then turned around by the microprocessor  12  selecting the controls lines so that N-channel FET  130  is turned ON, which turns OFF N-channel FET  160 , N-channel FET  150  is turned ON and N-channel FET  170  is turned OFF. Several inductor flybacks are then sent to the EL display  14 , causing the EL display  14  to be charged to the opposite polarity. 
     Once again, after the required number of pulses, the EL display  14  is discharged by turning ON N-channel FET  160 , which turns OFF N-channel FET  130 , and leaving N-channel FET  150  ON. N-channel FET  160  is used instead of the N-channel FET  170  because it has a series resistor  164  to limit the intensity of current flow. As previously mentioned, this helps minimize the presence of high current pulses so that Electro-Magnetic Interference (EMI) may be reduced, The H-bridge  19  is then turned around once more: N-channel FET  120  is turned ON, which turns OFF N-channel FET  140 , N-channel FET  150  is turned OFF and N-channel FET  170  is turned ON. Several inductor flybacks are then sent to the EL display  14 , causing the EL display  14  to be charged to the opposite polarity. 
     The purpose of diodes  142 ,  162  is to clamp the gates of N-channel FETs  120 ,  130  so that they do not become much more negative than the source voltage  9  when N-channel FETs  120 ,  130  are turned OFF. If they became more negative by, for example 20V, the gates oxides of N-channel FETs  120 ,  130  may be destroyed. Thus the microprocessor  12  provides a number of pulses to the EL display  14  with the H-bridge  19  in one configuration, discharges the EL display  14  and then provides a number of pulses with the H-bridge  19  in an opposite configuration. The flyback voltage caused by discharges of inductor  114  is applied to EL display  14  to determine the intensity of the display. 
     The microcontroller  12  controls the charging and discharging of inductor  114  by transmitting a pulse, on control line  210 , to N-channel FET  110 . By varying the charging and discharging durations, the intensity of the EL display  14  may also be varied.  FIG. 3  shows an example of a period of such a pulse line  100  where T is the period of the pulse  100  and T 1  is the time the pulse  100  is HIGH. When the pulse  100  is HIGH, N-channel FET  110  is turned ON and inductor  114  is charged for T 1  seconds, then, when the pulse  100  is LOW, N-channel FET  110  is turned OFF and inductor  114  is discharged for (T−T 1 ) seconds, which provides flyback energy to the EL display  14 .  FIG. 4  shows an example of the charging and discharging cycles of the EL display  14  in relation to a pulse  100  such as illustrated in  FIG. 3 . During the charging cycle of the EL display  14 , when the pulse  100  is LOW, the flyback energy from the discharge of inductor  114  causes a rise  102  in the charge of the EL display  14  and when the pulse is HIGH, the charge of the EL display remains constant  104  while the inductor  114  is being charged. This stepwise charging continues until a predetermined number of pulses per polarity P is attained, at which time the charge of the EL display  14  has reached the required maximum value V EL  across lines  16 ,  18  corresponding to the desired brightness level. The EL display  14  is then discharged  106 , the H-bridge  19  is turned around so that the polarity is inversed, and the process is repeated. This process creates an alternating voltage across lines  16 ,  18  from voltage source  9 . 
     Adjusting the duty cycle of the inductor  114  controls the amount of energy stored in the inductor  114  on each pulse. The duty cycle is the fraction of time the pulse  100  is HIGH during a pulse  100  period T 1  as described by Equation 1. 
     
       
         
           
             
               
                 
                   
                     duty 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     cycle 
                   
                   = 
                   
                     
                       T 
                       1 
                     
                     T 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     The amount of energy stored during each pulse is proportional to the inductance and the square of the current, as described by Equation 2. 
     
       
         
           
             
               
                 
                   E 
                   = 
                   
                     
                       LI 
                       2 
                     
                     2 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     where L is the inductance value of inductor  114 
         I is the inductor&#39;s  114  current during the pulse.       

     This current may be expressed by Equation 3: 
     
       
         
           
             
               
                 
                   I 
                   = 
                   
                     
                       VT 
                       1 
                     
                     L 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
           
         
       
     
     where V is the voltage of voltage source  9 ,
         T 1  is the time the pulse  100  is HIGH.       

     Varying the duty cycle varies the rate at which energy is transferred to the EL display  14 , which therefore controls the power and hence brightness. The power may be approximated by:
 
 P≈E·f   Equation 4
 
     where E amount of energy stored during each pulse,
         f is the frequency of the pulse  100 .       

     Combining Equation 2, Equation 3 and Equation 4 gives Equation 5 in which the power is proportional to the square of the voltage of voltage source  9 , the period and the square of the duty cycle of pulse  100 , and inversely proportional to the inductance value of inductor  114 . 
                         P   ≈       ⁢       1   2     ·   L   ·       (       V   L     ⁢     T   1       )     2     ·     (     1   T     )                   =       ⁢         1   2     ·       V   2     L     ·       T   1   2     T       =       1   2     ·       V   2     L     ·           T   2     ·   D     ⁢           ⁢     C   2       T                     =       ⁢         1   2     ·       V   2     L     ·   T   ·   D     ⁢           ⁢     C   2                     Equation   ⁢           ⁢   5               
where DC is the duty cycle of pulse  100 .
 
     Thus by varying the period and/or duty cycle the intensity of the EL display  14  may be varied. 
     Pulse  100  is generated by alternately setting control line  210  to HIGH for a duration of T 1  seconds and then to LOW for a duration of (T−T 1 ) seconds. The timing of the HIGH and LOW portions of the pulse  100  may be achieved by translating times T 1  and (T−T 1 ) into an equivalent number of instructions to be performed by the microcontroller  12 . The number of instructions microcontroller  12  executes per second is determined by dividing its frequency by 4. For example, a microcontroller running off its internal 4 MHz clock executes 1,000,000 instructions per second or, in other words, one instruction is executed each 1 μs. Thus, correct timing is achieved by executing a number of instructions equal to: 
                     number   ⁢           ⁢   of   ⁢           ⁢   instructions     =       T   1       1   ⁢           ⁢   µs               Equation   ⁢           ⁢   6               
for the HIGH pulse; and
 
                     number   ⁢           ⁢   of   ⁢           ⁢   instructions     =       (     T   -     T   1       )       1   ⁢           ⁢   µs               Equation   ⁢           ⁢   7               
for the LOW pulse.
 
     An example of an algorithm that may be executed by the microcontroller  12  to obtain pulse  100  is depicted by the flow chart shown in  FIG. 5 . In this example, an 8-bit microcontroller  12  is used, such as, for example, the PIC12C508A, which has a 4 MHz internal clock. The sequence of steps composing the algorithm is indicated by the sequence of blocks  302  to  314 . 
     In block  302  the algorithm starts by setting the output of control line  210  to HIGH. 
     At block  304 , a counter is set so that the proper number of instructions is executed in order to translate into the required time duration (e.g. T 1  or (T−T 1 ) seconds depending if the control line  210  is presently at HIGH or LOW respectively). In the case of an 8-bit microcontroller  12 , there is a circular counter that is increased every time an instruction is executed. The counter increases from 0 to 255, at which time it goes back to 0 and repeats the cycle. Thus, to count N instructions, the counter has to be set according to the following equation:
 
counter value=256− N   Equation 8
 
     If a duration of 50 μs is required, using either Equation 6 or Equation 7, this translates into 50 instructions, the counter is then set to 206 (256−50). Since the setting of the counter, as well as the operation of testing the counter, take a number of instructions to perform, an offset must be added to the counter in order to compensate for those operations which require a number of instructions to execute. Accordingly, if setting the counter requires N s  instructions and the testing of the counter N t  instructions, Equation 8 becomes:
 
counter value=256− N+N   s   +N   t   Equation 8
 
     N s  is added to the counter since the control line  210  has already been at either HIGH or LOW (depending on where in the pulse  100  sequence the algorithm is) for N s  instructions at the time the counter is started. Similarly, N t  is added to the counter because by the time the test is executed on the counter, the control line  210  will have already been at either HIGH or LOW (depending on where in the pulse  100  sequence the algorithm is) for N t  instructions. 
     At block  306 , the counter is tested, which operation takes N t  instructions, which in turn increases the counter by that number. 
     Then, at block  308 , the result of the test if checked to see if it the counter has reached or passed 0. If so, the algorithm proceeds to block  310 . If not, the algorithm repeats the testing operation by going back to block  306 . 
     Once the counter has reached or passed 0, at block  310  the algorithm determines if it has to execute additional instructions in order to compensate for testing operation the offset (N t ) that was added in setting the initial value of the counter. This is done because the testing of the counter is not instantaneous, which means that the counter may reach or pass 0 within a span of N t  instructions. Thus the counter is being stopped within N t  instructions of its desired value so that the next testing operation does not cause the counter to overshoot its desired value of N instructions. This requires from 1 to N t  additional instructions to be executed for the count to reach a total of N instructions. Simple NOP (no operation) instructions may be used for this purpose. 
     At block  312 , the algorithm checks if the control line  210  is presently at HIGH. If the control line  210  if not at HIGH, it will go back to block  302  where it will be set back to HIGH in order to start a new pulse  100  period. If the control line  210  is at HIGH, the algorithm proceeds to block  314  where the control line  210  if set to LOW and then proceeds to block  304  to complete the ongoing pulse  100  period. 
     Of course, depending on the implementation and the type of microcontroller used, the offsets may vary in order to provide for different coding structures which vary from one microcontroller type to another. As well, other timing methods may be used, such as, for example, a software loop with an interrupt when the counter reaches 0, depending once again on the type of microcontroller used. 
     The microcontroller  12  uses pulse  100  generated, for example, by the algorithm depicted in  FIG. 5 , to synchronize the discharging and turning around operations of the H-bridge. Building on the example depicted in  FIG. 5 ,  FIG. 6  depicts an algorithm that may be executed by the microcontroller  12  to obtain pulse  100  and synchronize the discharging and turning around operations of the H-bridge. The sequence of steps composing the algorithm is indicated by the sequence of blocks  301  to  322 . 
     At block  301 , the algorithm sets a pulse period counter to 1, this counter is used to count the number of pulse period that have been completed. 
     The sequence of blocks  302  to  314  behaves as described previously for the flow chart depicted by  FIG. 5 , with the exception of block  312  where the algorithm proceeds to block  316 , instead of going back to block  302 , whenever the condition of having the pulse set to HIGH is not met. 
     At block  316 , the algorithm checks if the number of pulse period per polarity has been reached by verifying the value of the pulse period counter. If the number of pulse period per polarity has not been attained, the algorithm proceeds to block  318  where the pulse period counter is increased by 1 and then goes back to block  302  to start a new pulse period. On the other hand, if the number of pulse period per polarity has been attained, the algorithm proceeds to block  320  where the H-bridge is discharged followed by block  320  where the H-bridge is turned around. The sequence of N-channel FET activation or deactivation is detailed in the description of  FIG. 2 . N-channel FETs  140 ,  150 ,  160 ,  170  are activated by setting control lines  240 ,  250 ,  260 ,  270 , respectively, to HIGH and deactivated by setting those same control lines to LOW. N-channel FETs  120 ,  130  are activated or deactivated by deactivating or activating associated N-channel FETs  140 ,  160  respectively. After the H-bridge  19  has been turned around, the algorithm goes back to block  301  where the pulse period counter is set back to 1 and the process starts over. 
     To provide for variation in intensity, the microcontroller  12  receives the values of T 1 , T and the number of pulses per polarity from the main processor  30  through control line  230 . The main processor  30  determines these values according to the brightness level selected by a user of the scanner  2 , using a manual dimmer control  7  on the keyboard  6 . Thus the primary adjustment of the intensity is through the manual dimmer control  7  on the keyboard  6 . 
     The adjustability of the intensity may also be used to compensate for other operating conditions or changes in external parameters that may be encountered in use. To compensate for variation in the voltage source  9  during use, a voltage level monitor  32  is connected through monitor line  34  to the voltage source  9 . The main processor  30  adjusts the values of T 1 , T and the number of pulses per polarity according to variations in the available voltage level as the voltage source  9  discharges. Furthermore, an ambient light level detector  40  may be connected to the main processor  30  through monitor line  36  so that the main processor  30  may adjust the values of T 1 , T and the number of pulses per polarity according to the level of ambient light, e.g. lowering the brightness level in low ambient light conditions and raising it in high ambient light conditions. The scanner  2  keyboard  6  may be provided with controls to enable or disable this feature. 
     The main processor  30  selects the appropriate values of T 1 , T and the number of pulses per polarity by accessing, for example, a lookup table, provided with the main processor  30 , that correlates the desired brightness level and optionally the available voltage level and ambient light level, to the required values for T 1 , T and the number of pulses. These values are then transmitted to the microprocessor through control line  230 . 
     When the main processor  30  sets control line  230  to LOW, the microcontroller  12  discharges the EL display  14  by enabling the appropriate FETs and the microcontroller  12  is put to sleep. Current consumption of the circuit  10  may be reduced if the microcontroller  12  discharges the EL display  14  for a few ms and then turns OFF N-channel FETs  140 ,  160  so that they do not allow current to flow through any H-bridge resistors  124 ,  134 ,  144 ,  164 . 
     Although the present invention has been described by way of a particular embodiment thereof, it should be noted that modifications may be applied to the present particular embodiment without departing from the scope of the present invention and remain within the scope of the appended claims.