Abstract:
A method and apparatus are provided for fast heating cold cathode fluorescent lamps (CCFL). Specifically, values corresponding to the actual luminance of a CCFL are compared to a desired luminance level and, if it is determined that the CCFL is operating under start-up conditions, a boost power supply is applied to the CCFL until either the CCFL outputs the desired luminance level, or a timer determines that start-up conditions no longer exist.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to controllers for lamps used to illuminate liquid crystal displays (“backlights”) and the like and, in particular, to a method and apparatus for fast heating a cold cathode fluorescent lamp. 
     2. Description of the Related Art 
     Liquid crystal displays (LCD) provide a rugged and flexible display suitable for use in automotive applications. The LCD is backlit typically by a cold cathode fluorescent lamp (CCFL). Such fluorescent lamps are bright and relatively efficient and can be fabricated to provide even illumination over a large area. CCFL&#39;s are particularly useful to provide backlighting for illuminated vehicular displays. 
     Unfortunately, CCFL&#39;s are sensitive to temperature and vary in luminance as the passenger compartment and console warms up. During cold start conditions, for example, the initial luminance level of the CCFL may be unacceptably low to an operator of the vehicle. One method for compensating for this low luminance is to use a high-pressure self-heating type CCFL and to supply a “boost current” to the CCFL during startup. The boost current is an additional amount of lamp current above the normal maximum levels, resulting in an increased power supply, which is converted by the CCFL into heat to raise the lamp temperature, thereby facilitating increased lamp efficiency and a corresponding increased lamp luminance. 
     However, supplying a boost current increases the rate at which the mercury (Hg) inside the lamp is expended, causing premature failure resulting in extreme and sudden loss in luminance of the CCFL. For example, the life reduction of the CCFL operating at an ambient temperature greater than 30° C. can be defined by the equation                L   B     =         (       I   N       I   B       )     1.5     *     L   N               (   1   )                                
     where L B  is the life span of the CCFL using boost current, I B ; and L N  is the normal CCFL life span under normal or recommended operating current, I N . The life of the CCFL under boost current is significantly reduced further when the ambient temperature is below 30° C., as would be experienced during cold startup conditions. For example, it has been determined that the life span of the CCFL may be reduced by over 150 hours per start when the boost current is unnecessarily applied upon startup of the CCFL. 
     Accordingly, what is needed is a method and apparatus for supplying a boost current to a CCFL only when necessary during cold startup conditions. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention, the luminance output of a backlight is dynamically controlled by supplying power to the backlight, and determining whether the actual luminance level of the backlight is less than a commanded luminance level, at which point a boost current is automatically supplied to the backlight to increase the actual output. 
     These as well as other features and characteristics of the present invention will be apparent from the description which follows. In the detailed description below, preferred embodiments of the invention will be described with reference to the accompanying drawings. These embodiments do not represent the full scope of the invention. Rather the invention may be employed in other embodiments. Reference should therefore be made to the claims herein for interpreting the breadth of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is hereby made to the following figures in which like reference numerals correspond to like elements, and in which: 
     FIG. 1 is a perspective, exploded view of an automotive control console; 
     FIG. 2 is a simplified block diagram of the control circuitry in accordance with the preferred embodiment; and 
     FIG. 3 is a flow chart generally illustrating a method used to carry out the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an automotive console  10  includes a bezel  12  supporting user controls  14  and a display opening  16 . Position behind the display opening  16  is a liquid crystal display (“LCD”)  18  followed by a fluorescent backlight  20 . The fluorescent backlight  20  surrounds a light pipe  21  to provide a large area, even illumination commensurate with the area of the LCD  18 . The backlight provides light passing through the LCD  18  so as to make figures displayed on the LCD  18  visible through the opening  16  to a driver or passenger for all lighting conditions ranging from full sunlight to conditions of low ambient light. 
     A circuit card  22  may be positioned behind the backlight  20  to support control electronics in accordance with the preferred embodiment as well as the necessary control electronics for the LCD  18 . 
     Referring now to FIG. 2, feedback circuitry  23  includes a light sensor, preferably a photodiode  22  that detects a level of luminance emitted by the CCFL  20 , and supplies current having a feedback voltage level to an amplifier  24 . The feedback voltage, corresponding to the CCFL luminance, then travels through a resistor  26  and into the negative terminal of an error amplifier  28  which operates as an integrator as will be described. A voltage level corresponding to a commanded luminance signal  30  is input into the positive terminal of the error amplifier  28 . The error amp  28  outputs an output voltage V E  to terminal  31  of an inverter  30 , and further includes a feedback loop  32  having a resistor  34  connected in series with a capacitor  36  that are, in turn, connected in parallel with the error amplifier  28 . Under normal steady state operating conditions, the sensed luminance from the CCFL  20  will be equal to the commanded luminance, and the error amplifier  28  will maintain the output voltage V E  in accordance with the steady state. Typically, the error amplifier output V E  is operating somewhere within the inverter  30  input dynamic range of 0.5 to 2.5 volts in accordance with the preferred embodiment. The Inverter dynamic range of 0.5V to 2.5V at terminal  31  corresponds to Inverter Pulse Width Time Modulation of 0% to 100% of the CCFL current level commanded at terminal  33 . 
     At cold temperatures, however, the CCFL efficiency is severely decreased from room temperature operation by as much as 25:1. Under these circumstances, the feedback luminance even in steady state will likely be less than the commanded luminance because of limits of CCFL output, and the error amplifier  28  will transition to the positive rail voltage of approximately 9 volts. Accordingly, the output voltage V E  may be examined to determine whether the CCFL  20  is achieving the steady state commanded luminance. If not, a boost current will be supplied to the inverter to supply heat to the CCFL  20 , thereby increasing its efficiency and resulting in accelerated increased luminance, as will be described below. 
     With continuing reference to FIG. 2, a boost current circuit  38  includes a “power on, time out” element that  40  controls a boost circuit switch  42  having a “off” position  44 , and a “on” position  46 . Upon start-up of the CCFL  20 , the circuit  40  will activate the boost switch  42  to the on position  46  for a predetermined length of time as defined by the time-out element  40 , at which point the switch  42  will revert to the off position  44 . As will be described below, even though the boost switch  42  is in the “on” position  46 , boost current may or may not be supplied to the CCFL, according to the voltage level V E  that is output by the feedback circuit  23 . The output from the boost switch  42  feeds into a resistor  48  that is connected in parallel with a diode  50 . 
     A boost current amplifier  52 , also an integrator in accordance with the preferred embodiment, includes a negative terminal that is connected in series with the resistor  48 , and receives voltage output from the boost switch  42 , and a positive terminal that receives voltage output from the error amplifier  28  via a diode  54 . The output from diode  54  is further grounded at ground  56 , as is well known in the art. A capacitor  58 , connected in series with diode  50 , is further connected in parallel with the boost current amplifier  52 , thereby providing a feedback loop  60 . A resistor  62  is further connected in series with the boost current amplifier  52  at a location downstream of the feedback loop  60 . Voltage dividers  62  and  64  are selected such that when amplifier  52  is at its positive rail, the boost current signal at terminal  33  is at the CCFL boost current maximum. 
     The operation of the preferred embodiment will now be described with reference to the above-described circuitry. Operation commences upon start-up of the CCFL  20 , which operates at a given luminance level that is detected by the photodiode  22 . The output voltage from photodiode  22  is input to the amplifier  24 , travels through the resistor  26 , and into the negative terminal of the error amplifier  28 . A predetermined commanded luminance level is fed into the positive terminal of the error amplifier  28  and the corresponding output voltage V E  is dependent upon the integral of the difference between voltage values being input into the negative and positive terminals. For instance, if the voltage levels corresponding to the sensed luminance of the CCFL  20  is less than the voltage corresponding to the commanded luminance, the error amplifier  28  will ramp up so as to produce an output voltage V E  having a maximum value of nine volts. Once the voltages being input to the positive and negative terminals of the error amplifier  28  are equal, thereby indicating a steady state condition, the feedback loop  32  will maintain the output voltage V E  at the necessary level to maintain the steady state condition. When the lamp is producing the desired luminance, V E  will fall within a range of 0.5 to 2.5 volts in accordance with the preferred embodiment. When V E  is not within this range, it is likely that the CCFL  20  is cold and unable to produce the desired light output. 
     The output voltage V E  is additionally input into the diode  54  having a voltage of 5.1 volts. Accordingly, the input into the positive terminal of the boost current amplifier  52  is the difference between V E  and 5.1 volts (V E− 5.1). Therefore, when the boost switch  42  is off at  44 , 7.5 volts will be input into the negative terminal of the boost current amplifier  52 . Accordingly, under these circumstances, the amplifier  52  will output a zero voltage. This is because the positive terminal of amplifier  52  will necessarily be less than 7.5 volts, given that the maximum value of V E  is 9 volts, and that V E  is dropped by 5.1 volts at diode  54 , thereby resulting in a maximum input of 3.9 volts into the positive terminal of amplifier  52 . Accordingly, when the switch  42  is in the off position  44 , no voltage will be input into terminal  33  of the inverter  30 , and no boost current will therefore be supplied to the CCFL  20 . 
     If, on the other hand, the switch  42  is in the “on” position  46 , 2.3 volts will be input into the negative terminal of the boost current amplifier  52 . Accordingly, the amplifier  52  will output a boost current to the inverter  30  and correspondingly to the CCFL  20  when the input the positive terminal of the amplifier  52  is greater than 2.3 volts. Therefore, boost current will be supplied when switch  42  is on, and V E  is greater than 7.4 volts (2.3+5.1), which will occur when the detected luminance level of the CCFL is less than the commanded luminance, and V E  has had time to ramp to more than 7.4 volts, indicating that a steady state condition has not yet been achieved. Accordingly, boost current will only supplied to the CCFL  20  when the luminance output from the CCFL  20  is sufficiently low so as to allow time for V E  to ramp to a level greater than 7.4 volts. 
     Therefore, even if the boost switch  42  is in the on position  46 , no boost current will be supplied to the CCFL  20  if the CCFL luminance is equal to the commanded luminance. Additionally, even when the CCFL luminance is less than the commanded luminance, once V E  begins ramping down, thereby indicating that the CCFL luminance is approaching the commanded luminance, no boost current will be sent to the CCFL  20  when 1) V E  has ramped down to less than 7.4 volts, or 2) V E  has ramped up to a value less than 7.4 volts, signifying that the CCFL is operating at a level lower than, but not sufficiently lower than, the commanded luminance. Additionally, as V E  approaches and surpasses 7.4V, such that V +  is infinitesimally greater than V 31   on boost current amplifier  52 , a boost current level will be desired that is less than the maximum boost to maintain the commanded brightness. Accordingly, if less boost is required, the output from amplifier  52  ramps to a voltage that controls terminal  33  to a boost level required to maintain the commanded brightness. Accordingly, only the necessary magnitude of boost current is applied to maintain the commanded brightness, thereby extending the life of the CCFL  20 . 
     Furthermore, it should be understood that boost conditions may exist when V E  is at 7.4V such that V +  and V −  are equal at 2.3V in accordance with the preferred embodiment. This will occur when a boost current level between a no boost condition and a full boost condition is necessary. Therefore, if less boost current is required than the maximum in order to maintain the commanded brightness, V E  goes to 7.4V and the output from amplifier  52  goes to a voltage which controls terminal  33  to a boost level to maintain the commanded brightness. Accordingly, only the necessary magnitude of boost current is commanded to obtain the commanded luminance, thereby extending the CCFL life. 
     In accordance with the preferred embodiment, the boost current transitions from a “on” state to a “off” state at a relatively slow rate of change so as to prevent drastic changes or flickering of the luminance of the CCFL  20 . As mentioned above, the boost current will be turned off in one of two situations. The first situation occurs when the timeout circuit sets the boost switch  42  to the off position  44 , thereby generating 7.5 volts to the negative terminal of the boost current amplifier  52 . It should be apparent that the time-out function will permit boost current to be supplied for a limited duration in case certain elements within the circuitry are not working properly, thereby maximizing the life of the CCFL  20 . Under a time-out condition, the rate of voltage change output from the boost current amplifier  52  is determined by the following equation:                  Δ                 V       Δ                 T       =         (       V   E     -   5.1     )     =     7.5                 V           (     R   48     )          (     C   58     )                 (   2   )                                
     where ΔV is the change in voltage levels across the positive and negative terminals of the boost current amplifier  52 ; ΔT is the time necessary to transition from a boost current “on” to the “off” state; R 48  is the resistance of the resistor  48  in accordance with the preferred embodiment; and C 58  is the capacitance of the capacitor  58  in accordance with the preferred embodiment. 
     Substituting the appropriate values for the variables in Equation (1) using the situation where the error amplifier  28  is at the positive rail (V E =9 volts),                  3.9   -     7.5                 V           (     2.1                 M                 Ω     )          (     1                 μF     )         =       -   1.71                     V          /        second             (   3   )                                
     Because the error amplifier  28  is at the positive rail for this calculation, the voltage level of the boost current will decrease at a maximum rate of 1.71 volts per second when transitioning from the “on” state to the “off” state. 
     The second condition whereby the boost current will transition from “on” to “off”is when 1) the boost switch  42  is in the “on” position  46 , thereby supplying 2.3 volts to the negative terminal of the boost current amplifier  52 , and 2) V E  begins to decrease, such as is the first case when the luminance of CCFL  20  begins to approach the commanded luminance. Because, in this situation, V E  will have a value less than 7.4 volts, a magnitude of less than 2.3 volts will be input into the positive terminal of the boost current amplifier  52 . Accordingly, when V E  is less than 5.1V, the rate of voltage change is determined by                  0   -     2.3                 V         2.1                 M                 Ω   *   1                 μF       =       -   1.1                     V          /        second             (   4   )                                
     The gradual rate of voltage change of the boost current is also desirable during a transitory condition, whereby V E  is ramping down at a value less than 7.4 volts but greater than the steady state condition of 0.5 to 2.5 volts. During this condition, the boost current will be decreasing while V E  is ramping down to the steady state. 
     It should be appreciated by one having ordinary skill in the art that the chosen voltage, resistance, capacitance, and voltage drop values for the various elements of the circuit illustrated in FIG. 2 may be varied without departing from the scope and the spirit of the present invention. It should further be appreciated that other suitable indicators corresponding to the luminance levels of the CCFL  20  may be relied upon as an alternative to luminance. For instance, a thermal detector on the CCFL can be used in conjunction with a look up table to control terminal  31  for the commanded brightness, as would be appreciated by one having ordinary skill in the art. Therefore, the present invention could use terminal  31  to control the boost current. Accordingly, the present invention is not intended to be limited to the detection of luminance signals from the CCFL  20 . 
     Additionally, it should be further appreciated that while hardware elements are shown, in the circuitry in accordance with the preferred embodiment, it should be apparent to one having ordinary skill in the art that the functions performed by the hardware elements, such as integrators  28  and  52 , could also be performed by appropriately programmed microprocessors or other alternative software apparatus. Accordingly, in another embodiment, the combination of the boost current circuit  38  and feedback loop  60  are illustrated as being part of a software, or microprocessor, based system  51  shown in broken lines in FIG.  2 . Specifically, the analog V E  is fed through an analog-to-digital converter (not shown) and input into the microprocessor along with digital inputs from the timer  40 . The microprocessor then outputs a digital boost current signal if necessary, as described above, which is then fed through a digital-to-analog-converter (not shown) and input into terminal  33 . It should be appreciated that the microprocessor could be modified to perform the function of the timer  40 . 
     With reference now to FIG. 3, a method for controlling boost current  68  begins at process block  70  where the luminance level of the CCFL  20  is determined using photodiode  22  or other suitable apparatus. Next, at decision block  72 , it is determined whether the CCFL luminance is less than the desired luminance, such as would be the condition during a cold-startup situation. If the CCFL luminance is greater than or equal to the desired luminance, process  68  will proceed to step  74 , whereby the boost current is transitional to the “off” condition, before reverting the CCFL luminance determination step  70 . If, however, it is determined at decision block  72  that the CCFL luminance is less than the desired luminance, process  68  will continue to step  76 , whereby it will be determined whether a startup condition exists, as would be indicated by a “on” position of the timeout circuit  40 . If a startup condition exists, the boost current is turned on at process  78  before once again determining the luminance of the CCFL  20  at step  70 . If, on the other hand, a startup condition does not exist, process  68  will once again revert to step  74  to ensure that the boost current is in a “off” condition. 
     The invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangements included within the spirit and scope of the invention, as set forth by the appended claims.