Abstract:
A backlighting circuit for user interface in an electronic device includes at least one light emitting diode optically coupled to the user interface wherein the at least one light emitting diode provides backlighting for the user interface. A current source is electrically coupled in series with the at least one light emitting diode wherein the current source controls a current through the at least one diode. In addition, a brownout detection circuit determines a brownout condition for the user interface responsive to the current through the diode. Related communications devices and methods are also discussed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of electronics and more particularly to backlighting circuits and methods for electronic devices. 
     BACKGROUND OF THE INVENTION 
     In general, a cellular radiotelephone includes a transceiver for transmitting and receiving radio communications to and from a radio base station, a controller for controlling the transmission and reception of the radio communications, and a user interface. More particularly, the user interface can include a keypad for accepting data input from a user and a visual display (such as a liquid crystal display) for providing information to the user. Furthermore, many cellular radiotelephones are battery operated allowing mobility during use. 
     In addition, backlighting can be used to illuminate the user interface. For example, one or more light emitting diodes (LEDs) can be used to provide backlighting to the user interface. In particular, keypad backlighting has been implemented using arrays of yellow-green (570 nm) light emitting diodes (LEDs). An array including a plurality of pairs of light emitting diodes (LEDs) 33 has been used wherein each of the LEDs in a pair are connected in series and wherein each of the series connected pairs of light emitting diodes are connected in parallel as shown in FIG. 1. The six parallel light emitting diode circuits are switched ON or OFF through the common NPN transistor 21. 
     The current through the collector of the common NPN transistor 21 is controlled using the voltage reference made up of the resistors 23 and 25, and the diode 27. A resistor 29 is also provided between the emitter of the transistor 21 and ground. Furthermore, a resistor 31 is connected in series with each of the pairs of series connected LEDs 33. As will be understood by one having skill in the art, the voltage at the base of the transistor 21 can be determined using the formula: 
     
         V.sub.BASE =V.sub.BE +V.sub.R 
    
     where V BASE  is the voltage at the base of transistor 21, V BE  is the voltage between the base and the emitter of transistor 21, and V R  is the voltage across the resistor 29. Increasing the collector current will thus increase V R  thereby reducing V BE  and limiting the collector current. The transistor 21 thus acts as a simple current source and operates in the linear forward active region of the transistor. 
     The collector current may be affected by a number of variables including the output impedance of the BACKLIGHT source signal; the process variations and temperature dependence of the forward voltage of diode 27; the process variations and temperature dependence of V BE  ; and the temperature coefficients of and tolerances of resistors 23 and 25. Given these uncontrolled process and environmental variables, the collector current through transistor 21 may be unreliable without allowing for relatively wide tolerances. 
     When using a 5 cell rechargeable battery to operate the cellular radiotelephone, sufficiently wide tolerances may be available. In general, a 5 cell rechargeable battery has a typical operating voltage of 5.0V to 7.0V. This operating range may provide ample voltage over the life of the battery to overcome the forward voltage (V F ) of the two series LEDs 33, the collector-emitter voltage of the NPN transistor 21, and the voltage across the degeneration resistor 29. Assuming that the saturation current (V SAT ) through transistor 21 is 200 mV, and ignoring the effects of the degeneration resistor 29, the minimum battery voltage required to guarantee backlighting can be calculated as follows: 
     
         V.sub.RD +2(V.sub.FD)+V.sub.CE =0.0V+2(2.2V)+2.2V+0.2V=4.6V. 
    
     where V RD  is the voltage across the diode resistor 31, V FD  is the forward voltage across one of the LEDs 33, and V CE  is the collector to emitter voltage of the transistor 21. 
     The forward voltage V FD  of a light emitting diode (LED) 33 is dependent on the conduction current through the LED, the ambient temperature, and the process variations from diode to diode. Accordingly, the LED forward voltage is typically less than the 2.2V listed in the manufacturer data sheets. FIG. 2 is a graph illustrating data collected in the laboratory using the backlighting circuit of FIG. 1 implemented with six pairs of series connected LEDs with the LED pairs being connected in parallel wherein each of the LEDs is a yellow-green 570 nm LED. The data used to generate this graph is provided below in Table 2. 
     
                                           TABLE 2__________________________________________________________________________V         V       V       V       V80 dgs I   60 dgrs         I   25 dgrs                 I   0 dgrs                         I   (-)30 dgrs                                  I__________________________________________________________________________10  3.548 10.357     3.604         10.091             3.729                 10.308                     3.819                         10.42                             3.94 10.1520  3.653 20.594     3.708         20.291             3.821                 20.32                     3.905                         20.312                             4.03 20.5530  3.727 30.522     3.782         30.555             3.886                 30.05                     3.968                         30.408                             4.09 30.30140  3.788 40.079     3.84         40.13             3.94                 40.413                     4.018                         39.801                             4.15 40.59950  3.846 50.413     3.894         50.012             4   50.815                     4.07                         50.808                             4.2  50.27560  3.899 60.461     3.95         60.645             4.04                 60.44                     4.11                         60.399                             4.25 60.56370  3.95 70.314     3.992         70.141             4.08                 70.033                     4.15                         70.105                             4.3  70.55980  3.99 80.111     4.04         80.213             4.13                 80.06                     4.19                         80.601                             4.34 80.665__________________________________________________________________________ 
    
     The voltage difference between the circuit input V SWDC  and the voltage at the collector of the transistor 21 was measured for various collector currents and temperatures for applications designed for a diode conduction current in the range of 8 mA to 12 mA (48 mA to 72 mA total) for a typical radiotelephone operating according to the DAMPS standard using a 5 cell rechargeable battery. As shown, a minimum voltage of 4.3 volts may be required to maintain forward conduction at cold temperatures in the range of -30° C. These curves also indicate that the compliance limits of the circuit may be exceeded as the voltage drops below 4.3V thereby reducing the current through the LEDs. In this condition, the user may notice keypad backlight dimming or &#34;brownout&#34;. 
     The backlighting circuit of FIG. 1 may provide acceptable performance for a radiotelephone powered by a 5 cell rechargeable battery as discussed above. This backlighting circuit, however, may not provide acceptable performance when used in a radiotelephone powered by a 4 cell NiCD/NiMH rechargeable battery which may provide a normal operating voltage in the range of 4.0V to 5.7V with an &#34;end-of-life&#34; voltage set at 4.2V. A typical discharge curve for a 4 cell battery is illustrated in FIG. 3. As shown, the end-of-life voltage is set at 4.2V. 
     Assuming that the saturation voltage of transistor 21 is 200 mV and assuming that there is a 4.3V drop across the LED array, a minimum of 4.5V is required to guarantee consistent backlighting operation. The LEDs would thus provide relatively consistent lighting at the upper end of the battery voltage range, but the LEDs could be expected to fade or turn off as the battery voltage drops below 4.5V. Furthermore, LED fading could be expected to occur at higher battery voltages in low temperature conditions and/or with LEDs having less than the average forward voltage as a result of standard process variations. 
     Raising the &#34;end-of-life&#34; voltage setting can reduce the occurrence of backlight brownout. For example, the nominal &#34;end-of-life&#34; voltage can be set to 4.6V to provide consistent backlighting operation. As shown in FIG. 3, however, this approach could reduce the useful operating time for the battery by as much as 25%. 
     Alternately, the LED array can be arranged with all of the LEDs in parallel thereby reducing the voltage drop across the LED array. This arrangement, however, may double the current consumed by the backlighting circuit and double the heat generated thereby. The power consumed by the backlighting circuit is thus undesirably increased. Accordingly, there continues to exist a need in the art for improved backlighting circuits. 
     SUMMRY OF THE INVENTION 
     It is therefore an object of the present invention to provide improved backlighting circuits and methods for user interfaces on electronic devices. 
     This and other objects are provided according to the present invention by a backlighting circuit including at least one light emitting diode optically coupled to a user interface of an electronic device wherein the at least one light emitting diode provides backlighting for the user interface. A constant current source is electrically coupled in series with the at least one light emitting diode wherein the current source controls the current through the at least one diode. In addition, a brownout detection circuit determines a brownout condition for the user interface responsive to the current through the at least one light emitting diode. The brownout detection circuit thus provides the information that the backlighting circuit has entered a brownout condition. Accordingly, a controller coupled to the brownout detection circuit can turn the current source off in response to a determination that the brownout condition has occurred. Alternately, the control circuit can turn off the electronic device allowing an orderly shutdown thereof. 
     More particularly, the brownout detection circuit can include an analog-to-digital converter, or a comparator. The analog-to-digital converter provides a signal representing the current through the at least one light emitting diode, while the comparator provides an indication that the current through the at least one diode has dropped below a predetermined threshold. Accordingly, the use of a comparator in the brownout detection circuit allows the use of an interrupt service routine in the controller thereby reducing the operations required of the controller to detect a brownout condition. 
     The current source can control the current through the at least one diode in response to a comparison between a reference signal and a feedback signal from the current source. More particularly, the current source can include a transistor electrically coupled in series between the diode and a feedback node, and a program resistor electrically coupled in series between the feedback node and a ground voltage node. In addition, an operational amplifier includes a first input electrically coupled to the reference signal, a second input electrically coupled to the feedback signal of the feedback node, and an output electrically coupled to a control electrode of the transistor. Accordingly, the operational amplifier drives the control node of the transistor in response to the comparison of the reference signal and the feedback signal. The current through the at least one light emitting diode can thus be controlled within precise tolerances as long as the battery voltage is above a predetermined threshold. Moreover, the at least one light emitting diode can include a plurality of pairs of series coupled light emitting diodes wherein each of the pairs of series coupled light emitting diodes is electrically coupled in parallel. By connecting LEDs in series, the current needed to drive the LED array can be reduced. 
     The backlighting circuit discussed above can thus be advantageously incorporated in a radio communications device. For example, the backlighting circuit can be used to provide illumination for a user interface such as a keypad or a liquid crystal display. Moreover, the backlighting circuit can be used to increase the operating life of a four-cell battery used to power the radio communications device. In other words, a four-cell NiCD/NiMH rechargeable battery having a normal operating voltage in the range of 4.0V to 5.7V can be used in combination with the backlighting circuit of the present invention to provide consistent illumination and to reduce brownout. 
     According to an alternate aspect of the present invention, a method for providing backlighting for an electronic device including a user interface and at least one light emitting diode optically coupled thereto includes the step of determining a brownout condition for the at least one diode responsive to a current through the at least one diode. Upon determination of a brownout condition, the diode can be turned off, or the electronic device can be turned off. In particular, the step of determining the brownout condition can include determining that the current through the at least one diode has dropped below a predetermined threshold. This method can further include the steps of comparing a feedback signal representative of a current through the at least one diode with the reference signal and controlling a current through the at least one diode in response to the comparison between the reference signal and the feedback signal. 
     According to the circuits and methods discussed above, consistent backlighting can be provided for an electronic device using batteries with relatively low operating voltages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating a backlighting circuit for a keypad of a cellular radiotelephone according to the prior art. 
     FIG. 2 is a graph illustrating the voltage drop across the light emitting diode array of the backlighting circuit of FIG. 1. 
     FIG. 3 is a graph illustrating a discharge curve for a 4 cell battery according to the prior art. 
     FIG. 4 is a block diagram illustrating a cellular radiotelephone according to the present invention. 
     FIG. 5 is a circuit diagram illustrating a first backlighting circuit for the cellular radiotelephone of FIG. 4. 
     FIG. 6 is a circuit diagram illustrating a second backlighting circuit for the cellular radiotelephone of FIG. 4. 
     FIG. 7 is a graph illustrating the operation of the brownout detection circuit of FIG. 6. 
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     FIG. 4 is a block diagram illustrating a cellular radiotelephone according to the present invention. As shown, this cellular radiotelephone includes a transceiver 51 for transmitting and receiving radio communications to and from a radio base station, a controller 53 for controlling the transmission and reception of radio communications, and a user interface 55 for accepting information from the user and/or for providing information to the user. The cellular radiotelephone of FIG. 4 also includes a speaker 57 for providing voice communications to the user, and a microphone 59 for accepting voice communications from the user. As will be understood by those having skill in the art, the term radiotelephone can also be defined to include portable electronic devices such as data phones and personal digital assistants that combine communications and computing capabilities. 
     More particularly, the user interface 55 includes a keypad 61, a visual display such as a liquid crystal display (LCD) 63, and a backlighting circuit 65. The backlighting circuit is used to illuminate the keypad 61 and/or the liquid crystal display 63 for use in the dark. A first embodiment of the backlighting circuit according to the present invention is illustrated in FIG. 5. As shown, the backlighting circuit includes an array of light emitting diodes 71 wherein pairs of the LEDs are connected in series and each of the series connected pairs of LEDs are connected in parallel. In addition, a LED resistor 73 is connected in series with each series connected pair of LEDs 71. Each of the LED resistors is connected to the battery voltage VBAT, and the second of each of the diodes of each pair is connected to the collector of the NPN transistor Q1 which is used to control the current through the diode array. The emitter of the NPN transistor Q1 is connected to a feedback node 75, and a program resistor 77 is connected between the feedback node 77 and the ground voltage. Accordingly, the current through the LED array passes through the NPN transistor Q1 and the program resistor 77 to ground. 
     The current through the LED array and the NPN transistor is controlled by providing a control signal at the base of the NPN transistor Q1. This control signal is generated by the operational amplifier 79 in response to a comparison of the reference signal from the reference signal generator 80 and the feedback signal from the feedback node 75. As shown, the operational amplifier includes a first input electrically coupled to the reference signal generator, a second input electrically coupled to the feedback node, and an output electrically coupled to the base of the NPN transistor Q1. Moreover, a brownout detection circuit such as an Analog-To-Digital converter (ADC) 81 can be used to detect that the backlighting circuit has entered a brownout condition. 
     In addition, the operational amplifier 79 and the ADC 81 can be implemented in an Application Specific Integrated Circuit (ASIC). As shown, the vertical dotted line of FIG. 5 separates the elements of the backlighting circuit implemented in the ASIC to the left, and the elements of the backlighting circuit implemented with discrete components to the right according to one embodiment of the present invention. In FIG. 5, the pin-outs 83a, 83b, and 83c indicate connections between portions of the circuit implemented inside the ASIC and portions of the circuit implemented outside the ASIC. 
     The operational amplifier 79 is preferably configured as a voltage follower wherein an output thereof drives the base of the NPN transistor Q1. The feedback node 75 is connected to the emitter of the transistor Q1, and the feedback signal provided from this node to the operational amplifier thus locks the emitter voltage V EMITTER  to the internal reference voltage V REF . The emitter current and the total current through the light emitting diode array can thus be set by selecting the program resistor 77 according to the following formula: 
     
         I.sub.EMITTER =V.sub.EMITTER /R.sub.PROGRAM =V.sub.REF /R.sub.PROGRAM 
    
     Because V REF  can be obtained using the ASIC bandgap reference, the emitter voltage and the output current will remain relatively constant over temperature and battery voltage until the current source begins to loose compliance as the battery voltage drops. As will be understood by those having skill in the art, an ASIC bandgap reference is a precision voltage reference which provides a stable output over temperature and input supply variations. 
     Once the battery voltage drops to the compliance limits of the current source, the opamp output current will increase saturating the external NPN transistor and the emitter voltage of the transistor will begin to drop. By maintaining the emitter voltage within relatively high tolerances over process and environmental conditions, the emitter voltage at the feedback node 75 can be measured and used to indicate that the backlighting circuit is in a brownout condition. In particular, the input of the ADC 81 can be coupled to the feedback node 75 allowing the feedback signal to be monitored by the controller 53 which can include the system processor. In other words, when the binary output of the ADC 81 drops below a predetermined threshold, a brownout condition is recognized by the controller 53. The controller can then either turn off the operational amplifier 79 thereby turning off the current through the backlighting circuit or turn off the whole radiotelephone allowing an orderly shutdown thereof. 
     The brownout detection can be made more accurate with a dynamic calibration using the internal non-volatile memory 67 such as an E2ROM. The emitter voltage detected by the ADC 81 can deviate from a nominal value as a result of: (i) variations in the reference voltage V REF  caused by internal resistor divider tolerances; (ii) input offset voltage in the operational amplifier causing V EMITTER  to vary; and (iii) offset error in the ADC 81. 
     A reference can be obtained for the emitter voltage by reading the output of the ADC when the battery is charged to a voltage of greater than 5.0V. This reference can then be stored in the memory and used as a relative comparison value. A software algorithm can then be implemented in the controller that compares current emitter voltage values generated by the ADC with the reference stored in memory. When the value read by the ADC is less than the reference stored in memory by a predetermined number of bits, the controller can recognize a brownout condition and determine that the battery has reached an &#34;end-of-life&#34; condition. For example, when implementing the operational amplifier 79 and the ADC 81 in an ASIC, the emitter voltage can be held at 120 mV+/-10%(+/-12 mV), and the ADC can have a resolution of 3.0V/255 which is equal to approximately 12 mV. Accordingly, a decrease in the ADC output by 4-bits relative to the reference could be used to indicate backlighting brownout. 
     An alternate embodiment of a backlighting circuit according to the present invention is illustrated in FIG. 6. In this embodiment, the brownout detection circuit is implemented using the comparator 91 which can also be implemented as a part of the ASIC. As shown, the positive input to the comparator is connected to the feedback node 75, and the negative input to the comparator is connected to the comparison node 93 wherein a comparison voltage V COMPARE  is generated by the voltage divider including resistors 95 and 97. Accordingly, the comparator will generate a high-to-low transition when the feedback signal (emitter voltage) falls below the comparison voltage thereby signaling a low-current or brownout condition for the backlighting circuit. 
     The comparison voltage can be derived using the V REF  signal generated by the reference voltage generator 80 (such as the ASIC bandgap reference) and the resistor divider including resistors 95 and 97. Because the resistor divider is implemented within the ASIC, the resistors 95 and 97 can have matched temperature coefficients. Accordingly, the voltage delta V COMPARE  -V REF  can be relatively constant over temperature and battery voltage, and any remaining errors would be due to the resistor tolerances of the ASIC manufacturing process and input offset voltages of the comparator and opamp. An effective brownout detection circuit can thus be implemented by setting the voltage delta V COMPARE  -V REF  to be greater than the cumulative error. 
     The brownout detection circuit of FIG. 6 has the advantage that the output of the comparator can be used to drive an interrupt of the controller. In other words, the controller is not required to poll the binary output of an ADC thereby reducing processing time required to detect the brownout condition. In other words, the comparator simply indicates to the controller whether the feedback signal (emitter voltage) is in tolerance or out of tolerance. This arrangement can simplify the controller software by reducing the need to read and interpret data generated by an analog-to-digital converter. The output of the comparator can thus be provided to an interrupt of the controller or multiplexed through interrupt control logic also included in the ASIC. The brownout response algorithm can thus be moved to an interrupt service routine (ISR) thus relieving the controller of the need to poll the brownout detection circuit. 
     The operation of the brownout detection circuit of FIG. 6 is illustrated in the graph of FIG. 7. As shown at time t=0, the feedback signal (emitter voltage or V EMITTER ) is slightly less than V REF . This difference is due to the error caused by the input offset voltage in the comparator and the operational amplifier, and in practice, the emitter voltage could be greater than V REF . In addition, the low-current indicator (output of the comparator) is high indicating that the emitter voltage is within tolerance. As the time increases, however, the battery discharges until the current source reaches the limits of compliance at time t=t a . In other words, the base current into the base of transistor Q1 has increased until the transistor has reached saturation and the emitter voltage begins to fall. For t&gt;t a , the emitter voltage decreases with the battery voltage until the emitter voltage is equal to the comparison voltage V COMPARE  at time t b . At this point, the output of the comparator transitions from high-to-low indicating a brownout condition for the backlighting circuit. This transition can be used to interrupt the controller. 
     The use of the brownout detection circuits discussed above allows the backlighting circuit to operate until the battery can no longer support its operation without regard to external conditions because the brownout is detected based on the current through the LED array as opposed to the battery voltage. The radiotelephone controller can thus monitor the battery voltage and/or the brownout detection signal. Accordingly, the controller can determine a battery end-of-life when the 4-cell battery reaches 4.2V. In addition, the controller can determine a battery end-of-life condition before the backlighting begins to dim. The brownout detection circuit thus allows consistent backlighting while reducing unnecessary determinations that the battery has reached an end-of-life condition. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.