Abstract:
A personal electronic device includes and inverter having a single inductor for powering an EL lamp and a buzzer. The lamp and the buzzer are coupled together to the output of the inverter and are in parallel with each other or are coupled in series between a source of direct current and ground.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to personal electronic devices, such as wristwatches, pocket pagers, calculators, and organizers, having an EL lamp and a piezoelectric buzzer powered from a single inductor. 
     An EL lamp is essentially a capacitor having a dielectric layer including a phosphor powder which glows in the presence of a strong electric field and a very low current. The dielectric layer is held between two electrodes, one of which is transparent. Because the EL lamp is a capacitor, an alternating current (AC) must be applied to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage and the current through the EL lamp ceases. 
     For personal electronic devices such as wristwatches, pocket pagers, and cellular telephones, an EL lamp is driven by an inverter which converts direct current from a small battery into alternating current. In order for an EL lamp to glow sufficiently, a peak to peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. 
     While there are many ways to increase voltage, e.g. by using a transformer or a voltage doubler, most applications for an EL lamp use what is known as a “flyback” inverter in which the energy stored in an inductor is supplied to the EL lamp as a small pulse of current at high voltage. The inverter typically operates at high frequency (4 khz. or more) to minimize the size of the magnetics, i.e. the inductor or transformer, in the inverter. 
     FIG. 1 is based upon the disclosure of U.S. Pat. No. 4,527,096 (Kindlmann). When transistor  14  turns on, current flows through inductor  15 , storing energy in the magnetic field generated by the inductor. When transistor  14  shuts off, the magnetic field collapses at a rate determined by the turn-off characteristics of transistor  14 . The voltage across inductor  15  is proportional to the rate at which the field collapses ( δi / δt ). Thus, a low voltage and large current is converted into a high voltage at a small current. 
     The current pulses are coupled through diode  16  to the DC diagonal of a switching bridge having EL lamp  12  connected across the AC diagonal. Assuming that transistors  18  and  19  are conducting, the same amount of energy is supplied to lamp  12  each time transistor  14  turns off and, therefore, the voltage on the lamp is pumped up by a series of current pulses from inductor  15  as transistor  14  repeatedly turns on and off. Diode  16  prevents lamp  12  from discharging through transistor  14 . If transistor  14  were switched on and off continuously, the pulses would charge lamp  12  to the maximum voltage available from inductor  15 , e.g. about 140 volts. Since an EL lamp needs an alternating current or a variable direct current, the lamp would glow initially and then extinguish when the capacitance of the lamp became fully charged. 
     To avoid this problem, the transistors in opposite sides of the bridge alternately conduct to reverse the connections to lamp  12 . The bridge transistors switch at a lower frequency than the frequency at which transistor  14  switches. The four bridge transistors are high voltage components, adding considerably to the size and cost of the circuit. In addition, the circuit is not single ended, i.e. one cannot ground one side of lamp  12 , which is preferred. 
     One could use separate inverters for driving an EL lamp and a buzzer. In many applications, particularly watches, a second inverter is difficult to add, primarily because of the cost of a second inductor. It is known in the art to power a piezoelectric buzzer and an EL lamp from a single flyback inverter. FIG. 2 is based upon the disclosure of U.S. Pat. No. 4,529,322 (Ueda). In inverter  20 , transistor  14  is switched on and off at about 8 khz. to charge lamp  12 . When transistor  21  is conducting, lamp  12  is discharged. 
     There is an average DC bias across lamp  12 , approximately equal to one half the maximum voltage, because the lamp is charged in only one direction and then discharged. DC bias on an EL lamp can cause corrosion and shorting of the electrodes of the lamp, particularly at elevated temperature and humidity, decreasing the life of the lamp. 
     Another problem with inverter  20  is that transistors  21  and  22  draw current from terminal  13  through inductor  15 . This current is wasted since it does not contribute to powering lamp  12 , thereby reducing the efficiency of the inverter and decreasing battery life. 
     A third problem with inverter  20  is that switch  25  is necessary for isolating piezoelectric buzzer  26  from the high voltage pulses applied to lamp  12 . High voltage pulses stress the piezoelectric element and can cause failure. In the Ueda patent, switch  25  is one of two ganged switches actuated by undisclosed means. 
     FIG. 3 is based upon the disclosure of U.S. Pat. No. 5,313,117 (Kimball). Inverter  30  includes transistor  31 , inductor  32 , and transistor  33  connected in series between voltage source  34  and ground. Inductor  32  is alternately connected through transistors  35  and  37  to lamp  27 . Diode  36  is connected in a series with transistor  35  for preventing the transistor from operating in the inverse active mode, i.e. preventing transistor  35  from conducting current from the ground terminal through the forward bias based-collector junction when the voltage on lamp  27  is negative. Similarly, diode  38  prevents transistor  37  from operating in the inverse active mode when the voltage on lamp  27  is positive and greater than the battery voltage. The transistors, resistors, and diodes are implemented on a single chip. The inductor and capacitors are external devices coupled to the chip on a printed circuit board. 
     External logic circuitry provides a series of pulse bursts alternately on output lines “X” and “Y”. These bursts are coupled to the bases of transistors  31  and  33  and cause the transistors to conduct alternately, thereby providing positive and negative half wave voltages to lamp  27 . Inverter  30  produces alternating current at a single ended output and one side of lamp  27  can be grounded. 
     It remains a problem in the art to drive an EL lamp and a buzzer from a single inverter with as few components as possible with no waste current and with no DC bias. 
     In view of the foregoing, it is therefore an object of the invention to provide a personal electronic device with an inverter having a single inductor for providing alternating current to an EL lamp and direct current pulses to a buzzer. 
     Another object of the invention is to provide a personal electronic device with an inverter having no waste current. 
     Another object of the invention is to provide a personal electronic device with an inverter having no DC bias. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved in this invention in which a personal electronic device includes an inverter having an output coupled to the junction of an EL lamp and a buzzer. The lamp and the buzzer are coupled in parallel with each other to ground or are coupled in series between a DC supply and ground. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic of an inverter of the prior art; 
     FIG. 2 is a schematic of an inverter of the prior art for powering an EL lamp and a piezoelectric buzzer; 
     FIG. 3 is a schematic of an inverter of the prior art having a single ended output; 
     FIG. 4 is a schematic of an inverter constructed in accordance with the invention; 
     FIG. 5 is a schematic of an inverter constructed in accordance with an alternative embodiment the invention; 
     FIG. 6 is a schematic of an inverter constructed in accordance with a preferred embodiment of the invention; and 
     FIG. 7 is a variation of the embodiment shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4 illustrates an inverter constructed in accordance with the invention in which an EL lamp and a buzzer are coupled in parallel between the output of an inverter and ground. SCR  41  isolates buzzer  42  from the pulses produced by inductor  43  and transistor  44 . The pulses are coupled by diode  46  to bridge  47  where they are converted into alternating current through lamp  48 . Transistor  51  operates in synchronism with transistor  44  to discharge any charge accumulated on buzzer  42 . Buzzer  42  sounds when a high voltage is applied to signal input  53 . Because of the alternating current through lamp  48 , no separate discharge circuitry is needed and there is no waste current. Transistor  51  does not produce waste current because the transistor operates in synchronism with transistor  44 . 
     FIG. 5 illustrates an inverter constructed in accordance with another aspect of the invention in which the EL lamp and the buzzer are coupled in series across the low voltage supply. In this embodiment, transistor  56  isolates buzzer  54  from the pulses produced by inductor  43  and transistor  44 . The pulses are coupled by diode  46  to the junction of bridge  47  and buzzer  54 . When transistor  56  is non-conducting, the pulses are applied to EL lamp  48  by bridge  47  to produce an alternating current through the lamp. 
     For many applications, lamp  48  has a capacitance of about 3 nf and buzzer  54  has a capacitance of 10-15 nf. When transistor  56  is conducting, the pulses are coupled substantially through buzzer  54 , which has a much lower impedance than lamp  48 . Thus, only the buzzer appears to operate. Because of the impedance difference, only transistor  56  is needed to select between operating the lamp and operating the buzzer, simplifying the circuit. The operation of transistor  56  need not be synchronized with the operation of transistor  44 , further simplifying the circuitry. 
     In FIG. 6, the EL lamp and the buzzer are coupled in series between supply and ground. Junction  61  is coupled to output  63  of an inverter constructed as illustrated in FIG.  3 . Lamp  64  is lit as long as the inverter is operating and as long as transistor  65  is not conducting. Applying a signal to input  66  causes transistor  65  to conduct and substantially all the current from the inverter goes through buzzer  69 , sounding the buzzer and extinguishing lamp  64 . 
     In FIG. 7, EL lamp  71  is connected to the AC diagonal of bridge  72  and the DC diagonal of the bridge is coupled to junction  74  by diode  75  and to ground. Buzzer  76  is coupled to junction  74  by SCR  77  and is discharged by transistor  78 , which is connected in parallel with the buzzer. The base of transistor  78  is coupled to the base of transistor  79  and the two transistors switch together, thereby avoiding waste current when discharging buzzer  76 . Buzzer  76  is sounded by turning on SCR  77 , causing pulses from inductor to be coupled to the buzzer. Bridge  72  is off, preventing lamp  71  from being charged or discharged. 
     The invention thus provides an inverter for powering an EL lamp or a buzzer by providing AC for the lamp and pulsed DC for the buzzer. Waste current is eliminated by eliminating transistors coupled in parallel with the switch transistor in series with the inductor. 
     Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, one can interchange power and ground or substitute PNP transistors for NPN transistors, and vice-versa.