In recent years, diverse types of wristwatches, from medium-priced to low-priced types, have been introduced in an effort to cope with numerous market needs. With the spread of personal computers, a lamp based on an electroluminescent (hereinafter EL) device has evolved as a thin lamp for liquid-crystal displays. The EL lamp is thin, adopts of field emission, and has a beautiful glow color. Clocks, in which an EL lamp is incorporated and which have a digital, combination, or analog displays, have been put on the market as multi-function clocks.
An example of the system configuration will be described in conjunction with a drawing. FIG. 2 shows an example of system configuration for an EL driver 1 including a clock mechanism with a known EL lamp. A general example of a system configuration for a clock is illustrated. Reference numeral 41 denotes a clock means. 42 denotes a time display means corresponding to a motor with indicators for an analog clock or a liquid-crystal display for a digital clock. 43 denotes a time display means driving means. S44 denotes a time display means driving signal that is provided by the time display means driving means 43. These components are involved in a time display in a clock. 1 denotes an EL driver. 2 denotes a battery serving as a first power source. 3 denotes a waveform shaping means. 4 denotes a second supply voltage producing means. 5 denotes an EL driving means. 9 denotes an EL lamp. The battery 2 serves as a first power source for the EL driver and a power source for the clock means 41. V11 denotes first supply voltage. V12 denotes second supply voltage. S14 is a driving input signal produced by the waveform shaping means 3. S15 denotes a driving output signal causing the EL lamp 9 to glow. S27 denotes a second supply voltage boosting signal produced by the waveform shaping means 3. The waveform shaping means 3 uses the first supply voltage 11 supplied from the battery 2 to produce the driving input signal S14 used for driving the EL lamp and the second supply voltage boosting signal S27. For causing the EL lamp to glow, alternating current (hereinafter AC) voltage having an amplitude of several tens of volts must be applied. The second supply voltage producing means 4 is used as a power source to produce the second supply voltage V12 for use in driving the EL lamp. The second supply voltage V12 is then fed to the EL driving means 5. The EL driving means 5 receives the driving input signal S14 from the waveform shaping means 3, produces the driving output signal S15 that is an AC signal having the same amplitude as the second supply voltage V12, applies the driving output signal S15 to the EL lamp 9, and thus causes the EL lamp 9 to glow.
Next, the system configuration will be described in detail in conjunction with drawings. FIG. 3 is a schematic diagram showing the circuitry of a general example constructed according to the system configuration shown in FIG. 2. When a semiconductor device is used, a CMOS device will be adopted. FIG. 5 is a timing chart concerning FIG. 3. To begin with, the configuration will be detailed in conjunction with FIG. 3. Reference numeral 41 denotes a clock means for providing timing pulses for small-sized electronic equipment. 53 denotes a crystal oscillator for generating a reference signal for the clock means 41. S21 denotes a reference signal provided by the crystal oscillator.
Reference numeral 3 denotes a waveform shaping means composed of a frequency divider 8, a NAND 60, and inverters 61 and 62. S22 denotes a fraction signal provided by the frequency divider and fed to the NAND 60 and inverters 61 and 62. S27 is a second supply voltage boosting signal provided by the NAND 60. 4 denotes a second supply voltage producing means. S14 denotes a driving input signal provided by the inverters 61 and 62. 5 denotes an EL driving means. 30 denotes a level shifter constituting the EL driving means. V12 denotes a second supply voltage provided by the second supply voltage producing means 4 and fed to the EL driving means 5. S15 denotes a driving output signal provided by the EL driving means 5. 9 denotes an EL lamp. 64 denotes a switching transistor that inputs the second supply voltage boosting signal S27 through the gate thereof. 63 denotes an inductor. 65 denotes a diode. 66 denotes a smoothing capacitor. When the EL driver 1 is constructed with a semiconductor device, the inductor 63 and smoothing capacitor 66 are connected externally because they cannot be mounted or cannot be efficiently mounted in a semiconductor device.
Operations will be described in conjunction with FIGS. 3 and 5. The reference signal S21 provided by the crystal oscillator 53 in the clock means 41 is fed to the frequency dividing means 8, whereby a plurality of fraction signals with different frequencies are produced. The NAND 60 receives one of or a plurality of signals of several to several tens of kilohertz, and supplies the second supply voltage boosting signal S27. The frequency and duty ratio of the second supply voltage boosting signal S27 are set to the values permitting optimal boosting efficiency in the second supply voltage producing means 4. With the second supply voltage boosting signal S27, the switching transistor 64 is turned on or off so that the inductor 63 can generate a counter electromotive force. The backflow of the counter electromotive force generated by the inductor 63 is prevented by the diode 65. At the same time, the counter electromotive force is smoothed by the smoothing capacitor 66 so that it can be used as supply voltage for driving the EL lamp, and then fed to the EL driving means 5.
The inverter 61 receives a timing signal of several tens to several hundreds of hertz and produces a driving input signal S14. The two inverters 61 and 62 are mounted because push-pull configuration is adopted for driving the EL lamp 9. The EL lamp 9 is equivalent to a large capacitor in an electric circuit. When AC voltage is applied to both the electrodes of the EL lamp 9, the EL lamp 9 glows. For more efficient glowing, it is better to apply voltage to both the electrodes at intervals of several tens to several hundreds of hertz. For driving both the electrodes, two EL driving means 5 are needed. The voltages at the electrodes must be 180.degree. out of phase. The driving input signal S14 has the same amplitude as the first supply voltage V11 (voltage level VE1). For driving the EL lamp, an AC voltage having the same amplitude as the second supply voltage V12 (voltage level VE2) is needed. The EL driving means 5 must therefore have the ability to upgrade the level of the driving input signal S14. The EL driving means 5 therefore has the level shifter 30 therein. As shown in FIG. 5, the driving input signal S14 fed to the level shifter 30 has the amplitude VE1. The amplitude VE1 is upgraded by the level shifter 30, so that the driving input signal S14 has the amplitude VE2. Having the amplitude VE2, the driving output signal S15 can drive the EL lamp 9 in terms of the voltage value. However, the level shifter 30 may not be able to supply the current required for driving the EL lamp. In this case, a large current passes through an inverter that can supply required current. The inverter is thus driven with the output of the level shifter 30, whereby the driving output signal S15 is produced.
An EL driver having the configuration shown in FIG. 2 or 3 can theoretically drive an EL lamp. However, for causing the EL lamp 9 to glow with sufficient luminance, the EL driver must be constructed with a high-voltage semiconductor device. Taking a transistor formed with a high-voltage semiconductor device for instance, when the threshold voltage (Vth) is set to a standard value, the leakage current may increase (or the current consumption may increase) or the transistor may malfunction and fail to operate practically. For realizing a high-voltage transistor and preventing occurrence of leakage current, the threshold voltage of a transistor formed with a semiconductor device must be higher than that of an ordinary semiconductor device or must be about 1V at a minimum. When a 1.5-V silver battery or 3-V lithium battery is employed as a power source, the margin between a threshold voltage and operating voltage is as small as 0.5 to 2V. As a result, a facility, which operates normally as long as the margin between a threshold voltage and operating voltage is sufficient, fails to operate properly. This is critical, especially, to the level shifter 30 in the EL driving means 5. This problem will be described in detail in conjunction with FIG. 3.
Assuming that the output of the level shifter 30 makes a low-to-high transition, a PMOS 34 and NMOS 35 are turned on simultaneously. For enabling the level shifter 30 to achieve precise level shifting, the potential at a junction 36 between the PMOS 34 and NMOS 35, which serve as a resistive divider, must be equal to the VSS value. Otherwise, the PMOS 32 is not turned on and a high-level signal is not supplied from a terminal Q. The on-state resistance of the NMOS 35 must therefore be smaller than that of the PMOS 34. However, unless the voltage (voltage level VE1) of the driving input signal S14 is too low to create a large difference from the threshold voltage of the EL driver, the NMOS 35 is not fully turned on. The on-state resistance of the NMOS 35 does not therefore decrease fully. Consequently, the on-state resistance is not sufficiently smaller than that of the PMOS 34. As a result, the potential at the junction 36 does not have the VSS value and the level shifter fails to operate normally.