Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-06-08
2002-04-30
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S224000, C315S226000
Reexamination Certificate
active
06380690
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a capacitive load drive circuit for selectively driving a plurality of capacitive loads such as EL (electroluminescence) elements.
2. Description of the Related Art
Heretofore, there has been a capacitive load drive circuit for driving a plurality of capacitive loads such as EL elements as shown by, for example, FIG.
4
. The drive circuit comprises a high voltage supply unit
31
for generating a voltage CHV suitable for driving EL elements E
1
and E
2
, an output unit
32
for selectively outputting the voltage CHV of the high voltage supply unit
31
to the EL elements E
1
and E
2
, a drive signal generating unit
33
for generating a drive signal EA, EB for driving the output unit
32
and a selecting unit
34
for generating a selecting signal for selecting either of the EL elements E
1
and E
2
to be driven and controlling the drive signal generating unit
33
to drive the selected EL elements.
The output unit
32
comprises inverters IVA and IVB and a common inverter IVC, each having the same construction. The inverters IVA and IVB and the common inverter IVC each comprise a P-channel MOS transistor P
1
and an N-channel MOS transistor N
1
having interconnected drains. The inverters form a plurality of output terminals A, B and C. A source of the P-channel MOS transistor P
1
is connected to the output terminal CHV of the high voltage supply circuit
1
and a source of the N-channel MOS transistor N
1
is connected to a reference terminal VSS (e.g., OV). The P-channel MOS transistor P
1
of each of the inverters IVA and IVB and the common inverter IVC is turned ON and OFF by a drive signal the level of which is shifted by a level shifter LS. The N-channel MOS transistor N
1
is turned ON and OFF by the drive signal and outputs the voltage generated by the high voltage supply unit
31
from each of the output terminals A, B and C as a drive voltage. There are also parasitic diodes D
1
and D
2
present across the outputs of the P-channel MOS transistor P
1
and the N-channel MOS transistor N
1
.
One pole of the EL element E
1
is connected to the output terminal A and other pole thereof is connected to the output terminal C to thereby form an H bridge circuit with respect to the EL element E
1
. One pole of the EL element E
2
is connected to the output terminal B and other pole thereof is connected to the common output terminal C (hereinafter, referred to as a common output terminal C) to thereby form an H bridge circuit with respect to the EL element E
2
.
According to a first drive method using the above-described circuit construction, there is a drive method which is characterized by bringing an output terminal of a non-selected EL element, other than the common output terminal, into a high impedance state by means of a selecting signal, even when the output signal is output to the common output terminal, so that a capacitive coupling effect is produced and the non-selected EL element is neither charged nor discharged.
An explanation will be given of operation of the drive circuit of
FIG. 4
with reference to the waveform diagrams shown in FIG.
5
.
At a point in time after a standby state has ended, the common output terminal C generates a drive voltage C. The phase of the drive voltage C is defined for reference purposes as a positive phase. When the EL element E
1
is instructed to turn on by a selecting signal EA output by the selecting unit
34
, the output terminal A generates a drive voltage A having a phase inverse to that of the common output terminal C. As a result, the EL element E
1
is charged and discharged to thereby turn on. Meanwhile, when the EL element E
2
is instructed to turn off by an EL element selecting signal EB, the output terminal B is brought into a high impedance state. Since the EL element is a capacitive load, the potential B′ at the output terminal B is varied by an amount that the potential variation at the common output terminal C and the EL element E
2
is not charged and discharged to thereby turn off.
Further, two EL elements can be simultaneously turned on by making phases of drive voltages of the output terminal A and the output terminal B inverse to the phase of the common output terminal C and two of the EL elements can be turned off by bringing both the output terminal A and the output terminal B into the high impedance state or turning OFF the high voltage supply unit or an IVC power source.
A second drive method for use with the above-described circuit is characterized in that when an EL element is to be turned on, a drive voltage having a phase inverse to the phase of the drive voltage at the common output terminal is output to the output terminal of the selected EL element (other than the common output terminal). When an EL element is to be turned off, a drive voltage having a positive phase relative to the drive voltage of the common output terminal is applied to the output terminal of the selected EL element (other than the common output terminal).
According to the first drive method described above, and as shown in the timing diagrams of
FIG. 5
, the selecting signals EA and EB non-synchronously supplied to the drive voltages A, B and C are used as signals for switching the drive voltages as they are. Accordingly, there is a high probability that the drive voltage is switched or stopped at a state in which electric charge is stored by the capacitive load. When this occurs and a drive voltage is applied to the common output terminal successively, there is brought about a phenomenon in which electric charge is made to flow or is drawn to the parasitic diodes across the output terminal of the H bridge circuit which is to be brought into the high impedance state. For example, when at timing t of
FIG. 5
, a state of simultaneously turning on both the EL element E
1
and the EL element E
2
is switched to a state of in which only one of the EL elements E
2
is turned on (i.e., the state of simultaneously turning on the two is changed to the state of turning on one EL element), in the case in which voltage Va of the high voltage supply unit is supplied to both ends of the EL element E
1
with the output terminal A as positive, when the drive voltage of the common output terminal C is successively changed to Va, the potential A of the output terminal A becomes 2×Va by virtue of capacitive coupling of the EL element, and potential difference between the two poles of the EL element E
1
becomes larger than the voltage Va generated by the high voltage supply unit. Accordingly, current is made to flow to the parasitic diodes. Also, when the polarity of voltage remaining at the EL element E
1
is reversed, the potential difference between the two poles of the EL element E
1
similarly becomes −2×Va. Accordingly, current is similarly made to flow to the parasitic diodes.
Therefore, unnecessary charging and discharging is carried out with respect to the EL element, and there is a problem in that a non-selected EL is nonetheless lighted.
Further, according to the second drive method described above, although the drive voltages applied to the two poles of an EL lamp which is turned off are provided with the same phase, there is a problem in that the EL lamp may be slightly lit due to a difference in power of driving signals applied at the output terminal or a slight deviation in timing.
SUMMARY OF THE INVENTION
In view of the foregoing problems, it is an object of the present invention to provide a drive system for bringing an output terminal used for supplying drive voltage to a capacitive load into a high impedance state when the capacitive load is caused to stop driving so that the capacitive load is prevented from being unnecessarily driven by the flow of electric charge to parasitic diodes at the output terminal, so that, for example, an EL element serving as a capacitive load is prevented from being turned on.
In order to achieve the above-described object, according to the present invention, there is provided a
Adams & Wilks
Nippon Precision Circuits Inc.
Vu David
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