Abstract:
A method and circuit for generating a voltage waveform across a capacitive load such as an electroluminescent device is described. The method includes the steps of charging and discharging the capacitive load using a constant current source. By sequentially performing the steps of providing a constant current through the capacitive load, eliminating the current, and discharging the capacitive load, a trapezoidal voltage waveform is achieved. In portable telephones and laptop computers with electroluminescent displays, the trapezoidal waveform results in reduced audible noise and power consumption.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of voltage waveform generators and more specifically to the generation of a voltage waveforms across a capacitive load. 
     BACKGROUND OF THE INVENTION 
     Electroluminescent (EL) devices require an AC drive signal in order to produce illumination. The frequency of the drive signal is typically in the audio frequency range. As a result, audible noise can be generated. Devices such as portable telephones which operate in the audio frequency range and have EL lamps can be adversely affected by the EL hum or background noise. Similarly, laptop computers with EL displays can exhibit undesirable background noise. 
     Decreased power consumption is realized for waveforms that approximate a sinusoid at the EL drive frequency. Drive circuits which generate square waveforms or saw-tooth waveforms can be used to drive the EL lamp to reduce power consumption and audible noise. These circuits typically use thyristors or high voltage metal oxide semiconductor field effect transistors (MOSFETs) biased in the triode region as switching devices, however, this voltage control mode does not allow for accurate control of the EL lamp charge and discharge rates. 
     SUMMARY OF THE INVENTION 
     This invention relates to a method and circuit for generating a voltage waveform across a capacitive load. A plurality of switches are used to control the current through the capacitive load. Synchronized operation of the switches allows the capacitive load to be linearly charged and discharged. The circuit can be used to generate a trapezoidal waveform for driving an electroluminescent device. The trapezoidal waveform reduces audible noise which can be detrimental in audio devices having electroluminescent displays, such as portable telephones and laptop computers. In addition, the circuit and electroluminescent device achieve a higher power efficiency. 
     The method includes the steps of charging a reactive load with a substantially constant current, terminating the substantially constant current and discharging the capacitive load to generate a substantially constant current. In another embodiment, the method includes the steps of closing a first switch between a first terminal of the capacitive load and a first terminal of a substantially constant current source and closing a second switch between a second terminal of the capacitive load and a second terminal of the substantially constant current source so that the capacitive load is charged at a substantially linear rate. The embodiment includes the additional steps of opening the first switch, opening the second switch and closing a third switch between the first and second terminals of the capacitive load so that the capacitive load is discharged at a substantially linear rate. In another embodiment, the substantially linear charge rate and the substantially linear discharge rate are approximately equal. 
     The circuit includes a first switch between the first terminal of the capacitive load and the first terminal of the substantially constant current source, a second switch between the second terminal of the capacitive load and the second terminal of the substantially constant current source and a third switch between the first terminal of the capacitive load and the second terminal of the substantially constant current source. A substantially constant current linearly charges the capacitive load when the first and second switches are closed and the third switch is open. A substantially constant current is generated by the linear discharge of said capacitive load when the first and second switches are open and the third switch is closed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is pointed out with particularity in the appended claims. The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which: 
     FIGS.  1 ( a-f ) is a series of schematic state diagrams which depict an embodiment of the invention during six states of its operation; 
     FIG. 2 is a voltage time diagram of the voltage produced across the capacitive load by the embodiment of the circuit shown in FIGS.  1 ( a-f ); 
     FIG. 3 is a transistor level schematic diagram of an embodiment of the invention; and 
     FIG. 4 is a switching clock diagram for the embodiment of the invention shown in FIG.  1 . 
     FIG. 5 is a transistor level schematic diagram of anotherr embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In brief overview and referring to FIG. 1 a,  an embodiment of the invention includes a first switch  10  and a second switch  12  connected together through their first terminals to one terminal of a resistor  14 . The other terminal of the resistor  14  is connected to one terminal of a capacitor  16  and one terminal of a coil  18  through a diode  20 . A switch  23  is connected at one terminal to the common node between the coil  18  and diode  20 . The other terminal of the switch  23  is connected to a reference voltage Vs  32 . The combination of coil  18 , diode  20 , switch  23  and capacitor  16  form a boost converter  21  which charges capacitor  16  to a high voltage. In one embodiment the boost converter  21  operates at a frequency of 32 kHz. In another embodiment the boost converter is replaced by a DC voltage source. 
     The respective second terminals of switches  10  and  12  are connected to a respective side of a capacitive load  22  such as an electroluminescent lamp. Each respective second terminal of switches  10  and  12  is connected to a respective first terminal of a respective pair of switches  24 ,  26  and  28 ,  30 . The second terminal of one switch  24 ,  30  of each pair of switches is connected to Vs  32 . The second terminal of the other switch  26 ,  28  of each pair of switches is also connected to Vs  32  through a respective resistor  34 ,  38 . A pair of diodes  42 ,  44  are arranged with their anodes connected to Vs  32 , and with their respective cathodes connected to a respective second terminal of switches  10 ,  12 . Typically Vs  32  is ground. 
     In operation, and referring again to FIG. 1 a , initially switches  10 ,  28  and  30  are closed and switches  12 ,  24 , and  26  are open causing capacitive load  22  to charge through resistor  14 . Once the capacitive load  22  is charged, switch  10  opens (FIG.  1 ( b )) thereby causing the charging to cease. Referring to FIG.  1 ( c ), at this time switches  28  and  30  are opened and switch  26  is closed, providing a discharge path across capacitive load  22  through resistor  34  and diode  44 . Because resistor  34  has substantially the same value as resistor  14 , the discharge rate of the capacitive load  22  is substantially the same as its rate of the charging. 
     Next (FIG.  1 ( d )) switches  12 ,  24  and  26  are closed permitting the capacitive load  22  to charge through resistor  14  as in FIG.  1 ( a ) only with the opposite polarity. Once the capacitive load  22  is charged, switch  12  opens (FIG.  1 ( e )) thereby causing the charging to cease. Referring to FIG.  1 ( f ), at this time switches  24  and  26  are opened and switch  28  is closed, providing a discharge path across capacitive load  22  through resistor  38  and diode  42 . Again, because resistor  38  has substantially the same value as resistor  14 , the discharge rate of the capacitive load  22  is substantially the same as its rate of the charging. 
     The voltage waveform across the capacitive load  22  as a result of the operation of the switches shown in the embodiment of FIG.  1 ( a-f ) is shown in FIG.  2 . When the switches are positioned as shown in the first state (FIG.  1 ( a )), the capacitive load  22  is charging and the voltage across the load  22  rises substantially linearly  50 . 
     In actual implementation, the switches in one embodiment are MOSFETs (see FIG.  3 ). In this embodiment switches  100  and  106  are connected such that their sources are in communication with resistor  14  and their drains are in communication with the capacitive load  22 . As the voltage on the capacitive load  22  rises, it approaches the voltage value on the capacitor  16  of the boost converter  21 . When this occurs, the voltage difference across the source and drain of the switches  100  and  106  becomes small and the switches transition to a shut off state. The time to reach this shut off state is approximately proportional to the current charging the capacitive load  22 . Thus the value of resistor  14  determines the start of the shutoff state. 
     When switch  10  is opened as shown in the second state (FIG.  1 ( b )), the charging ceases and the voltage takes on a substantially constant value  52 . When switch  26  is closed (FIG.  1 ( c )) the capacitive load  22  discharges  54  at substantially the same rate at which it was charged until the voltage returns to zero  56 . 
     At this point the capacitive load  22  is recharged by the closing of switch  12  (FIG.  1 ( d )) with an opposite polarity to that shown in FIG.  1 ( a ) and the voltage becomes linearly increasingly negative  58 . When switch  12  is then opened (FIG.  1 ( e )) the voltage across the capacitive load  22  becomes substantially constant  60 . Then the closing of switch  28  discharges the capacitive load  22  (FIG.  1 ( f )) at substantially the same rate at which it was charged  62 , until zero voltage appears across the capacitive load  22 . The cycle then repeats. In one embodiment these six states are repeated every 4 ms. 
     Referring to FIG. 3, a device constructed in accordance with the invention as shown in FIG. 1, includes a resistor  14 ′ one terminal of which is connected to a boost converter  21  which includes a coil  18 , a diode  20 , and a capacitor  16 , as previously described. A MOSFET device  23 ′ controlled at its gate by a 32 KHz signal switches the coil  18  in the boost converter  21 . The other terminal of resistor  14 ′ is connected to the respective first terminals  89 ,  91  of two switches  10 ′ and  12 ′. Each switch  10 ′,  12 ′ has a second terminal  93 ,  95 , respectively, which is in communication with a respective terminal  97 ,  99  of capacitive load  22 , and a respective control terminal  96 ,  98 , by which the respective switch  10 ′,  12 ′ is turned on and off. 
     Switch  10 ′ includes a resistor  90  connected between the boost converter  21  and the drain of a transistor  92 . The source of transistor  92  is connected to reference voltage  32  through resistor  94 . The gate of transistor  92  is the control terminal  96  of the switch  10 ′ and is connected to a clocking line designated CK 5 . A voltage corresponding to a clocking cycle is applied to the gate of transistor  92 , turning the transistor  92  on and off. The turning on and off of this transistor  92  controls the current flow through resistors  90 ,  94 . A transistor  100 , whose source is the input terminal  89  of switch  10 ′ and whose drain is the output terminal  93  of switch  10 ′, has a gate which is connected to the common node of resistor  90  and drain of transistor  92 . As transistor  92  is turned on and off by the clocking voltage CK 5  applied to its gate through control terminal  96 , the resulting voltage drop across resistor  90  turns transistor  100  on and off, thereby alternately connecting and disconnecting the capacitive load  22  to the boost converter  21  as described with respect to FIG.  1 . 
     Similarly, switch  12 ′ includes a resistor  102  connected between the boost converter  21  and the drain of a transistor  104 . The source of transistor  104  is connected to reference voltage  32  through the same resistor  94  by which the source of transistor  92  is connected to the reference voltage  32 . The gate of transistor  104  is the control terminal  98  of the switch  12 ′ and is connected to a clocking line designated CK 4 . A voltage corresponding to a clocking cycle is applied to the gate of transistor  104 , turning transistor  104  on and off. The turning on and off of this transistor  104  controls the current flow through resistors  102 ,  94 . A transistor  106 , whose source is the input terminal  91  of switch  12 ′ and whose drain is the output terminal  95  of switch  12 ′, has a gate which is connected to the common node of resistor  102  and drain of transistor  104 . As transistor  104  is turned on and off by the clocking voltage CK 4  applied to its gate through control terminal  98 , the resulting voltage drop across resistor  102  turns transistor  106  on and off, thereby alternately connecting and disconnecting the capacitive load  22  to the boost converter  21  as described with respect to FIG.  1 . 
     Switch  24 ′ in this embodiment is a transistor having a drain connected to the terminal  97  of capacitive load  22  and a source connected to reference voltage  32 . The gate of switch  24 ′ is the control terminal for the switch  24 ′ and is connected to control terminal  98 . As such the same clocking voltage CK 4  applied to the control terminal of switch  12 ′ is applied to switch  24 ′. Switch  30 ′ in this embodiment is a transistor having a drain connected to the terminal  99  of capacitive load  22  and a source connected to reference voltage  32 . The gate of switch  30 ′ is the control terminal for the switch  30 ′ and is connected to control terminal  96 . Again, the same clocking voltage CK 5  applied to the control terminal of switch  10 ′ is applied to switch  30 ′. 
     The remaining switches  26 ′,  28 ′ are transistors and each transistor has its respective drain terminal connected to a respective terminal  97 ,  99  of the capacitive load  22 , and its source terminal connected to reference voltage  32  through its respective resistor  34 ′,  38 ′. The gate of each respective transistor is the respective control terminal for the respective switch. Each control terminal is connected to a respective clocking circuit which places a clocking voltage designated CK 3  and CK 2  respectively on the respective gate, thereby turning the respective switch  26 ′,  28 ′ on and off. The two diodes  42 ,  44  shown in FIG. 1, are provided by the parasitic diodes (from source to drain) of the transistors of switches  24 ′,  30 ′. In one embodiment the transistors are MOSFETs. 
     Although the embodiment shown contemplates switches  10 ′ and  12 ′ which include two resistors and two MOSFETs, other embodiments are contemplated. In one such embodiment switches  10 ,  12  are single transistors, replacing transistors  100  and  106 , whose gates are directly driven by clocking signals CK 4  and CK 5 , rather than being controlled by the switching of additional transistors  92 ,  104  which are driven by clocking signals CK 4  and CK 5  as in the embodiment shown. 
     Referring to FIG. 4, the clocking cycles (CK 2 , CK 3 , CK 4 , and CK 5 ) for the embodiment of the invention shown in FIG. 3 are depicted. It is important to note that the voltages for clock cycles CK 2 , CK 4  and CK 5  have been offset to permit their display. Specifically clock cycle CK 2  has been offset by 4 volts; clock cycle CK 4  has been offset by −4 volts; and clock cycle CK 5  has been offset by −8 volts. 
     Referring to FIG. 5, another embodiment based on a current mirror configuration includes switches  10 ″,  12 ″,  26 ″ and  28 ″. Switch  10 ″ includes transistors  92 ,  100  and  108 , switch  12 ″ includes transistors  104 ,  106  and  110 , switch  26 ″ includes transistors  112 ,  114 ,  116 ,  118  and  120 , and switch  28 ″ includes transistors  122 ,  124 ,  126 ,  128  and  130 . The value of resistor  132  determines the current charging and discharging the capacitive load  22 . 
     While the invention has been shown and described with reference to specific referred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the following claims.