Abstract:
A driver for a piezoelectric actuator includes a pulse width modulator and an output amplifier packaged as a single semiconductor device, preferably on a single semiconductor die. The driver includes a first boost converter that supplies power to the output amplifier, which preferably has programmable gain. A second amplifier, for driving the gate of a switching transistor in the first boost converter, is powered by a second boost converter. The piezoelectric actuator provides tactile feedback for the keyboard or the display in a battery operated electronic device.

Description:
BACKGROUND 
       [0001]    This invention relates to a battery powered driver and, in particular, to a single chip driver for a piezoelectric actuator. 
         [0002]    A piezoelectric actuator requires high voltage, greater than typical battery voltages of 1.5 to 12.6 volts. A “high” voltage is 20-200 volts, with 100-120 volts currently being a typical drive voltage. Some line driven power supplies for actuators provide as much as 1000 volts. Producing high voltage from a battery is more difficult than producing high voltage from a power line. As noted in U.S. Pat. No. 7,468,573 (Dai et al.), the high voltage required “to drive piezoelectric actuators in today&#39;s small electronic devices is undesirable.” The solution proposed in the &#39;573 patent is to use two pulses of “lower” voltage instead of a single pulse at high voltage. The “lower” voltage is not disclosed. Single layer actuators generally require a higher voltage than multilayer actuators. Multilayer actuators have the advantage of providing greater feedback force than single layer actuators. 
         [0003]    A voltage boost circuit can be used to convert the low voltage from a battery to a higher voltage for the driver. In a boost converter, the energy stored in an inductor is supplied to a capacitor as pulses of current at high voltage. 
         [0004]      FIG. 1  is a schematic of a circuit including a known boost converter; e.g. see U.S. Pat. No. 3,913,000 (Cardwell, Jr.) or U.S. Pat. No. 4,527,096 (Kindlmann). Inductor  11  and transistor  12  are connected in series between supply  13  and ground. When transistor  12  turns on (conducts), current flows through inductor  11 , storing energy in the magnetic field generated by the inductor. Current through inductor  11  increases quickly, depending upon battery voltage, inductance, internal resistances, and the on-resistance of transistor  12 . When transistor  12  shuts off, the magnetic field collapses at a rate determined by the turn-off characteristic of transistor  12 . The rate of collapse is quite rapid, much more rapid than the rate at which the field increases. The voltage across inductor  11  is proportional to the rate at which the field collapses. Voltages of one hundred volts or more are possible. Thus, a low voltage is converted into a high voltage by the boost converter. 
         [0005]    When transistor  12  shuts off, the voltage at junction  15  is substantially higher than the voltage on capacitor  14  and current flows through diode  16 , which is forward biased. Each pulse of current charges capacitor  14  a little and the charge on the capacitor increases incrementally. At some point, the voltage on capacitor  14  will be greater than the supply voltage. Diode  16  prevents current from flowing to supply  13  from capacitor  14 . The voltage on capacitor  14  is the supply voltage for other components, such as amplifier  21 . 
         [0006]    As used herein, “supply” provides the operating power for a circuit, as opposed to “bias” that provides control or offset. For example, it is known in the memory art to provide a boost circuit for biasing the gate of a field effect transistor; U.S. Pat. No. 4,660,177 (O&#39;Conner). 
         [0007]    The output of amplifier  21  is coupled to piezoelectric actuator  22 . The input to amplifier  21  can receive an alternating current signal, for bidirectional movement, or a direct current signal, for unidirectional movement or as half of a complementary drive (two amplifiers, one for each polarity, coupled to opposite terminals of piezoelectric actuator  22 ). In a complementary drive, the absolute magnitudes of the boosted voltages are greater than the absolute magnitude of the battery voltage. A complementary drive can use half the high voltage (or be provided with twice the high voltage) of a single drive but requires two boost converters. 
         [0008]    In  FIG. 1 , the gate drive for transistor  12 , illustrated as pulse width modulator  24 , transistor  12 , and amplifier  21  are separate semiconductor devices. Diode  16  is often on the same die as switching transistor  12 . This construction is necessarily large and expensive. 
         [0009]    Thus, there is a need for a battery powered driver that is a single chip power supply for piezoelectric actuators. Although die size is increased and the die is more expensive, the total cost for semiconductors can be reduced. There is also a problem of combining devices without reducing efficiency. An external supply voltage of three volts (two batteries), typical for today&#39;s portable electronics, restricts circuit design and reduces efficiency. 
         [0010]    In view of the foregoing, it is therefore an object of the invention to provide a single chip driver for a piezoelectric actuator that is as efficient as battery powered drivers using several semiconductor devices. 
         [0011]    Another object of the invention is to reduce the component count in drivers for piezoelectric actuators. 
         [0012]    A further object of the invention is to improve the efficiency of a driver powered by a low voltage external supply. 
       SUMMARY OF THE INVENTION 
       [0013]    The foregoing objects are achieved in the invention in which a driver for a piezoelectric actuator includes a pulse width modulator and an output amplifier packaged as a single semiconductor device, preferably on a single semiconductor die. The driver includes a first boost converter that supplies power to the output amplifier, which preferably has programmable gain. A second amplifier, for driving the gate of a switching transistor in the first boost converter, is powered by a second boost converter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a schematic of a driver, constructed in accordance with the prior art, coupled to a piezoelectric actuator; 
           [0016]      FIG. 2  is a perspective view of an electronic device having a display and a keypad, either or both of which include a piezoelectric actuator; 
           [0017]      FIG. 3  is a schematic of a driver, constructed in accordance with the invention, coupled to a piezoelectric actuator; 
           [0018]      FIG. 4  is a more detailed schematic of a driver, constructed in accordance with a preferred embodiment of the invention, coupled to a piezoelectric actuator; 
           [0019]      FIG. 5  is a schematic of a driver, constructed in accordance with an alternative embodiment the invention, coupled to a piezoelectric actuator; 
           [0020]      FIG. 6  is a schematic of a driver having complementary outputs, constructed in accordance with an alternative embodiment the invention and coupled to a piezoelectric actuator; and 
           [0021]      FIG. 7  is a schematic of a driver having complementary outputs and a single voltage supply. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 2  illustrates electronic device  25  including display  26  and keypad  27 . Either the display or the keypad, or both, can be provided with a piezoelectric device (not shown) for providing tactile feedback when a key or a portion of the display is depressed slightly. Devices for providing feedback are known in the art. As described above, such devices can be single layer or multi-layer and unidirectional or bidirectional. 
         [0023]      FIG. 3  illustrates a driver for a piezoelectric actuator in which the circuitry for driving the gate of a switching transistor is on the same semiconductor die as the amplifier for controlling the device. Die  31  includes pulse width modulator  33  and amplifier  34 , which is powered by high voltage from capacitor  14 . By powering amplifier  34  from a high voltage supply, input  36  can receive voltages greater than external supply voltage  13 , e.g. greater than three volts. 
         [0024]    The output of amplifier  34  is coupled to piezoelectric actuator  22  for driving the device either unidirectionally or bidirectionally, depending upon input signal. 
         [0025]    Although pulse width modulator  33  is a low voltage device and amplifier  34  is a high voltage device, the two are readily isolated on a die by techniques long known in the art for processing a semiconductor wafer. 
         [0026]    In accordance with another aspect of the invention, die  31  includes at least two pads (not shown) coupled to inputs  38  and  29 . These inputs are optionally grounded to provide at least four (2 2 ) levels of gain in amplifier  34 . If the invented driver is produced in large numbers, the pads can be grounded, or not, internally, thereby reducing pin count and package size. For small production runs, the pads can be coupled to external pins to allow a customer to set gain as desired. 
         [0027]      FIG. 4  is a block diagram of a preferred embodiment of the invention in which the switching transistor is included on the die with the pulse width modulator and the amplifier. In this embodiment, die  41  includes internal boost converter  42  for generating a local supply voltage on the die. Boost converter  42  is preferably a capacitive pump, known per se in the art, storing energy on external capacitor  43 . The output from boost converter  42  is, for example, five volts, for powering buffer amplifier  51 . By providing an internal supply voltage that is higher than V cc , the battery voltage, one can drive the gate of switching transistor  52  at a higher voltage, thereby increasing the efficiency of the high voltage boost converter. 
         [0028]    A voltage divider including resistor  55  and resistor  56  is coupled in parallel with capacitor  14  to provide feedback for controlling the voltage on capacitor  14 . 
         [0029]    Clock  44 , which can include an oscillator and dividers or counters (not shown), is coupled to pulse width modulator  46  and boost converter  42 , which need not operate at the same frequency. 
         [0030]    A clock rate greater than 100 kHz. or higher is preferred for pulse width modulator  46 . A clock rate in this range of frequencies enables one to use inductors that are physically small and less expensive. Current increases with inductance and decreases with frequency. The clock signal into boost converter  42  is preferably lower in frequency than the clock signal into pulse width modulator  46 ; e.g. one half or one fourth. 
         [0031]    Input amplifier  61  and output amplifier  62  are powered by the supply voltage on capacitor  14 . Output  63  of amplifier  62  is coupled to piezoelectric actuator  22 . There can be more than two amplifying stages between input  64  and output  63 . Amplifier  61  preferably includes at least two pads (not shown) coupled to inputs  67  and  68 . As with the embodiment of  FIG. 3 , these inputs are optionally grounded to provide at least four levels of gain in amplifier  61 . 
         [0032]      FIG. 5  is a block diagram of an alternative embodiment of the invention that differs from the embodiment of  FIG. 4  in two respects. Die  71  includes isolation diode  72  and amplifier  74  is powered by internal boost converter  42 . Otherwise, the operation of the embodiment is the same as for  FIG. 4 . 
         [0033]    In  FIG. 6 , neither side of piezoelectric actuator  22  is grounded. Instead, the actuator “floats,” coupled between the output of amplifier  81  and the output of amplifier  82 . Amplifier  82  is powered by capacitor  14 , which is charged positively relative to ground. Amplifier  81  is powered by capacitor  84 , which is charged negatively relative to ground. The absolute values of the voltages on capacitors  82  and  84  are much greater than the absolute value of V. Inductor  11 , piezoelectric actuator  22 , capacitor  85  and capacitor  85  are preferably the only components not included in a single semiconductor die. 
         [0034]    The operation of the two polarity boost converter is very similar to that disclosed in U.S. Pat. No. 5,313,141 (Kimball). Briefly, while transistor  86  conducts, transistor  87  turns on and off, causing positive pulses to be coupled to capacitor  14 . After a predetermined time, or number of pulses, the situation reverses and transistor  87  conducts while transistor  86  turns on and off, causing negative pulses to be coupled to capacitor  84 . Diode  88  prevents current flowing from capacitor  84  to supply or ground. Diode  89  prevents current flowing from capacitor  14  to supply or ground. 
         [0035]    The time constants associated with capacitors  14  and  84  are long enough that the voltage on the capacitors remains high, although fluctuating slightly because the voltage will decrease when a capacitor is not receiving charge pulses from the boost converter. The polarity of the boost pulses changes at a lower frequency than the pulse frequency of transistors  86  and  87 . If the pulse frequency is greater than 500 kHz, for example, polarity can reverse at tens of kilohertz and the voltage on capacitors  14  and  84  is constant to within a few percent. 
         [0036]    Aspects of the invention shown in other figures are omitted from  FIG. 6  for the sake of simplicity, including the dashed line representing a single semiconductor die. This is not to say that the other aspects cannot be part of an implementation of the invention in accordance with  FIG. 6 . Techniques for biasing gate drive amplifiers  93  and  94  are not shown but are known in themselves in the art. Pulse width modulator  96  includes logic for driving the gates of transistors  86  and  87 , in addition to generating a pulse width modulated signal. 
         [0037]    The embodiment of  FIG. 6  can drive the piezoelectric actuator over a range from +HV to −HV.  FIG. 7  is a variation of this embodiment, using a single voltage supply. The embodiment of  FIG. 7  can drive the piezoelectric actuator over a range from +HV to 0 (zero). This is one tradeoff. Another is that the embodiment of  FIG. 6  requires dielectric isolation (DI) construction on a die, which is a more expensive process than the process needed to make the embodiment of  FIG. 7 . 
         [0038]    The invention thus provides a single chip driver for a piezoelectric actuator that is as efficient as battery powered drivers using several semiconductor devices, thereby reducing the component count in drivers for piezoelectric actuators. 
         [0039]    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, the specific values given are by way of example only. One could enclose more than one semiconductor die in a single package. The pads for programming gain can be distributed among more than one amplifier in the embodiments of  FIG. 4  and  FIG. 5 . Internal boost converter  42  ( FIG. 4 ) can be added to die  31  ( FIG. 3 ) also. More generally, while aspects of the invention have been described in certain combinations, this is not to imply that other combinations are not included in the invention. Although a two polarity boost converter, using a single inductor, is shown in  FIG. 6 , separate boost converters, using two inductors, could be used instead. Inductor  11  is illustrated as a simple coil but is intended to cover more complex alternatives as well, e.g. an autotransformer or a transformer with more than one winding.