Abstract:
The invention concerns drive circuitry for generating a plurality of independent drive voltage signals for a plurality of electrodes of at least one liquid lens, the liquid lens comprising first and second miscible liquids, an interface between the first and second liquids being movable by electro wetting by application of a drive voltage signal to at least one electrode of the liquid lens, the drive circuitry including generating circuitry ( 406, 408, 410, 412 ) arranged to generate a fixed drive voltage (V dc ); a duty cycle controller ( 418 ) arranged to receive data values indicating a duty cycle to be applied to each of the plurality of electrodes, and to generate a control timing signal for each of the plurality of electrodes, each control timing signal having the duty cycle indicated by the controller; and driving means ( 426, 428, 430, 432 ) arranged to generate an independent drive voltage signal for each of the plurality of electrodes by selectively applying the fixed drive voltage, based on the control timing signal associated with each liquid lens.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a circuit for driving the electrodes of one or more liquid lenses, and in particular to a driving circuit for driving electrodes of liquid lenses having liquids moveable by electrowetting. 
       BACKGROUND OF THE INVENTION 
       [0002]    A number of embodiments of variable focus liquid lenses are described in European patent N° 1166157. FIG. 1  of the present application corresponds to  FIG. 12  of that patent and illustrates an example of a variable focus liquid lens according to the prior art. As shown in  FIG. 1 , a variable focus lens comprises a fluid chamber with two insulating transparent plates  1  and  2  and an optical axis Δ. Plate  2  comprises a conical or cylindrical recess, with a side face  4  inclined with respect to the optical axis Δ of the device, and which receives a drop of a first liquid  6  which is an insulating liquid. The remainder of the chamber extending up to transparent plate  1  is filled with a second liquid  8 , which is conductive. The liquids are immiscible, and have different refraction indexes but roughly the same density, forming a refractive interface or meniscus (A,B). A transparent electrode  10  is formed over the outer surface of insulating plate  2 . Another electrode  12  is provided in contact with the second liquid  8 . 
         [0003]    Due to the electrowetting effect, it is possible, by applying a voltage between electrodes  10  and  12 , to change the curvature of the interface between the first liquid  6  and the second liquid  8 , for example, from an initial concave shape as shown by line A, to a convex shape as shown by line B. Thus rays of light passing through the cell perpendicular to the plates  1  and  2  in the region of the drop  6  will be focused more or less depending on the voltage applied. 
         [0004]    A driver circuit is required to generate the voltages for controlling liquid lenses such as the liquid lens of  FIG. 1 . Examples of driving circuits are described in International Patent Application WO 2005/052654 and US Patent Application US 2005/0213653. However, such circuits are designed to drive only one pair of electrodes in a single lens. 
         [0005]    One proposal by the present applicant is to provide multiple liquid lenses in a lens module of an optical system in order to allow for the correction of aberrations, as well as to provide other features such as a zoom function. Another proposal is to provide multiple electrodes in a liquid lens which can be driven independently to provide more complex functions of the liquid interface in the lens, such as providing tilt or astigmatism. In order to independently drive multiple electrodes of one or more liquid lenses, one solution would be to duplicate the driver circuitry disclosed in the prior art. However, this solution is disadvantageous as it is costly, and consumes a large surface area of an integrated circuit or circuit board. In many environments in which space is limited, for example in the case of liquid lens driving circuits for driving one or more liquid lenses incorporated in a mobile telephone, it is important that space occupied by components of the driving circuitry is minimized. There is thus a need for a driving circuit that is able to independently drive multiple electrodes of one or more liquid lenses, without using excessive space and whilst remaining economical. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention aims to at least partially address the above needs. 
         [0007]    According to one aspect of the present invention there is provided drive circuitry for generating a plurality of independent drive voltage signals for driving a plurality of electrodes of at least one liquid lens, the liquid lens comprising first and second immiscible liquids, an interface between the first and second liquids being movable by electrowetting by application of a drive voltage signal to at least one electrode of the liquid lens, the drive circuitry comprising generating circuitry arranged to generate a fixed drive voltage; a duty cycle controller arranged to receive data values indicating a duty cycle to be applied to each of the plurality of electrodes, and to generate a control timing signal for each of the plurality of electrodes, each control timing signal having the duty cycle indicated by the controller; and driving means arranged to generate an independent drive voltage signal for each of the plurality of electrodes by selectively applying the fixed drive voltage, based on the control timing signal associated with each liquid lens. The fixed drive voltage signal for each electrode can be applied to an output to each of the plurality of electrodes in turn. Thus only one fixed drive voltage needs to be generated to drive a plurality of electrodes. The independent drive voltage signal for each of the plurality of electrodes is for example applied between each of the plurality of electrodes and one or more further electrodes, and each for example independently control at least part of a liquid interface, for example different liquid interfaces of different lenses, or different parts of a same liquid interface. 
         [0008]    According to an embodiment of the present invention, the drive circuitry is arranged to generate an independent drive voltage signal for each of a plurality of liquid lenses, each of the liquid lenses consisting in a pair of electrodes, the independent drive voltage signal applied between the pair of electrodes. 
         [0009]    According to another embodiment of the present invention, the drive circuitry is arranged to generate a plurality of independent drive voltage signals applied to each of a plurality of electrodes one of the liquid lenses. 
         [0010]    According to another embodiment of the present invention, the control circuitry further comprises control means for receiving a feedback value of the fixed drive voltage via a feedback path and controlling the level of the fixed drive voltage based on a comparison between the feedback value and a reference value. 
         [0011]    According to another embodiment of the present invention, the generating means comprises at least one inductor, at least one capacitor and at least one transistor. 
         [0012]    According to another embodiment of the present invention, the duty cycle controller comprises at least one counter arranged to count edges of a reference timing signal. 
         [0013]    According to another embodiment of the present invention, the drive circuitry further comprises frequency generating means for generating the reference timing signal and providing the reference timing signal to the duty cycle controller and the control means. 
         [0014]    According to another embodiment of the present invention, the drive circuitry further comprises an interface for receiving the data values, and at least one register for storing the data values. 
         [0015]    According to another embodiment of the present invention, the interface is a serial bus decoder. 
         [0016]    According to another embodiment of the present invention, each liquid lens comprises first and second electrodes, and the driving means comprises separate pairs of transistors connected to the first electrodes and a common pair of transistors connected to the second electrodes. 
         [0017]    According to another embodiment of the present invention, each of the control timing signals comprises pulses, the width of the pulses determining the duty cycle of that control timing signal. 
         [0018]    According to another embodiment of the present invention, each of the control timing signals comprises a plurality of fixed length pulses, the number of the fixed length pulses in a given period determining the duty cycle of that control timing signal. 
         [0019]    According to a further aspect of the present invention, there is provided an optoelectronics module comprising processing means; a plurality of liquid lenses; an image sensor; and the above drive circuitry arranged to drive the plurality of electrodes. 
         [0020]    According to a further aspect of the present invention, there is provided a mobile device comprising the above optoelectronic module. 
         [0021]    According to a further aspect of the present invention, there is provided a method of driving a plurality electrodes of at least one liquid lens each with an independent drive voltage signal, each liquid lens comprising first and second immiscible liquids, an interface between the first and second liquids being movable by electrowetting by application of a drive voltage signal, the method comprising: generating a fixed drive voltage; receiving data values indicating a duty cycle to be applied to each of the plurality of liquid lenses and based on the data values generating a control timing signal for each of the plurality of liquid lenses; and generating an independent drive voltage signal for each of the plurality of liquid lenses by selectively applying the fixed drive voltage, based on the control timing signal associated with each liquid lens. 
         [0022]    Further objects, features and advantages of the present invention will become apparent from the following detailed description of exemplary preferred embodiments, when considered together with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The foregoing and other purposes, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
           [0024]      FIG. 1  illustrates schematically an embodiment of a known variable focus liquid lens; 
           [0025]      FIG. 2  illustrates schematically circuitry for controlling and driving multiple liquid lenses according to a first embodiment of the present invention; 
           [0026]      FIG. 3  illustrates the driving circuitry of  FIG. 2  in more detail according to the first embodiment of the present invention; 
           [0027]      FIG. 4  illustrates the DC-DC generator of  FIG. 3  in more detail according to the first embodiment of the present invention; 
           [0028]      FIG. 5A  illustrates one of the H-bridges of  FIG. 3  in more detail according to the first embodiment of the present invention; 
           [0029]      FIG. 5B  illustrates an alternative embodiment of the H-bridges of  FIG. 3 ; 
           [0030]      FIG. 5C  illustrates yet an alternative embodiment of the H-bridges of  FIG. 3 ; and 
           [0031]      FIGS. 6A and 6B  illustrate timing diagrams relating to the circuitry of  FIG. 3  according to first and second examples respectively. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]      FIG. 2  illustrates an optoelectronic module  300  comprising a plurality of liquid lenses. Module  300 , which is for example incorporated in a mobile telephone, comprises a processor  302 , which is for example the baseband processor of a mobile phone, an image signal processor (ISP)  304 , LCD display  306 , a lens module  308  and a battery  310  which is for example a mobile phone battery. 
         [0033]    Lens module  308  comprises a driver circuit  312 , for driving the plurality of variable focus liquid lenses  314 . In this example four liquid lenses are provided, one of which is used for focusing, a second for zooming, and third and fourth lenses for the correction of chromatic and field curvature aberrations. In alternative embodiments, different liquid lens arrangements having a different number of liquid lenses could be driven. 
         [0034]    The variable focus liquid lenses  314  are for example liquid lenses as described in relation to  FIG. 1  above, or any similar liquid lens having a focus variable by application of a drive voltage. Each lens comprises first and second electrodes which receive an independent drive voltage signal from driver circuit  312  via lines  315 . Alternatively, the four liquid lenses could be replaced by a single liquid lens having four electrodes and a common electrode, the independent drive voltages from driver circuit  312  being applied between the common electrode and each of the four electrodes. For example, a plurality of the electrodes  10  of  FIG. 1 , which are insulated from the liquids, could be provided and independently driven to control different parts of the liquid interface. Independent voltages are then for example applied between the electrode  12  and each of the plurality of electrodes  10 . Examples of this type of lens are discussed for example in U.S. Pat. No. 6,538,823, which is hereby incorporated by reference to the extent allowable by the law. Alternatively, the single liquid lens could comprise a plurality of electrodes  12  in contact with the conducting liquid  8  of  FIG. 1 , to control different parts of the liquid interface. In this case, independent voltages are for example applied between the electrode  10  and each of the plurality of electrodes  12 . Examples of such lenses are for example discussed in currently unpublished European Application No. 06301000. 
         [0035]    The lens module  308  preferably further comprises a number of fixed lenses  316 , and a CMOS (complementary metal oxide semiconductor) sensor  318  for receiving images received via the variable focus lenses and fixed lenses. In alternative embodiments a different type of sensor could be used, such as a CCD (charge-coupled device) sensor. The CMOS sensor  318 , fixed lenses  316  and variable focus lenses  314  are arranged along an optical axis Δ of the lens module, and a variable diaphragm  320  provides means for adjusting the aperture of the lens module, allowing the light level entering the lens to be controlled. 
         [0036]    As illustrated, the processor  302 , LCD display  306 , ISP  304 , CMOS sensor  318  and driver  312  are for example powered by a DC voltage level V bat  from battery  310 . 
         [0037]    In operation, ISP  304 , under control of processor  302 , determines and generates data signals indicating the required drive voltages for each of the liquid lenses  314 , or in the case of a liquid lens with a common electrode and a plurality of electrodes in contact with the conducting liquid or a plurality of electrodes insulated from the conducting liquid, for each of the plurality of electrodes. The drive voltage levels for each lens/electrode are for example determined based on algorithms processed by the ISP, which optimises focusing, zoom, and correction of optical aberrations such as chromatic aberration and field curvature aberration of the lens module. ISP preferably receives signals from CMOS sensor  318 , which are used in particular to indicate when focusing has been achieved. 
         [0038]    ISP  304  provides these control signals on a serial bus  324  to driver circuit  312 , which generates drive signals for driving each of the electrodes of the liquid lenses based on these control signals. The drive signals are provided to each of the electrodes of the liquid lenses via lines  315 . In particular, the driver circuit  312  is connected via lines  315  to the first and second electrodes in each liquid lens, or in the case of multiple electrodes in a single lens, to one common electrode and each of the plurality of electrodes. The drive voltage to each liquid lens/electrode is preferably an oscillating (AC) voltage. 
         [0039]    ISP  304  also controls CMOS sensor  318  to capture the image received via the lenses, at an image formation region of the sensor. ISP  304  receives signals generated by CMOS sensor  318  based on the captured image on lines  322 . The captured image can then be displayed on LCD display  306 . 
         [0040]    Diaphragm  320  is a mechanical diaphragm controlled by independent circuitry under control of the ISP  304 . In alternative embodiments diaphragm  320  could be a liquid diaphragm comprising an opaque liquid moveable by electrowetting, and could be driven by the driver circuit  312  in the same way as the liquid lenses  314 . 
         [0041]      FIG. 3  illustrates the driver circuit block  312  of  FIG. 2  in more detail. 
         [0042]    With reference to  FIG. 3 , driver circuit  312  comprises an integrated circuit (IC)  404 , an inductor  406 , and a capacitor  408 . IC  404  comprises a diode  410  and a MOSFET (metal oxide semiconductor field effect transistor)  412 . IC  404  further comprises an RC (resistor-capacitor) oscillator  414  for generating a reference oscillating signal of a given frequency which is provided to a frequency generation block  416 . Frequency generation block  416  generates a timing signal based on the reference oscillating signal, and outputs this signal on line  417  to a duty cycle controller  418  and to a DC-DC generator  420 . 
         [0043]    Serial bus  324  is connected to IC  404 , and a serial bus decoder  421  is provided on IC  404  for decoding the serial data signals received via serial bus  324  and storing the data in first, second, third and fourth registers  422 , each of these registers storing drive voltage data associated with a respective one of the first, second, third and fourth liquid lenses/electrodes (not shown in  FIG. 3 ). 
         [0044]    DC-DC generator  420  controls MOSFET  412  based on a reference voltage V REF  received on line  423  and a feedback signal received on line  424 . In particular, DC-DC generator  420  generates a switch control voltage signal V SC , based on the feedback and reference voltages, and provided to the gate terminal of MOSFET  412 . The main current terminals of MOSFET  412  are connected to ground and to a first terminal of inductor  406 . The first terminal of inductor  406  is also connected to a first terminal of capacitor  408  via diode  410 . The second terminal of inductor  406  is connected to V bat  and the second terminal of capacitor  408  is connected to ground. 
         [0045]    The first terminal of capacitor  408  is also connected to first, second, third and fourth H-bridges  426 ,  428 ,  430  and  432  for driving the first, second, third and fourth liquid lenses. In particular, the first H-bridge  426  provides output voltages V oA1  and V oB1  on lines  434  and  436  respectively for driving a first liquid lens. In a similar fashion, second H-bridge  428  provides output voltages V oA2  and V oB2  on lines  438  and  440  respectively for driving a second liquid lens. The third H-bridge  430  provides output voltages V oA3  and V oB3  on lines  442  and  444  for driving a third liquid lens, and the fourth H-bridge  432  provides output voltages V oA4  and V oB4  on lines  446  and  448  for driving a fourth liquid lens. 
         [0046]    In operation, MOSFET  412  is switched by DC-DC generator  420  such that current is driven through inductor  406  to capacitor  408  via diode  410 . In particular, when MOSFET  412  is switched on, current is driven through inductor  406  to ground. When MOSFET  412  is switched off, current continues to flow through inductor  406 , and is driven through diode  410  to charge capacitor  408 . Diode  410  prevents capacitor  408  from discharging back through MOSFET  412 . In this way a DC voltage V dc  is generated across capacitor  408  which can be much higher than V bat . This DC voltage is for example fixed at 60 volts. 
         [0047]    In the present embodiment the voltage V dc  is fixed by the reference voltage V REF . Based on this reference voltage the required fixed DC voltage across capacitor  408  is provided. V REF  will generally be constant to maintain the same fixed DC voltage across capacitor  408 , but in some embodiments this reference voltage could be increased slightly when high load is expected from the liquid lenses, to prevent the fixed DC voltage V dc  dropping. 
         [0048]    In order to provide the fine voltage control of the drive voltage to each of the liquid lenses, rather than varying the level of the DC voltage generated, a duty cycle controller  418  is provided. 
         [0049]    Duty cycle controller  418  controls the duty cycle of the drive signal to each of the H-bridges such that the duty cycle of the drive signal to each liquid lens is varied. Duty cycle controller  418  generates the duty cycle signal for each liquid lens based on the data from the four registers  422 , which store data received via serial bus  324  and decoded by serial bus decoder  421 . The peak-to-peak voltage of the drive signal to each liquid lens is preferably fixed at 2V dc , which is for example at 120 V, however due to the variation in duty cycle of the drive voltage, the RMS (root mean squared) voltage of each of the drive voltages is varied, thus varying the power provided to each liquid lens. In the case that the drive voltage is a square wave, a peak-to-peak voltage of 120 V provides an RMS voltage of 60 V. Given sufficient control of the duty cycle, each liquid lens can thus be controlled with the required precision. An example of the required precision is a drive voltage that can be controlled in steps of 60 mV RMS between 0and 60 V RMS. Thus approximately 1000 steps are required between a duty cycle of 0 percent and a duty cycle of 100 percent. Furthermore, 10 or more bits of data, converted and transmitted from the ISP  304  to driver  312  via the serial bus  324 , is for example provided for each lens to determine the required duty cycle. 
         [0050]      FIG. 4  illustrates the DC-DC generator  420  of  FIG. 3  in more detail. As shown in  FIG. 4 , DC-DC generator  420  preferably comprises a comparator  500 , in this example an operation amplifier (Op amp), having two differential inputs, a first receiving the reference voltage V REF  on line  423  and the second connected to a node  502  which is connected to ground via a first resistor  504 , and to V dc  on line  424  via a second resistor  506 . The first and second resistors  504 ,  506  act as a potential divider dividing the voltage V dc  to a suitable value for comparison with V REF , for example dividing V dc  by thirty if a value of 2 V for V REF  corresponds to 60 V of V dc . Op amp  500  provides an output on line  508  to a switch control block  510 . 
         [0051]    Switch control block  510  also receives the timing signal on line  417  generated by frequency generation block  416 , and adjusts this signal based on the output of Op amp  500  to provide the switch control signal V SC  to control MOSFET  412 . 
         [0052]    The first H-bridge  426  of  FIG. 3  is shown in more detail in  FIG. 5A . The other H-bridges of  FIG. 3  comprise identical circuitry connected to the corresponding control lines. H-bridge  426  comprises four MOSFETs  601 ,  602 ,  603  and  604 . First and second MOSFETs  601 ,  602  are connected in series via their main current terminals between the fixed voltage V dc  and ground. Third and fourth MOSFETs  603 ,  604  are also connected in series via their main current terminals between the fixed voltage V dc  and ground. A first liquid lens  605  is connected between output line  434  of IC  404  which is connected to the node between the third and fourth MOSFETs  603 ,  604  and output line  436  of IC  404  which is connected to the node between the first and second MOSFETs  601 ,  602 . Alternatively, nodes  434  and  436  could be connected between one of a plurality of first electrodes of a liquid lens and a common second electrode, where the first electrodes are either in contact with the conducting liquid and the common electrode insulated from the conducting liquid, or vice versa. 
         [0053]    The gate terminals of the first and third MOSFETs  601 ,  603  are connected to lines  450 ,  452  respectively and thus directly receive the signals generated by the duty cycle controller  418 . The gate terminals of the second and fourth MOSFETs  602 ,  604  are connected to outputs of first and second two-input OR gates  606 ,  607  respectively. Each OR gate  606 ,  607  comprises a first input connected to lines  450 ,  452  respectively. A second input of each OR gate is connected to the output of a two-input NAND gate  608 , which comprises first and second inputs connected to lines  450 ,  452  respectively. 
         [0054]    In operation, when the control signal on line  452  is high whilst the control signal on line  450  is low, the first and fourth MOSFETs  601 ,  604  are switched on and line  436  is connected to V dc  whilst line  434  is connected to ground. On the other hand, when the control signal on line  452  is low whilst the control signal on line  450  is high, the second and third 
         [0055]    MOSFETs  602 ,  603  are switched on and line  434  is connected to V dc  whilst line  436  is connected to ground. When both control signals on lines  450  and  452  are low, the output of NAND gate  608  is high, and thus both the second and fourth MOSFETs  602 ,  604  will be switched on, connecting both lines  434  and  436  to ground. 
         [0056]      FIG. 5B  illustrates an alternative embodiment of the first, second, third and fourth H-bridges  426  to  432 , in which the second electrode of each of the liquid lens is connected to a single pair of common MOSFETs. Alternatively, in the case that a single lens comprises a plurality of electrodes and a common electrode, the common electrode is for example connected to a single pair of common MOSFETs. This circuitry comprises ten MOSFETs labelled  610 ,  612 ,  614 ,  616 ,  618 ,  620 ,  622   624 ,  634  and  636  respectively. The first and second MOSFESTs  610 ,  612 , are connected in series between V dc  and ground via their main current terminals. Likewise, the third and fourth MOSFETs  614 ,  616 , fifth and sixth MOSFETs  618 ,  620 , and seventh and eighth MOSFETs  622 ,  624  are also connected in series between V dc  and ground via their main current terminals. The gates of the first and second MOSFETs  610 ,  612  are connected to input lines  450  and  452  respectively. The gate terminals of the third, fourth, fifth, sixth, seventh and eighth MOSFETs are connected to input lines  454 ,  456 ,  458 ,  460 ,  462  and  464  respectively. The node between the first and second MOSFETs  610 ,  612  is connected to output line  434 , providing the output voltage V oA1  to an electrode of a first liquid lens  626 . The node between the third and fourth MOSFETs  614 ,  616  is connected to output line  438  connected to an electrode of a second liquid lens  628  providing output voltage V oA2 . The node between the fifth and sixth MOSFETs  618 ,  620  is connected to output line  442  providing output voltage V oA3  to an electrode of a third liquid lens  630 . The node between the seventh and eighth MOSFETs  622 ,  624  is connected to output line  446  providing output voltage V oA4  to an electrode of a fourth liquid lens  632 . Alternatively, output lines  434 ,  438 ,  442  and  446  could each be connected to respective electrodes of a single lens. 
         [0057]    The ninth and tenth MOSFETs  634 ,  636  of the circuit of  FIG. 5B  are common MOSFETs providing the connection to the second electrode of each of the first, second, third and fourth liquid lenses, or in the case of a single lens having multiple electrodes, to a common electrode of that lens. MOSFETs  634  and  636  are connected in series between V dc  and ground via the main current terminals. The node between MOSFETs  634  and  636  is connected to each of the first, second, third and fourth liquid lenses, effectively providing output lines  436 ,  440 ,  444  and  448  of the H-bridges. The gate terminal of MOSFET  634  is connected to the output of a first four-input OR gate  638 , having its four input pins connected to the input lines  452 ,  456 ,  460  and  464  respectively such that MOSFET  634  is switched on when the signal on any of these lines is high. The gate terminal of MOSFET  636  is connected to the output of a second four-input OR gate  640 , having its four input pins connected to the input lines  450 ,  454 ,  458  and  462  such that MOSFET  636  is switched on when the signal on any of these lines is high. 
         [0058]    In operation, when any of the signals on lines  450 ,  454 ,  458  or  462  is high while the corresponding signal on lines  452 ,  456 ,  460 ,  464  is low, the first electrode of the corresponding liquid lens will be connected to V dc , and the second electrode to ground via MOSFET  636 . On the other hand, when any of the signals on lines  450 ,  454 ,  458  or  462  is low while the corresponding signal on lines  452 ,  456 ,  460 ,  464  is high, the corresponding first electrode of the liquid lens will be connected to ground, and the second electrode to V dc  via MOSFET  634 . It will be apparent that that if a given lens is to be off during a certain period and during this period both control signals associated with the given lens are low, it is possible that one of the other signals on lines  452 ,  456 ,  460 ,  464  is high at the same time. This will result in the given lens having a first electrode that is floating and a second electrode connected to V dc . In alternative embodiments, floating nodes can be avoided in this case when a given lens is to be off by instead connecting the first electrode of the given lens to V dc  for the period when the second electrode is connected to V dc . 
         [0059]    Operation of the circuitry of  FIG. 5B  is equivalent to the operation of four H-bridges arranged according to the circuitry of  FIG. 5A  described above, and thus this circuitry reduces the number of MOSFETs required for four H-bridges by six. Given that sixteen MOSFETs are required in total in the H-bridge arrangement of  FIG. 5   a , and ten in  FIG. 5B , this results in a reduction in circuit area of the H-bridge of approximately 40 percent. 
         [0060]      FIG. 5C  illustrates an alternative embodiment of the H-bridges of  FIG. 3 , in this case showing the example when applied to a single lens having multiple electrodes. 
         [0061]    As illustrated, a liquid lens  650  comprises a plurality of electrodes  652 ,  654 ,  656  and  658  of a first type, which are all either the electrodes in contact with the conducting liquid in the lens, or the electrodes insulated from the liquids arranged close to the edge of the liquid interface. A common electrode  660  of a different type to the first type is provided, i.e. contacting the conducting liquid if the electrodes of the first type are insulated from the conductive liquid, or insulated from the conducting liquid if the electrodes of the first type contact the conducting liquid. In this example, a first full H-bridge  662  is provided, allowing the voltage Vdc or ground to be connected to output lines  663   a  and  633   b  of H-bridge  662 . Lines  663   a  and  663   b  are connected to four half H-bridges  664 ,  666 ,  668  and  670 . Each electrode  652 ,  654 ,  656  and  658  is connectable via a respective one of the half H-bridges to either line  663   a  or  663   b.  Electrode  660  is permanently connected to line  663   b.  The full H-bridge  662  and the half H-bridges  664 ,  666 ,  668  and  670  are controlled by the control signals from the duty cycle controller, as will be apparent to those skilled in the art. 
         [0062]    The H-bridge arrangement in  FIG. 5C  could also be used for driving a plurality of lenses. 
         [0063]      FIGS. 6A and 6B  illustrate the timing of signals on line  417 , lines  450  and  452 , lines  434  and  436 , lines  454  and  456 , and lines  438  and  440  in the driving circuitry of  FIG. 3 .  FIG. 6A  illustrates a case in which the duty cycle controller  418  controls the width of each pulse, this width providing the variation in duty cycle for controlling the power to each of the different lenses.  FIG. 6B  illustrates an embodiment in which the duty cycle controller  418  varies the number of standard-width pulses provided during a determined period to each of the lenses, the number of these pulses providing the variation in the duty cycle for controlling the power to each of the different lenses. This second duty cycle control is particularly relevant to embodiment of the H-bridges  426 ,  428 ,  430 ,  432  of  FIG. 5B  or the embodiment of  FIG. 5C . 
         [0064]    Firstly with reference to  FIG. 6A , the timing signal in the form of a square wave generated by the frequency generator  416  on line  417  is shown labelled  417 . The duty cycle controller operates by counting pulses of this square wave in order to generate a square wave of required pulse width for controlling each H-bridge. In particular, a data value associated with each liquid lens is received via the serial bus  324 , the serial bus decoder  421  and registers  422 , and used by the duty cycle controller  418  to determine the required pulse width for driving each liquid lens. In  FIG. 6A , the signals on lines  450  and  452  from the duty cycle controller are illustrated, in each case the count being set at three rising edges of the timing signal on line  417 . In the example shown, a 4-bit counter is used, such that the counter counts from 0 to 15 and then resets. In practice, however, in order to generate the required precision for controlling each of the H-bridges, a 10 or 12-bit counter could be used, providing, for example, between 1 and 1024 rising edges of the timing signal to be selected. 
         [0065]    With reference to the signals on lines  450  and  452 , the solid line illustrates the signal on line  450 , used for controlling the positive pulse of the drive signal for the first liquid lens, whilst the dashed line illustrates the signal on line  452  used for controlling the negative pulse of the drive signal for the first liquid lens. According to the embodiment of  FIG. 6A , a fixed period of sixteen rising edges of the timing signal on line  417  is provided for the pulses, eight rising edges for the positive pulse and eight for the negative pulse. In practice such a period would be for example 1024 or 2048rising edges of the timing signal in order to provide sufficient precision, and this period would represent, for example, a frequency of approximately 500 or 1000 Hertz, whilst the timing signal on line  417  has a frequency of, for example, approximately 1 MHz. 
         [0066]    In alternative embodiments the time period between rising edges of the positive and negative pulses is not fixed, but instead a second counter could be used to determine the rest duration between the end of one pulse and the start of the next. 
         [0067]    The output signal across lines  434  and  436  at the output of H-bridge  426  is also shown in  FIG. 6A . The positive pulse is thus amplified to V dc , which is for example 60 volts. The negative pulse on line  452  is also amplified and provided across a liquid lens electrode in the opposite direction, thus providing a negative pulse of −V dc  across the liquid lens. The peak-to-peak voltage is thus 2V dc . 
         [0068]    In a second example, the second liquid lens is controlled such that the pulse width of the positive and negative pulses is five cycles of the timing signal on line  417  plus one half cycle. This is possible if the counter counts positive and negative edges of the timing signal. Again, the solid line illustrates signal line  454 , and the dashed line illustrates the signal line  456 . The amplified signal at the output of H-bridge  428  across lines  438  and  440  are also shown. 
         [0069]      FIG. 6B  illustrates the timing of signals in a second embodiment of the duty cycle controller  418 . The timing signal on line  417  is again a square wave, again with a frequency of approximately 1 MHz. In this embodiment, the signal to the first liquid lens on line  450 , shown by the solid line, is a fixed-length pulse occurring at every other rising edge of the timing signal, thus five times in a ten pulse reference period. The signal for controlling the negative side of the liquid lens on line  452 , as shown by the dashed wave, is also the same fixed-length pulse, but occurring after each alternate falling edge of the timing signal. As shown on lines  434  and  436 , the positive and negative pulses, respectively, at the output of the first H-bridge  436  are thus fixed length pulses occurring at regular intervals. 
         [0070]    An alternative pattern is generated by the duty cycle controller  418  on lines  454  and  456 , in which three pulses are provided in within the ten-pulse reference period. 
         [0071]    Whilst for clarity in the example illustrated in  FIG. 6B  a reference period comprises only ten cycles of the timing signal, in practice, the reference period comprises for example 1024 or more cycles of the timing signal, allowing the required precision in control of the duty cycle, and thus the required precision in the control of the power to each lens. The selected pulses may be at regular intervals throughout the 1000 cycles of the timing signal, or alternatively grouped into smaller blocks. 
         [0072]    Thus circuitry has been described that independently drives a plurality of electrodes of one or more liquid lenses, the circuitry comprising a common means for generating a fixed voltage level, and a duty cycle controller for controlling the power provided to each lens/electrode. Thus an independent drive voltage signal can be provided to each of the plurality of electrodes. By independent drive voltage signals, it is meant drive voltage signals that control either different liquid interfaces, or different parts a liquid interface if there are multiple electrodes of the same type in the same lens. The independent drive voltage signal for a particular electrode are applied between that electrode and a further electrode, thus a liquid lens having just two electrodes receives only one independent drive voltage. 
         [0073]    Providing a common means for generating a fixed voltage level has the advantage that only one set of generating circuitry (inductor  406 , capacitor  408 , diode  410  and MOSFET  412 ) is required, minimizing the required resources. Furthermore, reliability and durability of the driving circuit are improved by limiting the number of components required, and particularly by limiting the number of active components. 
         [0074]    Whilst an optoelectronic module incorporating the present invention has been described above as being incorporated in a mobile telephone, embodiments of the invention could be incorporated in different applications, in particular applications in which area is restricted. This includes mobile devices in general, for example laptop computer, PDAs (personal digital assistants) or wireless local area network devices, or other devices such as barcode readers. 
         [0075]    The type of electrowetting device that may be driven by driving circuitry according to embodiments of the invention is not limited to devices in which a refractive interface between liquids is directly moveable by electrowetting, but includes lenses in which electrowetting is used to move a secondary interface, which in turn causes a refractive interface to move. Furthermore, electrowetting devices driven by embodiments of the invention could include alternative devices to lenses, such as variable liquid diaphragms. 
         [0076]    Whilst the optoelectronic module  300  has been described as comprising an ISP  304  for determining the values of the drive voltages to each lens, in other embodiments alternative processing means could be used for determining these values, such as the baseband processor of a mobile phone, or a CPU (central processing unit). 
         [0077]    Furthermore, whilst a number of examples have been provided in  FIGS. 6A and 6B  for the duty cycle control signals, alternative duty cycle formats are possible. 
         [0078]    Whilst a serial bus  324  has been described for connecting the ISP  304  to the driver circuit  312 , in alternative embodiments, a parallel bus or an alternative interface could be used, for example a wireless interface such as bluetooth. 
         [0079]    Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalent thereto.