Abstract:
A circuit for managing current for a reversible thin film stack is provided. The circuit is able to block or substantially restrict leakage current from the film stack when the circuit is in a power-off state. The circuit, in one arrangement, is also able to provide a more steady flow of charge into the film stack, thereby facilitating fast transition, while maintaining sufficient power to other parts of the system. In one arrangement, the circuit is in the form of an integrated circuit, and is positioned in or on an optical disc. The circuit connects to a thin-film optical shutter, which may be set in a clear state that allows the disc to be played, or set in dark state that makes the disc unplayable. The circuit reduces leakage current, allowing the optical shutter to maintain the desired state.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. patent application No. 60/807,387, filed Jul. 14, 2006, and entitled, “Reduced Leakage EC Film Driver”, which is incorporated herein by reference. 
     
    
     FIELD  
       [0002]     The present invention relates to a circuit and process for providing power to a thin film stack and limiting the stack&#39;s leakage current. The circuit may be integrated into an integrated circuit device or constructed of discrete components, and may responds to control functions provided by a processor.  
       BACKGROUND  
       [0003]     A thin film stack, such as an electrochromic (EC) film, changes color states by the application of a voltage potential across its electrodes. The voltage causes a charge transfer to occur, which changes the optical properties of the film. By reversing the potential across the film, it can be switched to the opposite optical state. Once the film has accepted sufficient charge to change states, the drive voltage can be removed and the film will remain in that state, as long as no leakage current paths exist to discharge the film to its rest state. In some cases, a very low leakage may be acceptable, according to the application requirements. Discharge currents cause the accumulated charge across the film to dissipate, and the film changes its optical properties in an undesirable way. Shorting the film for instance will cause the film to seek an intermediate rest state between each of the fully charged states, either positive, or negative. This state cannot be allowed for some applications of EC Films.  
         [0004]     Difficulty arises, for example, when an electronic circuit in integrated chip form is interfaced to the EC Film. Not only must the circuit drive the film in a bipolar mode when the chip is powered by an external RF field, it must not allow any currents (or only extremely small currents) to flow when the chip if powered off. The EC Film will be in one of the two charged states. Either positive, or negative. In these states, the EC Film acts somewhat like a battery and has voltage present across the EC Film. This voltage will be present across the output terminals of the chip which drives it, even after the chip is powered off. Current chip technologies which utilize silicon as the semiconductor material to implement the chip circuitry, have substrate diodes present between the input and output pins of the chip, and the supply pins, typically labeled Vcc and Vss. In some cases the diodes exist as a result of how the circuit is fabricated on the silicon, and in other cases the diodes are purposely included as a way of making the chip tolerant of static discharge events to the input/output pins, which could damage the chip.  
         [0005]     These substrate diodes are normally reversed biased when the chip is powered by an external supply. The supply could be a battery, a DC supply powered by the AC line, or RF power. Under this condition, the substrate diodes are reversed biased and do not represent leakage current paths for the EC Film. However, this condition changes when the chip loses RF power, or voltage between Vcc and Vss. In this condition, the substrate diodes appear as back to back diodes between each I/O pin and Vss. If the film is connected between any I/O pin and Vss, or between 2 I/O pins, it effectively has a discharge path thru the substrate diode, which is now forward biased. This causes a current to flow in the EC Film, which changes its optical state.  
       SUMMARY  
       [0006]     Briefly, the present invention provides a circuit for managing current for a reversible thin film stack. The circuit is able to block or substantially restrict leakage current from the film stack when the circuit is in a power-off state. The circuit, in one arrangement, is also able to provide a more steady flow of charge into the film stack, thereby facilitating fast transition, while maintaining sufficient power to other parts of the system. In one arrangement, the circuit is in the form of an integrated circuit, and is positioned in or on an optical disc. The circuit connects to a thin-film optical shutter, which may be set in a clear state that allows the disc to be played, or set in dark state that makes the disc unplayable. The circuit reduces leakage current, allowing the optical shutter to maintain the desired state. 
     
    
     BREIF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is an illustration of an optical disc having a reversible optical shutter with reduced leakage current, with the optical shutter in the dark state.  
         [0008]      FIG. 2  is an illustration of an optical disc having a reversible optical shutter with reduced leakage current, with the optical shutter in the clear or bleached state.  
         [0009]      FIG. 3  is a block diagram of a circuit for providing reduced leakage current.  
         [0010]      FIG. 4  is the circuit of  FIG. 3  shown transitioning the optical shutter.  
         [0011]      FIG. 5  is a block diagram of a circuit for providing reduced leakage current.  
         [0012]      FIG. 6  is the circuit of  FIG. 5  shown transitioning the optical shutter to the clear or bleached state.  
         [0013]      FIG. 7  is the circuit of  FIG. 5  shown transitioning the optical shutter to the dark state.  
     
    
     DETAILED DESCRIPTION  
       [0014]     Referring now to  FIGS. 1 and 2 , an optical disc  1  is illustrated. Optical disc  1  may be, for example, a DVD, a Blu-ray disc, an HD-DVD, a music CD, a data CD, or a game CD. The optical disc  1  has an optical shutter  40 . The optical shutter  40  is typically a reversible thin-film stack that has a dark state ( FIG. 1 ) or a bleached or clear state ( FIG. 2 ). The materials, processes, and uses of a such a thin-film stack are fully described in copending U.S. patent application Ser. No. 11/460,827, filed Jul. 28, 2006, and entitled, “Persistent Electro-Optic Devices and Processes for Optical Media”, which is incorporated herein in its entirety. The optical shutter is reversible, that is, it can transition between its states under control of the processor  20 . The shutter has an electro-optic or electro-chromic material that in a clear or bleached state allows the disc to be played in an associated disc player, and in the dark state, disrupts the player&#39;s ability to read the disc.  
         [0015]     Processor  20  connects to an antenna  30  for receiving an RF signal. The RF signal may be a UHF signal or an HF signal. It will be understood that the particular antenna design will be according to the RF band being used. The processor  20  also has a radio transceiver for receiving and sending data to an associated reader or scanner, and also has power convertor circuitry for converting the RF signal to power. In this way, the RF signal received from the reader/scanner is able to power-up the processor  20 , and the processor  20  and the reader/scanner are able to communicated data and instructions. Processor  20  also has output lines connected to the optical shutter  40 . In this way, the processor  20  is able to send a power signal to the optical shutter to effect a state transition. Typically, sending power with one polarity arrangement will transition in one direction, while sending power in the other polarity arrangement will transition in the other direction. The construction and use of RF circuits for controlling an optical shutter is fully described in U.S. patent application Ser. No. 11/457,428, filed Jul. 13, 2006, and entitled, “Devices and Methods for RF Communication with an Optical Disc”, which is incorporated herein in its entirety.  
         [0016]     Optical disc  1  may be manufactured and initially shipped with the optical shutter  40  in its dark state. Since the disc will not play in a typical disc player, the disc is less likely to be stolen, and may be packaged, shipped, and displayed with reduced security measures. At a point of sale, a reader/scanner provides an RF signal that powers-up processor  20 . Processor  20  and the reader/scanner communicate data packages to authenticate the disc  1  and to confirm that the disc is ready for activation. If the disc is ready, then the reader/scanner sends an activation key to the processor  20 . If the key is correct, then the processor  20  applies power to the optical shutter. The power is applied with the polarity set to transition the optical shutter from the dark state to the clear state.  
         [0017]     At a later time, the consumer may return the disc to the retailer. To accept the disc back into stock, the retailer uses another reader/scanner device to power-up and communicate with the disc. However, this time the reader/scanner provides the instructions for the processor to provide the power at the polarity to cause the optical shutter  40  to transition back to the dark state. Then, the disc may be returned to stock. It will be appreciated that many applications may benefit from a reversible thin film stack.  
         [0018]     The processor  20  has circuitry to more stably maintain the optical shutter in its desired state. Since a reversible thin-film stack has a potential, this circuitry acts to block or substantially restrict the amount of current that can leak when the processor is not powered.  
         [0019]     Referring now to  FIG. 3 , a reduced-leakage current circuit is illustrated. Circuit  50  has a processor  52  connected to an antenna  54 . When in the presence of a proper RF field, a rectifier provides the processor  52  with a power signal, and allows the processor and a scanner/reader to communicate data. The data is used by logic and memory  58  to determine when it is appropriate to transition the thin-film stack  68  to its other state. The circuitry  50  has a current control module  62  that determines which of two current lines  59  or  61  is used. Each current line  59  and  60  requires a different isolated return path, which is selected by the return path control  64 . To transition in one direction, the processor  52  selects line  59  and its associated isolated return path. To transition in the other direction, the processor selects line  61  and its associated isolated return path. Also, current control  62  may act to control the amount of current flowing into the film stack. When power is first applied to the film-stack, the amount of charge transferred to the film stack is limited through a relatively large resistive load. By limiting the amount of power drawn by the film stack, the risk of deactivating the processor is reduced. As the film stack charges, the resistive load is reduced, but current stays relatively constant to the film stack. By keeping the current to the film stack relatively constant, the film can be transitioned rapidly, and the risk of killing the processor is reduced. It will be apperceived that the current regulation may be done in an open loop arrangement, or may be adjusted responsive to the measured level of current moving into the film stack. Also, although the current regulation may be implemented using resistive or other loads, it will be appreciated that current can be limited or adjusted in other ways.  
         [0020]      FIG. 4  shows the circuit  50  of  FIG. 3  transitioning from a first state to a second state. In this transition, the processor uses the current control module and the return control module to set current path  71 . Even when power is removed from the processor, the selected return path continues to act block or substantially limit leakage current when the thin film is in its second state.  FIG. 4  also shows the circuit  50  of  FIG. 3  transitioning from the second state to the first state. In this transition, the processor uses the current control module and the return control module to set current path  73 . Even when power is removed from the processor, the selected return path continues to act block or substantially limit leakage current when the thin film is in its first state.  
         [0021]     Referring now to  FIGS. 5, 6 , and  7 , a specific implementation of a reduced-leakage circuit  100  is illustrated. Although a specific example is shown, it will be understood that any integrated circuit device may be isolated by this technique, and that the circuit could be integrated into an integrated circuit chip.  
         [0022]     The example circuit ( 100 ) as shown in  FIG. 5  is implemented as a discrete-component design. It will be understood that the implementation of  FIG. 5  may be modified in many ways consistent with this disclosure, and that it is readily adaptable to integration into an integrated circuit chip. Referring to  FIG. 5 , U 1  ( 102 ) is a microcontroller (uC) from TI, a MSP430. Other processors may be used. It is powered from an external power source (not shown). The power source applies a positive voltage to the Vcc terminal with respect to the Vss terminal. This voltage is typically +3.6 Volts, but will depend on specific components selected. The microcontroller  102  receives input data signals via port P 1 . 1  from an external source, which instructs the microcontroller to change its I/O port pins (P 1 . 0  to P 1 . 7 , and P 2 . 0  to P 2 . 7 ) in accordance with its internal coded instructions. Note that in alternate implementations the instructions may be hard coded into the chip as a hardware state machine, as opposed to firmware instructions residing in internal memory. The ports of the microcontroller which are used, (P 2 . 0 , P 2 . 1 , P 1 . 0 , P 1 . 3 , P 1 . 5 , and P 1 . 7 ) are configured as digital outputs. They switch levels between Vss and Vcc. Substrate diodes exist on each of these port lines to Vcc and Vss within the uC.  
         [0023]     A current limiting module  105  acts to linearize the charge passed to the EC film stack  111 . R 1  thru R 4  ( 105 ) serve to limit the current that the film can draw from the output ports during switching. The EC Films behave similar to both batteries and capacitors in that they work by a transfer of charge. If the film is switched by a constant voltage source, it draws a very high initial current which then decreases in approximately exponential fashion to zero. This is similar to hanging an uncharged capacitor across the output port of the IC. The resulting large current would cause the voltage supply of the IC to collapse since there is a very limited amount of RF power available to power the IC. Therefore, it is desirable to limit the current draw, and R 1  thru R 4  ( 105 ) provide this function. However, if a single resistor is used to limit the current, the charge transfer takes about 3 time constants (3RC), where R is the limiting resistor, and C is the capacitance of the EC Film. This is undesirable, so two different value resistors are connected to 2 ports, so that either or both of them can limit the current. By properly switching the outputs, the current draw of the film can be maintained at relatively constant value. This allows the time constant for switching to be reduced, while still limiting the current to an allowable level that can be supply via the RF source. It will be understood that more or fewer resisters may be used to provide the current curve needed for a particular application.  
         [0024]     Diodes D 1  and D 2  ( 106 ) provide isolation from the substrate diodes in the microcontroller  102 . Q 1  ( 113 ) and Q 2  ( 115 ) are N channel enhancement mode FETS which allow the EC Film  111  to be driven in a bipolar fashion. As will be further described, these FETS act as isolation switches. The substrate diodes in the FET&#39;s ( 113  and  115 ) provide another isolation barrier for leakage currents from the EC film stack  111 .  
         [0025]     J 2  ( 107 ) may be a physical connector, or simply pads on the optical media, chip, or EC film  111  that allows an external DC supply to bias the EC Film in either of two states, based on the connection polarity. This can be achieved with the microcontroller in an un-powered condition. This may be very desirable during the manufacturing process in order to initialize the EC film in its dark state.  
         [0026]     As an example, if the microcontroller is not powered, all of its I/O ports are at zero volts, and further, are in a high impedance condition. However, the substrate diodes are effectively connected back to back between each I/O port and Vss. Since the ports are at zero volts, Q 1  ( 113 ) and Q 2  ( 115 ), which are enhancement mode FET&#39;s, are turned off. If a battery is connected across J 2  ( 107 ) in such a way that the one terminal is driven positive with respect to the other terminal of the EC film stack, then the film will be charged to its dark state. Note that there is no current path for positive current to flow from the film stack. Diode D 2  ( 106 ), and the substrate diode of Q 1  ( 113 ) are both reversed biased, and Q 1  ( 113 ) is off. When the battery is removed, this situation remains the same. Without any current path, the EC film  111  retains its charge. In reality, there will be some very small leakage current thru D 1  ( 106 ), the substrate diodes in the microcontroller, and the substrate diode of Q 1  ( 113 ). Since this is a series path, the magnitude of the current can be controlled by selecting D 2  ( 106 ) to be very low leakage. If the polarity of the battery across J 2  ( 107 ) is reversed, the EC film  111  is charged to the alternate state (Light). Since the circuit is symmetric, the same analysis applies. D 1  ( 106 ), and the substrate diode of Q 2  ( 115 ) are reversed biased, and Q 2  ( 115 ) is off, so no current flow can occur.  
         [0027]     Note that the EC film  111  will not be discharged by the un-powered circuit in either state. Further, it does not matter if the film was charged by an external battery, or the circuit itself, when powered.  
         [0028]     The circuit can change the state of the EC film stack from its initial dark state to the Light state, in the following manner (see  FIG. 6 ). The uC powers up from an external RF power source, and receives a command to activate the optical media by switching the EC Film state to Light. The uC asserts P 2 . 1  which causes the LIGHTDRIVE signal to go high, which turns on Q 1  ( 113 ). At the same time, P 1 . 0  is asserted and goes high. This in turn causes a current to flow thru R 1  and deliver charge to the EC film  111 . As the EC film  111  accepts the charge, its terminal voltage increases, which reduces the voltage drop across R 1 . In order to keep the current relatively constant, P 1 . 3  is asserted high, and P 1 . 0  is asserted low. This increases the current by about a factor of 2. At some later time, P 1 . 0  is asserted high again, and the current is now increased once again, since R 1  and R 2  are effectively in parallel. This keeps the current approximately constant and allows the EC film  111  to charge approximately 3 times faster than using a single resistor to limit the current. Once the EC film is charged, P 1 . 0  and P 1 . 3  can remain high until the uC powers down, or P 1 . 0  and P 1 . 3  can be asserted low, along with P 2 . 1 .  
         [0029]     The EC Film can also be switched by the circuit to the dark state for the Light state by asserting P 2 . 0 , P 1 . 5 , and P 1 . 7  while all the other ports are low (see  FIG. 7 ). Asserting P 2 . 0  and P 2 . 1  both high is not allowed, as that would turn on both Q 1  and Q 2  and effectively short the EC Film to zero volts, which is not a desired optical state.  
         [0030]     A truth table for the system is shown in Table 1 below.  
                                                 TABLE 1                           System States            Chip                           Film       State   P1.0   P1.3   P2.1   P1.5   P1.7   P2.0   State               No   0   0   0   0   0   0   Either -       Pwr                           Hold       Pwr   1   0   1   0   0   0   &gt;Light       Pwr   0   1   1   0   0   0   &gt;Light       Pwr   1   1   1   0   0   0   LIGHT       Pwr   0   0   0   0   0   0   LIGHT       Pwr   0   0   0   1   0   1   &gt;Dark       Pwr   0   0   0   0   1   1   &gt;Dark       Pwr   0   0   0   1   1   1   DARK       Pwr   0   0   0   0   0   0   DARK       Pwr   x   x   1   x   x   1   Shorts                                   the Stack                  
 
         [0031]     In some cases it may be desirable to discharge the thin film stack. For example, if the film stack is charged in one state, the film stack can quickly, and without the application of power, transition to its rest state. If both P 2 . 0  and P 2 . 1  are turned on, then both Q 1  ( 113 ) and Q 2  ( 115 ) are activated. In this way the terminals of the film stack are shorted to ground, and the film stack quickly discharges to its rest state. After the stack is in its rest state, then the circuit  100  can resume operation as discussed above to set the film into its desired state. By first shorting the film, the overall transition may be completed more quickly and with less power. In some cases, the rest state may be one of the desired states. In this case, if the film stack is in the other state, then simply shorting the film stack as described will transition the stack.  
         [0032]     While particular preferred and alternative embodiments of the present intention have been disclosed, it will be appreciated that many various modifications and extensions of the above described technology may be implemented using the teaching of this invention. All such modifications and extensions are intended to be included within the true spirit and scope of the appended claims.