Abstract:
Methods and apparatus are provided for generating low EMI display driver power supply. The methods and apparatus include switching circuits that utilize two groups of parallel circuit traces, each of which is coupled to one end of a switching device. The two groups of traces are configured to be interleaved with each other such that no two traces from either group are next to any other traces from the same group. When the switching device is activated, current flows through the circuit and charges an energy storage element. When the switching device is deactivated, the energy storage element discharges a portion of its energy to a second energy storage element and to the driver circuits. In another embodiment, an additional circuit trace is provided which is only connected on one end and is free floating on the other end to capture the majority of EMI remaining that was generated by the switching circuit.

Description:
BACKGROUND OF THE INVENTION 
   This relates to switching circuitry that may be used to drive display drivers, and particularly to providing switching circuitry that operates at switching high speeds while producing low EMI output. 
   There are various well known techniques for generating supply voltages to display driver circuits. In one instance, for example, a charge pump circuit may be used to act as a high voltage power source for a display driver. In that instance, the charge pump could be configured to first charge a capacitor to a given voltage from a battery. Once charged, the capacitor may be placed in a series connection with the battery to effectively double the output voltage. For example, a 3 volt battery may be used to charge a capacitor, which could then be placed in series with the battery to provide a 6 volt output. Charge pumps often operate at relatively high energy efficiencies, but often don&#39;t provide as much current as other methods, such as a switching regulator. For example, typical charge pumps provide energy at power conversion efficiency on the order of about 90%. 
   Another well known technique for providing energy to display driver circuits is to use a switching regulator circuit. In a switching regulator circuit, a switch is used to charge and discharge an active element, such as an inductor, to provide an output voltage. Switching regulators are often used to supply high current, however, such circuits typically generate radiated energy as part of the switching process. The radiated energy is often observed as noise on the circuits surrounding the switching regulator. Switching regulator circuits often produce lower power conversion efficiency, which can be on the order of 80-85% efficiency. 
   Charge pump circuits may provide energy without the introduction of noise, however, that energy is produced at a lower current driving capability due to the large internal resistance of such circuits. This may not be an issue in instances where the display itself is relatively small, such as the display on an Apple iPod Nano product. However, conventional charge pump circuits may not be able to provide the current necessary to drive a larger display, such as the ones used on Apple&#39;s iPhone and iPod Touch products. 
   SUMMARY OF THE INVENTION 
   In accordance with embodiments of the invention, methods and apparatus are provided for generating supply voltages for display driver circuits at very high efficiencies and with low quantities of radiated energy (i.e., low noise). In particular, the methods and apparatus are provided to utilize switching regulator circuits that have been modified such that multiple circuit paths are created which carry electric current in opposite directions in order to cancel out the radiated noise of each path. In addition, additional terminal lines are provided which act to sink any electromagnetic interference (EMI) generated in the outermost paths that are actively coupled to the circuit (e.g., the paths in which current flows). 
   Embodiments of the present invention provide the capability to produce relatively large amounts of current, which can be used in driver circuits for relatively large displays such as the Apple iPhone display, without incurring the typical penalties associated with EMI or noise in such implementations. In conventional implementations of chip on glass (COG), an integrated circuit (IC) may be located on one side of the glass used in displays. The IC may include a transistor which operates as the switch in the switching regulator. The transistor may include multiple parallel leads connected to the source and multiple parallel leads connected to the drain. The leads may be connected to a piece of flex circuitry to complete the circuit via circuit elements formed of indium tin oxide (ITO). ITO is particularly useful in display applications because it is a transparent material, but it has a high resistance (it may be on the order of about 10 ohms or so), which can result in a voltage drop of about 500 millivolts. 
   In one embodiment of the present invention, the parallel source and drain paths are configured in an alternating relationship, such that a source path to ground is placed between each two drain paths which are configured to provide the output voltage. In this manner, the EMI generated in the source paths is cancelled by the EMI generated in the drain paths, because the currents through them flow in the opposite direction to each other. 
   In another embodiment of the present invention, the reduction in EMI is more pronounced by the use of a terminal lead (i.e., a lead that is only connected at one end) at the periphery edges of the circuit. The terminal leads act essentially as RF antennas to pick up any leaking fields generated by the last fully-connected paths in the circuit. 
   Various other alternative embodiments are possible. 
   Therefore, in accordance with the present invention, there is provided methods and apparatus for producing sufficient current to drive circuits for relatively large displays, such as the Apple iPhone, which do not generate the electromagnetic interference (EMI) typically associated with such circuits. In addition, the reduction in EMI can be increased through the use of terminal leads. 
   Media player apparatus operating in accordance with the methods and circuits of the present invention are also provided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
       FIG. 1  is a schematic diagram of a switching regulator which may be used in accordance with an embodiment of the present invention; 
       FIG. 2  is a timing diagram depicting the operation of a switching regulator such as the switching regulator shown in  FIG. 1  in accordance with an embodiment of the present invention; 
       FIG. 3  is a schematic diagram of a conventional implementation of a switching regulator to provide drive current to a digital display in accordance with an embodiment of the present invention; and 
       FIG. 4  is a schematic diagram illustrating various embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a switching regulator circuit  100  that can be implemented in accordance with the principles of the present invention. Switching regulator  100  may include a voltage source  102  that produces a voltage V, an inductor  104  that stores a current I, a diode  106  that prevents energy from the output device from being drained by the switching regulator, and a transistor switch  110 . Diode  106  is coupled to capacitor  108 , which provides the output voltage to the display driver circuit (not shown). As shown, voltage source  102  is configured to be connected between ground and inductor  104 . Inductor  104  may be coupled to both diode  106  and to the drain of transistor  110  to provide operation as described below. The source of transistor  110  is coupled to ground, while the gate of transistor  110  is coupled to a control line. This configuration is commonly known as a boost regulator. 
     FIG. 2  shows a control timing diagram  200  that may be used to show the operation of switching regulator  100 . Timing diagram  200  may include, for example, control trace  202 , which would be the control signal applied to the gate of transistor  110  of  FIG. 1 . Timing diagram  200  may also include current trace  204 , which shows the current being conducted by inductor  104  of  FIG. 1 . If the current passing through inductor  104  remains constant, there will be essentially no voltage drop across inductor  104  (a negligible drop related to the copper used to form the windings of inductor  104  will occur). 
   Switching regulator  100  may be operated in the following manner. When the control signal  202  is HIGH, for example at time  206 , the voltage on the gate of transistor  110  causes current to flow from the drain to the source of transistor  110  (and then on to ground). Thus, voltage source  102  provides an input voltage to inductor  104  that causes the current flowing through inductor  104  to ramp up, as shown at time  208  in current trace  204  (as shown by arrow  112  in  FIG. 1 ). Once the control signal at the gate of transistor  110  switches to a LOW state, as shown at time  210  in  FIG. 2 , the switch end of inductor  104  (i.e., the end coupled to diode  106  and to transistor  110 ) swings positive, which causes diode  106  to become forward-biased. This causes current to flow through diode  106  and through capacitor  108  to ground, thereby enabling capacitor  108  to be charged to a voltage that is higher than the voltage of source  102 . Thus, at that time, the circuit follows the path shown by arrow  114  in  FIG. 1 . 
   The output voltage V 2  across capacitor  108  may vary slightly as the switch turns ON and OFF. However, the speed at which the switching occurs may result in little variance in the output voltage V 2 . This is why the “efficiency” of switching is so high (90% or higher). While the gate of transistor  110  is in the LOW (or OFF) state, the current flowing from inductor  104  will actually flow to both capacitor  108 , as well as to the load connected to capacitor  108 . In order to limit the current flowing from diode  106  from falling below a certain level, at time  212 , for example, the control signal applied to the gate of transistor  110  switches back to a HIGH state, once again causing the circuit to operate as indicated by arrow  112  in  FIG. 1 . During that time, the output load is provided energy solely from capacitor  108 , as inductor  104  is charged back up. 
     FIG. 3  shows one implementation of a switching regulator circuit  300  used to generate direct voltage (DC) for a digital video display (not shown). Switching regulator  300  may include inductor  304 , diode  306  and transistor  310  (elements  304 ,  306  and  310  may be similar to those previously described with respect to  FIG. 1 ). Instead of using a substance such as copper or gold for the bonding wire, however, it may be preferable to use indium tin oxide (ITO) because it is transparent (which is needed since the circuit is being used to drive a display). ITO, unlike gold, has a relatively high resistance, which can be something on the order of about 10 ohms, but can be as high as 50 ohms or more. In order to reduce the resistance, multiple traces are used for a single switch. For example, by breaking up a signal which would have had a resistance of 50 ohms into four paths, the resistance of each path drops to 12.5 ohms (50 divided by 4). 
     FIG. 3  also shows a series of resistors  320 - 328  that are coupled in parallel between the source of transistor  310  and ground, as well as a series of resistors  330 - 338  that are coupled between the drain of transistor  310  and inductor  304  and diode  306 . Each of these “resistors” is not an actual, physical, resistor that has been coupled into regulator  300 . Instead, each of these resistors represents the resistance of the ITO material that is used as a “bonding wire” in regulator  300 . In addition to the components shown, regulator  300  also includes voltage source  302  and capacitor  308 , both of which operate as previously described with respect to  FIGS. 1 and 3  (in which similarly numbered elements were described—e.g., voltage source  102  in  FIG. 1  versus voltage source  302  in  FIG. 3 ). The division between glass and flex circuitry is shown generally by dashed line  340 , such that the “glass” side is represented by arrow  342 , while the “flex” side is represented by arrow  344 . 
   As generally described above, regulator  300  operates in a manner similar to that of regulator  100 . As the gate of transistor  302  is switched from LOW to HIGH, current flowing through inductor  304  will ramp up causing diode  306  to become reverse-biased (and thereby to act as a blocking diode). Current will continue to flow through parallel “resistors”  330 - 338 , through transistor  310 , and through parallel “resistors”  320 - 328 . When the gate of transistor  310  is switched from HIGH to LOW, current flows directly from inductor  304  through diode  306  (which is then forward-biased), to capacitor  308 , which charges capacitor  308  to a voltage higher than the voltage of voltage source  302 , as well as providing current from inductor  304  directly to the load attached to capacitor  308 . 
   One of the problems associated with the use of regulators like regulator  300  is the relatively large amount of EMI produced by the circuit. This is particularly troublesome in instances where the regulator circuit is being used to drive a display of a device that may be susceptible to such interference, such as a cellular or WIFI communications device (although the EMI problems could, in fact, negatively affect such operations as the playback of audio or video files). In those instances, the interference may cause an unacceptable degradation in the quality of the transmitted and/or received signals that the user&#39;s experience becomes virtually intolerable. Alternatively, the generation of EMI may require the hardware designers to implement complicated and potentially expensive solutions to deal with the EMI. These solutions could also potentially add to the overall weight and/or size of the device that the regulator is to be used in. 
     FIG. 4  shows a switching regulator  400  which has been configured to operate in accordance with the principles of the present invention. Switching regulator  400  provides a high efficiency output which is capable of driving relatively large digital video displays with low EMI emissions. The displays can be on the order of the size of, for example, an Apple iPhone of Apple iPod Touch, or even larger. 
   Switching regulator  400  includes voltage source  402 , inductor  404 , diode  406 , capacitor  408  and transistor  410 . Each of these components operates in a similar manner as described above with respect to  FIGS. 1 and 3 . In addition, switching regulator  400  includes source “resistances”  420 - 428  and drain “resistances”  430 - 438  which, as described above, are not discrete, physical resistors, but are, in fact, representative of the resistance which occurs from the use of indium tin oxide instead of gold for the bonding wire. The division between the glass and the flex circuitry is generally indicated by dashed line  440 , with arrow  442  indicating generally the glass side, and arrow  444  generally indicating the flex side. 
   Unlike the configuration shown in  FIG. 3 , switching regulator  400  produces little to no electromagnetic interference. This is accomplished by configuring the parallel source paths and the parallel drain paths in a specific manner. In particular, in accordance with the principles of the present invention, the parallel source paths are interleaved with the parallel drain paths. For example, drain path  430  is configured to be in between parallel source paths  420  and  422 . Source path  422  is between parallel drain paths  430  and  432 . Drain path  432  is between parallel source paths  422  and  424 , and so on. 
   The interleaving of source and drain paths provides the positive result that EMI produced on one path is substantially cancelled by the EMI produced on one or more adjacent paths. This is illustrated in  FIG. 4  by arrows  470  and  472 . Arrows  470  show that, when the control signal applied to the gate of transistor  410  is HIGH (and current is flowing through transistor  410 ), the current through the source paths is flowing downward, from the glass area to the flex area. At the same time, however, the current flowing through drain paths is flowing upward, from the flex to the glass, as shown by arrows  472 . Since the current flowing through a source path should be substantially the same as the current flowing through a drain path, but in the opposite direction, the EMI generated in one path should be substantially cancelled out by the EMI generated in the other path. 
   Operation of switching regulator  400  is similar to the operation described previously with respect to  FIGS. 1-3 , except that switching regulator produces significantly less EMI and/or noise than the previously described switching regulators. When the control signal applied to the gate of transistor  410  is HIGH, such that current flows through transistor  410 , EMI produced through the source paths is essentially canceled by the EMI produced through the drain paths, which is traveling in the opposite direction. When the control signal applied to the gate of transistor  410  is LOW, current flows from inductor  404  and does not pass through transistor  410 . Accordingly, little to no EMI is generated in that instance as well. 
   An additional embodiment of the present invention is also shown in  FIG. 4 . It may be additionally advantageous, in accordance with the principles of the present invention, to provide two additional paths, shown as dashed components  450  and  460 , to further reduce EMI effects, while maintaining a highly efficient switching regulator. In particular, it may be advantageous to add an additional drain path shown by “resistance”  452 , as well as an additional source path shown by “resistance”  462 . These paths are configured such that they are “terminal” paths, in that they are only connected at one end. Moreover, because of this configuration, there will not be any current flowing through these paths. However, the paths will still operate to pick up any leaking EMI field generated by the adjacent paths. This pick up effect is indicated by arrows  480  and  482 . For example, arrow  480  is shown to be pointing toward the bottom of  FIG. 4 , to indicate that it will absorb any counter leaking EMI in the opposite direction as indicated by arrow  472  on path  438 . The terminal paths would only be necessary next to the outer most fully functional paths (i.e., in  FIG. 4 , the outer most fully functional paths are shown by reference numerals  420  and  438 ). 
   Thus it is seen that methods and apparatus for producing low EMI energy at levels necessary to drive varying sizes of digital displays are provided. The present invention produces current sufficient to drive relatively large digital displays, such as the touch screen on the Apple iPhone, without generating the negative effects of high EMI radiation. It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention, and the present invention is limited only by the claims that follow.