Abstract:
A combined touch sensor and light-emitting-diode (LED) driver comprises a touch sensor circuit configured to detect a touch, where the touch sensor circuit is coupled to a common node and configured to operate with a first operating voltage, an LED driver circuit configured to drive an LED if the LED is coupled to the common node, where the LED driver circuit is also coupled to the common node and configured to operate with a second operating voltage is higher than the first operating voltage, and an n-type field effect transistor (FET) connected in series between the common node and the touch sensor. The n-type FET prevents the higher operating voltage of the LED driver from affecting the operation of the touch sensor, when a port of the combined touch sensor and LED driver IC is used to drive an LED. The touch sensor may be a capacitance-to-digital converter.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a combined touch sensor and LED (Light-Emitting Diode) driver. 
         [0003]    2. Description of the Related Arts 
         [0004]    Modern electronic devices often have both a display device to display information and touch sensors to receive input data. There are a variety of types of touch sensor applications, such as touch screens, touch buttons, touch switches, touch scroll bars, and the like. For example, a cellular telephone or personal digital assistant often has a touch screen and a liquid crystal display (LCD) device overlaid with the touch screen. 
         [0005]    LCDs typically require a backlight to provide a light source for the LCD display. White LEDs are being used increasingly as the backlight for LCDs. These white LEDs for backlighting LCDs are typically driven by an LED driver that feeds high, constant sink current through the white LEDs to provide constant luminescence, while the anode of the white LED is typically driven by a charge pump circuit. 
         [0006]    Touch sensors have a variety of types, such as resistive type, capacitive type, and electromagnetic type. A capacitive touch screen is coated with a material, typically indium tin oxide, that conducts a continuous electrical current across a sensor. The sensor exhibits a precisely controlled field of stored electrons in both the horizontal and vertical axes of the display to achieve capacitance. The human body is also an electrical device which has stored electrons and therefore also exhibits capacitance. When the sensor&#39;s normal capacitance field (its reference state) is altered by another capacitance field, e.g., by the touch with someone&#39;s finger, capacitive type touch sensors located at each corner of the touch screen panel measure the resultant distortion in the characteristics of the reference field and send the information about the touch event to the touch screen controller for mathematical processing. There are a variety of types of capacitive touch sensors, including Sigma-Delta modulators (also known as capacitance-to-digital converters (CDCs)), charge transfer type capacitive touch sensors, and relaxation oscillator type capacitive touch sensors. 
         [0007]    Because of the small size required in mobile electronic devices such as cellular telephones, LED drivers are sometimes combined with touch sensors on one integrated circuit (IC) chip. In this case, one or more ports of the combined touch sensor and LED driver IC may be used for the touch sensors in one instance and the LED driver in another instance depending upon the settings on the IC. These common, shared ports on the combined touch sensor and LED driver IC are beneficial, because (i) the size of the IC may be reduced and (ii) the same port may be conveniently used with the touch sensor or the LED driver depending upon the user&#39;s settings and needs. However, combining the LED driver with touch sensor on one IC with shared ports may present problems due to different operating voltages used in the LED driver and the touch sensor. Touch sensors typically operate on an operating voltage of 1.65-1.95 volt, while LED drivers typically operate on a much higher operating voltage of 3.0-4.3 volt in order to drive the LED. Since the LED driver is fabricated on the same IC as the touch sensor and both the LED driver and touch sensor may be connected to a shared port of the combined touch sensor and LED driver IC, the higher operating voltage of the LED driver may affect the operation of the touch sensor circuit and thereby cause malfunction in the touch sensor circuit or even damage the touch sensor circuit. 
         [0008]    Thus, there is a need for a combined touch sensor and LED driver IC without such problems. 
       SUMMARY OF THE INVENTION 
       [0009]    Embodiments of the present invention include a technique for electrically separating the different operating voltages of an LED driver circuit and touch sensor circuit in a combined touch sensor and LED driver IC. The touch sensor circuit may be a capacitance-to-digital converter (CDC) circuit. More specifically, in one embodiment, a combined touch sensor and light-emitting-diode (LED) driver comprises a touch sensor circuit configured to detect a touch, where the touch sensor circuit is coupled to a common node and configured to operate with a first operating voltage, an LED driver circuit configured to drive an LED if the LED is coupled to the common node, the LED driver circuit also coupled to the common node and configured to operate with a second operating voltage that is higher than the first operating voltage, and an n-type field effect transistor connected in series between the common node and the touch sensor. The n-type field effect transistor may be an n-type MOSFET (Metal Oxide Silicon Field Effect Transistor). The present invention has the advantage that the higher operating voltage of the LED driver circuit is prevented from affecting the operation of the touch sensor circuit, when a port of the combined touch sensor and LED driver IC is used to drive an LED. 
         [0010]    The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The teachings of the embodiments of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. 
           [0012]      FIG. 1  illustrates a combined capacitance-to-digital converter (CDC) and LED driver used as the CDC circuit, according to one embodiment of the present invention. 
           [0013]      FIG. 2  illustrates a combined capacitance-to-digital converter (CDC) and LED driver used as the LED driver, according to another embodiment of the present invention. 
           [0014]      FIG. 3  illustrates a combined capacitance-to-digital converter (CDC) and LED driver used as the LED driver, according to still another embodiment of the present invention. 
           [0015]      FIG. 4A  illustrates the CDC circuit and how an n-type MOSFET is added to the CDC circuit, according to one embodiment of the present invention. 
           [0016]      FIG. 4B  illustrates the operation of the CDC circuit of  FIG. 4A  in one phase, according to one embodiment of the present invention. 
           [0017]      FIG. 4C  illustrates the operation of the CDC circuit of  FIG. 4A  in another phase, according to one embodiment of the present invention. 
           [0018]      FIG. 5A  is a timing diagram illustrating the operation of the CDC circuit of  FIG. 4A , when the capacitance on the touch screen is not disturbed by a touch on the touch screen. 
           [0019]      FIG. 5B  is a timing diagram illustrating the operation of the CDC circuit of  FIG. 4A , when the capacitance on the touch screen is disturbed by a touch on the touch screen. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0020]    The Figures (FIG.) and the following description relate to preferred embodiments of the present invention by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed invention. 
         [0021]    Reference will now be made in detail to several embodiments of the present invention(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
         [0022]      FIG. 1  illustrates a combined capacitance-to-digital converter (CDC) and LED driver used as the CDC circuit, according to one embodiment of the present invention. The combined CDC and LED driver IC  100  includes both a CDC module  102  and an LED driver module  104 . The CDC driver module  102  operates to sense touches on a touch screen (not shown herein). The CDC driver module  102  includes the actual CDC circuit  106  that operates with a touch screen (TS) operating voltage of VDD 1  (e.g., 1.65 V-1.95 V), and an n-type MOSFET (NMOS)  110  connected in series with the CDC circuit  106 . The LED driver  104  operates with an operating voltage of VDD 2  (e.g., 3.0 V-4.3 V). The LED driver  104  can be any conventional type of LED driver that provides a regulated current to an LED. For example, one such LED driver is illustrated in U.S. patent application Ser. No. 11/855,904 filed on Sep. 14, 2007 entitled “Progammable LED driver,” which is assigned to the same assignee as the present application and is incorporated by reference herein in its entirety. Likewise, the CDC circuit  106  may be any type of CDC circuit that detects touches on a touch screen and converts the charges stored in a capacitor to digital values. One example of a CDC circuit is illustrated in  FIG. 4A , as will be explained below in more detail. Although the embodiment of  FIG. 1  is illustrated as a combined CDC and LED driver with the CDC being a type of touch sensor, the present invention can be used with any type of combined touch sensor and LED driver IC with the touch sensor and the LED driver connected to a common node, as long as the touch sensor includes a component configured to operate on a voltage lower than the voltage used by the LED driver. 
         [0023]    Both the CDC module  102  and the LED driver  104  are connected to the port  108  of the IC  100 , so that the port  108  can be used to control either the CDC module  102  or the LED driver  104  depending upon the application of the IC  100 . The example of  FIG. 1  illustrates the IC  100  being used as a CDC application. Thus, the sense capacitor C sensor  that detects the touches on the touch screen is connected in series to the port  108 . Although only one port  108  is shown in  FIG. 1  for simplicity of illustration, the IC  100  may have many such ports, some of which are shared between the CDC module  102  and the LED driver  104  as in  FIG. 1  and others of which are dedicated to either the CDC module  102  or the LED driver  104 . 
         [0024]    The NMOS  110  is connected between the port  108  and the CDC circuit  106 . As will be explained below, a non-overlapping 2-phase clock (P 1 , P 2 ) is applied to the gate of NMOS  110 , so that the NMOS  110  is maintained in the “on” state most of the time except during the transitional periods of the non-overlapping 2 phase clock (P 1 , P 2 ). The NMOS  110  prevents the operating voltage VDD 2  of the LED driver from affecting the CDC circuit  106  when an LED driver  104  is connected to the port  108  and the IC  100  is used as an LED driver. More specifically, when VDD 1  is applied to the gate of NMOS  110 , the voltage at node  112  is clamped and does not exceed VDD 1 −Vt(n), where VDD 1  is the operating voltage of the CDC circuit  106  and Vt(n) is the threshold turn-on voltage of NMOS  110 . Note that a p-type MOSFET may not be used in the place of NMOS  110 , because such p-type MOSFET would pass a voltage higher than VDD 1  to the CDC circuit  106 . 
         [0025]      FIG. 2  illustrates a combined capacitance-to-digital converter (CDC) and LED driver used as the LED driver, according to another embodiment of the present invention. The IC  100  of  FIG. 2  is the same as the IC  100  of  FIG. 1 , except that the IC  100  is used as an LED driver application in the example of  FIG. 2 . Thus, an LED  116  is connected between port  108  and ground. The LED driver  104  includes a current source  114  that provides regulated current to the LED  116  through the port  108  of the IC  100 . The current source  114  is connected between the operating voltage VDD 2  and the port  108 . However, NMOS  110  prevents the operating voltage VDD 2  of the LED driver  104  from affecting the CDC circuit  106 . As explained above, when VDD 1  is applied to the gate of NMOS  110 , the voltage at node  112  is clamped and does not exceed VDD 1 −Vt(n), where VDD 1  is the operating voltage of the CDC circuit  106  and Vt(n) is the threshold turn-on voltage of NMOS  110 . Note that a p-type MOSFET may not be used in the place of NMOS  110 , because such p-type MOSFET would pass a voltage higher than VDD 1  to the CDC circuit  106 . 
         [0026]      FIG. 3  illustrates a combined capacitance-to-digital converter (CDC) and LED driver used as the LED driver, according to still another embodiment of the present invention. The IC  100  of  FIG. 3  is the same as the IC  100  of  FIGS. 1 and 2 , except that the IC  100  is used as an LED driver application with the LED driver  104  functioning as a current sink in the example of  FIG. 2 . Thus, an LED  116  is connected between port  108  and the operating voltage VDD 2  of the LED driver  104 . The anode of the LED  116  is connected to the operating voltage VDD 2  and the cathode of the LED  116  is connected to the port  108 . The LED driver  104  includes a current source  114  that functions as a current sink sinking regulated current from the LED  116  through the port  108  of the IC  100 . The current source  114  is connected between the port  108  and ground. The NMOS  110  prevents the operating voltage VDD 2  from affecting the CDC circuit  106  through the port  108 . As explained above, when VDD 1  is applied to the gate of NMOS  110 , the voltage at node  112  is clamped and does not exceed VDD 1 −Vt(n), where VDD 1  is the operating voltage of the CDC circuit  106  and Vt(n) is the threshold turn-on voltage of NMOS  110 . Note that a p-type MOSFET may not be used in the place of NMOS  110 , because such p-type MOSFET would pass a voltage higher than VDD 1  to the CDC circuit  106 . 
         [0027]      FIG. 4A  illustrates the CDC circuit and how an n-type MOSFET is added to the CDC circuit, according to one embodiment of the present invention. The example of  FIG. 4A  illustrates the situation when the IC  100  of  FIG. 1  is used as a CDC application. 
         [0028]    Referring to  FIG. 4A , the CDC circuit  106  includes reference capacitor C ref , switches  410 ,  404 ,  406 ,  402 , amplifiers AMP 1 , AMP 2 , capacitor C int , an inverter  408 , and a D-type flip flop  400 . N-type MOSFET  110  is connected in series with the CDC circuit  106  at node B between the two switches  402 ,  406  and the sense capacitor C sensor . Node B is equivalent to node  112  in  FIGS. 1 ,  2 , and  3 . The sense capacitor C sensor  is connected in series with the NMOS  110 , between NMOS  110  and ground. Switch  402  is connected between node B and ground. Switch  406  is connected between nodes B and C. Switch  404  is connected between nodes A and C. Switch  410  is connected in parallel with the reference capacitor C ref , between voltage VH and node A. Amplifier AMP 1  receives the voltage at node C at its negative input terminal and a DC voltage VM that is lower than the DC voltage VH at its positive voltage terminal. Amplifier AMP 1  and capacitor C int  form an integrator integrating the voltage at node C and outputs an integrated output voltage VOUT. Amplifier AMP 2  compares VOUT at its positive input terminal to the voltage at node C at its negative input terminal, and outputs POL. POL is the data input to the D type flip flop  400 . The D type flip flop  400  is operated by a clock signal that is an inverted from the oscillator signal OSC by the inverter  408 . The non-inverted output of the D type flip flop  400  is the PHASE signal and the inverted output of the D type flip flop  400  is the PHASEB signal. 
         [0029]    A non-overlapping 2-phase clock signal (P 1  or P 2 ) formed by clock signals P 1  and P 2  is applied to the gate of NMOS  110  to control the turning on and off of the NMOS  110 . As will be explained in more detail below, the clock signals P 1  and P 2  are non-overlapping in the sense that they are not at logic high at the same time. In other words, if the clock signal P 1  is at logic high, the clock signal P 2  is at logic low. If the clock signal P 2  is at logic high, the clock signal P 1  is at logic low. Switches  402 ,  404  are turned on and off according to the clock signal P 1 , while switches  406 ,  410  are turned on and off according to the clock signal P 2 . 
         [0030]      FIG. 4B  illustrates the operation of the CDC circuit of  FIG. 4A  in one phase, according to one embodiment of the present invention. The example of  FIG. 4B  illustrates the situation where the clock signal P 1  is at logic high and the clock signal P 2  is at logic low. Accordingly, switches  402 ,  404  are turned on and switches  406 ,  410  are turned off. NMOS  110  is turned on due to clock signal P 1 . Thus, the charges stored in the sense capacitor C sensor  are discharged  414  to ground through the NMOS  110  and the switch  402 , thereby resetting the sense capacitor C sensor . Since switch  406  is turned off, the sense capacitor C sensor  is disconnected from node C. In contrast, the reference capacitor C ref  is connected to node C through the switch  404 . Positive DC voltage VH charges  412  capacitor C int  connected to the negative input of the amplifier AMP 1 , whose voltage is integrated to generate VOUT. Thus, VOUT is negative and POL is also negative, resulting in the PHASE signal of “0” and PHASEB signal of “1” sampled at the clock frequency of the D-type flip flop  400 . 
         [0031]      FIG. 4C  illustrates the operation of the CDC circuitry of  FIG. 4A  in another phase, according to one embodiment of the present invention. The example of  FIG. 4C  illustrates the situation where the clock signal P 1  is at logic low and the clock signal P 2  is at logic high. Accordingly, switches  402 ,  404  are turned off and switches  406 ,  410  are turned on. NMOS  110  is turned on due to clock signal P 2 . In this situation, the sense capacitor C sensor  is connected to node C through NMOS  110  and the switch  406 . Thus, the charges from the integration capacitor C int  are stored  416  in the sense capacitor C sensor  through the NMOS  110  and the switch  406 . Thus, VOUT is positive and POL is also positive, resulting in the PHASE signal of “1” and PHASEB signal of “0” sampled at the clock frequency of the D-type flip flop  400 . Since switch  404  is turned off, the reference capacitor C ref  is disconnected from node C and is discharged (reset)  418 . 
         [0032]      FIG. 5A  is a timing diagram illustrating the operation of the CDC circuitry of  FIG. 4A , when the capacitance on the touch screen is not disturbed by a touch on the touch screen.  FIG. 5A  is explained in conjunction with  FIG. 4A . As shown in  FIG. 5A , the oscillator signal OSC provides the inverted clock signal for the D-type flip flop  400 . The PHASE signals are sampled  502 ,  504 , . . . ,  514  by the D type flip flop  400  at the falling edge of the OSC signal, due to the inverter  408 . Signals P 1  and P 2  together form a non-overlapping 2-phase clock signal, where P 1  is at logic high while P 2  is at logic low, and P 2  is at logic high while P 1  is at logic low. Break-before-make intervals  520 ,  522  are built into the clock signals P 1 , P 2  so that clock signals P 1 , P 2  are not at logic high at the same time. 
         [0033]    The voltage at node A transitions from VH to VM when P 1  transitions to logic high, and transitions from VM to VH when P 2  transitions to logic high. VH is a DC voltage applied to one end of the reference capacitor C ref , and VM is another DC voltage lower than VH and applied to the positive input of the amplifier AMP 1 . The voltage at node B transitions from VM to ground when P 1  transitions to logic high, and transitions from ground to VM when P 2  transitions to logic high. This is because the voltage at node C is approximately the same as VM with ripples  524  occurring when P 1  transitions to logic high and ripples  526  occurring when P 2  transitions to logic high. That is, the DC components of the voltage at node C are the same as the voltage VM. 
         [0034]    As explained above, the output VOUT of the integrator (AMP 1 , C int ) transitions to logic low when P 1  transitions to logic high, and transitions to logic high when P 2  transitions to logic high. In this manner, VOUT alternates between low voltage and high voltage when the capacitance on the sense capacitor C sensor  is not disturbed by a touch on the touch screen. Likewise, the output POL of the amplifier AMP 2  transitions to logic low when P 1  transitions to logic high, and transitions to logic high when P 2  transitions to logic high. In this manner, POL alternates between logic low and logic high when the capacitance on the sense capacitor C sensor  is not disturbed by a touch on the touch screen. As a result, PHASE outputs a data stream  502 ,  504 ,  506 ,  508 ,  510 ,  512 ,  514  of “1010101 . . . ” when the capacitance on the sense capacitor C sensor  is not disturbed by a touch on the touch screen. 
         [0035]      FIG. 5B  is a timing diagram illustrating the operation of the CDC circuitry of  FIG. 4A , when the capacitance on the touch screen is disturbed by a touch on the touch screen. The timing diagram of  FIG. 5B  shows the same signals as those shown in  FIG. 5A , except that the voltages at nodes A, B, and C are not shown for simplicity of illustration. When the capacitance on the sense capacitor C sensor  is disturbed by a touch on the touch screen, VOUT starts to increase in each cycle  552 ,  554 ,  556 ,  558 ,  560 ,  562 ,  564 ,  566 ,  568 ,  570  and maintains the high voltage  572 ,  574 ,  576  saturated at the supply voltage VDD 1  of the CDC circuit  106 . POL alternates between logic high  580  and logic low  582  as explained previously with reference to  FIG. 5B  until the point where VOUT does not fall below the voltage at node C (see  558 ). At that point, the POL also does not return to logic low (i.e., maintains logic high (see  586 )). As a result, PHASE outputs a continuous data stream of 1&#39;s soon after the capacitance on the sense capacitor C sensor  is disturbed by a touch on the touch screen. The PHASE data stream shown in  FIG. 5B  would be “101011111111111 . . . ” Thereafter, when the touch is removed, the PHASE signal will revert to an alternating data stream of “1010101 . . . ” as shown in  FIG. 5A , although not shown in  FIG. 5B . 
         [0036]    Upon reading this disclosure, those of skill in the art will appreciate still additional alternative structural and functional designs for a combined touch sensor and LED driver IC. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims.