Patent Publication Number: US-2004047107-A1

Title: ESD protection circuit for touch button

Description:
FIELD OF THE INVENTION  
       [0001] The invention relates to a touch button adopted for a touch operation interface of electronic devices, and particularly a touch button with ESD protection.  
       BACKGROUND OF THE INVENTION  
       [0002] The touch button is a widely used button key structure. Compared with the conventional press button key, it responds faster, is more reliable and can reduce a user&#39;s fatigue. In general, the touch button has a touch sensor IC connecting to the button key. When a user touches the button key, the touch sensor IC detects the change of the capacitance and activates the internal circuit. Although it is convenient, the touch sensor IC is sensitive to electrostatic discharge.  
       [0003] Electro-Static Discharge (ESD) mainly comes from the human body. When people walk, friction occurs on the clothes or the casings of electronic devices that are on the path of the air convection. This friction generates electrostatic discharges, which are accumulated on the surfaces of the electronic devices. When the accumulated electrostatic discharges reach a certain level, an electrostatic discharge takes place and may cause malfunction of the electronic devices or even damage the electronic devices. Hence the electronic devices often require a special element or circuit to avoid such incidents.  
       [0004] On the touch button, when the electrostatic discharge hits from the human body or the surface of an electronic device, the touch sensor IC  40  is easily damaged, as FIG. 1. The commonly adopted approach to protect electronic devices from the harmful effect of the electrostatic discharge is painting. A conductive material is added to non-conductive acetate-based baking paints to make the paint layer become a semiconductor. The paint layer is properly grounded so that electrostatic discharges generated by the friction between the clothes of people and the air convection can be channeled to the ground. However such an approach cannot be adopted on the touch button because once the touch button is painted with electrostatic resistant paint, it becomes non-conductive and cannot function properly.  
       [0005] Therefore the most commonly used method is to add a protective IC  41  inside or outside the touch sensor IC  40 , as FIG. 2. But such an approach significantly increases the cost. In addition, the general protection design has the capacity of only 2 KV, while electrostatic discharge can be easily greater than 5 KV. Thus the conventional protection design is not effective.  
       SUMMARY OF THE INVENTION  
       [0006] The object of the invention is to provide an ESD protection circuit for touch button to overcome the problems set forth above and to flow the electrostatic discharge into the ground before reaching the touch sensor IC so that the touch sensor IC may be protected.  
       [0007] The ESD protection circuit for touch button of the invention adopts the point discharge principle to flow the electrostatic discharge to the ground without affecting the functions of the touch button key.  
       [0008] According to the invention, the circuit board of the touch button is laid out at least one point. And the point is corresponding to another point connecting to the ground. Electrostatic discharge entering into the button key generates a point discharge between the two points. Thus the electrostatic discharge flows into the ground circuit before reaching the touch sensor IC.  
       [0009] In order to ensure that the electrostatic discharge is point discharged, the circuit of the touch sensor IC further includes a resistor. So the effective resistance of the touch sensor IC is greater than the effective resistance between the two points. Such a design can effectively trigger point discharge and channel the electrostatic discharge to the ground circuit to protect the touch sensor IC.  
       [0010] The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 is a schematic view of the circuit of the touch button.  
     [0012]FIG. 2 is a schematic view of the circuit with a protection IC.  
     [0013]FIG. 3 is a schematic view of the invention.  
     [0014]FIG. 4 is a schematic view of the invention, including a resistor.  
     [0015]FIG. 5 is a schematic view of another embodiment of the invention with a symmetrical structure.  
     [0016]FIG. 6 is a schematic view of another embodiment of the invention to form protection circuit by attaching copper foil. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0017] The ESD protection circuit for touch button according to the invention employs the point discharge principle based on the considerations that the touch button cannot be directly connected to the ground. The touch sensor IC would be protected from being damaged by the electrostatic discharge without affecting its function and performance.  
     [0018] Referring to FIG. 3, the touch button of the invention includes a circuit board  10 , which has a touch control section  20 , a touch sensor IC  40  and a discharge section. The touch control section  20  links to the exterior for receiving the touch actions of users and is electrically connected to the touch sensor IC  40 . The discharge section includes a second point  31 , which connects to a ground circuit  30  and corresponds to a first point  201  extended from the touch control section  20 . The first point  201  and the second point  31  face each other and may be the naked copper foil on the circuit board  10 .  
     [0019] The touch control section  20  receives user&#39;s touch actions from an exposed button key portion. In the event that an electrostatic discharge occurs and is transmitted to the touch control section  20 , the first point  201  of the touch control section  20  and the second point  31  of the discharge section generate point discharge therebetween. The electrostatic discharge flows into the ground circuit  30 . As the touch sensor IC  40  is activated by detecting the change of the capacitance, transmission of the electrostatic discharge to the ground circuit  30  does not affect the function and performance of the touch sensor IC  40 .  
     [0020] In order to ensure that the electrostatic discharge is discharged through the point discharge, the principle that electricity flows to where the resistance is lowest may be adopted. Thus a resistor  50  may be included between the electrostatic sensor IC  40  and the touch control section  20 , as shown in FIG. 4, so that the effective resistance of the electrostatic sensor IC  40  is greater than the effective resistance between the first point  201  and the second point  31 . Therefore the electrostatic discharge is forced to discharge between the first point  201  and the second point  31 , and is channeled to the ground circuit  30 .  
     [0021] Taking into account structural symmetry and ensuring that discharge is accomplished in a short time, a plurality of points may be adopted. Referring to FIG. 5, the touch control section  20  has four points  201 ,  202 ,  203 , and  204  located on the periphery in a symmetrical fashion and connected to the touch sensor IC  40 . The discharge section has four points  31 ,  32 ,  33  and  34  corresponding to the four points  201 ,  202 ,  203 , and  204  of the touch control section  20 , and connecting respectively to the ground circuit  30 . As a result, the electrostatic discharge can be effectively channeled to the ground circuit  30 .  
     [0022] The four points  31 ,  32 ,  33  and  34  of the discharge section and the four points  201 ,  202 ,  203 , and  204  of the touch control section  20  are formed by layout circuit on the circuit board  10 , as shown in FIG. 5. It also attaches copper foil on the circuit board  10  to form the four points  31 ,  32 ,  33  and  34  of the discharge section and the four points  201 ,  202 ,  203 , and  204  around the touch control section  20 , referring to FIG. 6.  
     [0023] While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.