Patent Publication Number: US-2023142717-A1

Title: Electrostatic discharge protection circuit

Description:
BACKGROUND 
     Technical Field 
     The disclosure relates to an electrostatic discharge protection circuit, and particularly relates to an electrostatic discharge protection circuit that can increase a discharge time of an electrostatic discharge current. 
     Description of Related Art 
     Please refer to  FIG.  1   .  FIG.  1    is a circuit diagram of an electrostatic discharge circuit in the prior art. An electrostatic discharge protection circuit  100  includes a transistor T 1 , a resistor R 1 , a capacitor C 1 , and an inverter INV 1 . The transistor T 1  is coupled between power rails PWL 1  and PWL 2 , and is controlled by a control voltage Vg to be turned on or cut off. 
     The resistor R 1  and the capacitor C 1  may be configured to sense whether an electrostatic discharge phenomenon occurs on the power rail PWL 1 . When the electrostatic discharge phenomenon occurs, the inverter INV 1  may generate a high-level control voltage Vg according to a voltage at a coupling point of the resistor R 1  and the capacitor C 1 , so that the transistor T 1  is turned on. Through the turned-on transistor T 1 , an electrostatic discharge current on the power rail PWL 1  may be discharged to the power rail PWL 2 . 
     In the conventional electrostatic discharge protection circuit  100 , a turn-on time of the transistor T 1  is determined by a charging speed of the capacitor C 1 . To extend the discharge time of the electrostatic discharge current, it is necessary to increase the area of the capacitor C 1 , which will increase the area of the circuit and increase the production cost of the circuit. 
     SUMMARY 
     The disclosure provides an electrostatic discharge protection circuit, which can increase a discharge time of an electrostatic discharge current. 
     The electrostatic discharge protection circuit of the disclosure includes a discharge switch, a first transistor, an inverter, and a feedback circuit. The discharge switch is coupled between a first power rail and a second power rail, and is turned on or cut off according to a control voltage. The first transistor has a first end coupled to the first power rail. A control end of the first transistor receives the control voltage. The inverter is coupled between a second end of the first transistor and a control end of the discharge switch. The feedback circuit is coupled between an output end and an input end of the inverter and is configured to determine whether to provide a turn-on path between the input end of the inverter and the second power rail according to the control voltage. 
     Based on the above, the embodiment of the disclosure executes an electrostatic discharge phenomenon on the first power rail through the first transistor and the feedback circuit. The first transistor is configured to provide an active load. The feedback circuit is configured to extend the discharge time of the electrostatic discharge current in an electrostatic discharge protection mode. The feedback circuit is configured to keep the discharge switch from being turned on in a normal working mode. Through the architecture of the electrostatic discharge protection circuit of the embodiment of the disclosure, the area of the circuit can be effectively reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a circuit diagram of an electrostatic discharge circuit in the prior art. 
         FIG.  2    is a schematic diagram of an electrostatic discharge protection circuit according to an embodiment of the disclosure. 
         FIG.  3    is a schematic diagram of an electrostatic discharge protection circuit according to another embodiment of the disclosure. 
         FIG.  4 A  and  FIG.  4 B  are schematic diagrams of equivalent circuits of the electrostatic discharge protection circuit in action according to the embodiment of the disclosure. 
         FIG.  5    is a current waveform diagram of the electrostatic discharge protection circuit according to an embodiment of the disclosure and the conventional electrostatic discharge circuit in an electrostatic discharge mode. 
     
    
    
     DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS 
     Please refer to  FIG.  2   .  FIG.  2    is a schematic diagram of an electrostatic discharge protection circuit according to an embodiment of the disclosure. An electrostatic discharge protection circuit  200  includes a discharge switch  210 , a transistor M 1 , an inverter INV 1 , and a feedback circuit  220 . The discharge switch  210  is coupled between a power rail PWL 1  and a power rail PWL 2 . The discharge switch  210  may be constructed by a transistor T 1 . A control end of the transistor T 1  receives a control voltage Vg and may be turned on or cut off according to the control voltage Vg. 
     In the embodiment, in a normal working mode, the power rail PWL 1  may be configured to receive an operating power supply, and the power rail PWL 2  may be configured to receive a ground voltage. 
     In addition, a first end of the transistor M 1  is coupled to the power rail PWL 1 , and a second end of the transistor M 1  is coupled to the feedback circuit  220  and an input end of the inverter INV 1 . A control end of the transistor M 1  is coupled to an output end of the inverter INV 1  to receive the control voltage Vg. On the other hand, the feedback circuit  220  is coupled to the input end and the output end of the inverter INV 1 . The feedback circuit  220  is configured to determine whether to provide a turn-on path between the input end of the inverter INV 1  and the power rail PWL 2  according to the control voltage Vg. 
     In terms of action details, the transistor M 1  and the feedback circuit  220  are configured to sense whether an electrostatic discharge phenomenon occurs on the power rail PWL 1 . The transistor M 1  may be used as an active load, and when the electrostatic discharge phenomenon occurs, the transistor M 1  may be cut off and provide a relatively high resistance value. At this time, the voltage at the input end of the inverter INV 1  may be a relatively low ground voltage, and the inverter INV 1  generates a relatively high control voltage Vg at the output end. The feedback circuit  220  provides the turn-on path on the power rail PWL 2  and at the input end of the inverter INV 1  according to the control voltage Vg with the relatively high voltage and keeps the input end of the inverter INV 1  equal to the ground voltage. 
     At the same time, the discharge switch  210  composed of the transistor T 1  may be turned on according to the control voltage Vg and form a current discharge path of an electrostatic discharge current between the power rails PWL 1  and PWL 2  to achieve the ability of electrostatic discharge protection. 
     Please note here that in an electrostatic discharge mode, the feedback circuit  220  of the embodiment of the disclosure may effectively extend a turn-on time of the discharge switch  210  and increase a discharge time of the electrostatic discharge current through providing a turn-on path between the input end of the inverter INV 1  and the power rail PWL 2  and keeping the inverter INV 1  equal to the ground voltage. 
     On the other hand, when the electrostatic discharge phenomenon does not occur, the transistor M 1  may be turned on and provide a relatively low resistance value. In the normal working mode, the input end of the inverter INV 1  is substantially equal to the operating power supply, and the inverter INV 1  generates the control voltage Vg with a relatively low voltage (for example, equal to the ground voltage) at the output end thereof, and the transistor T 1  in the discharge switch  210  is cut off. At this time, the feedback circuit  220  may cut off connection path between the power rail PWL 2  and the input end of the inverter INV 1  according to, for example, the control voltage Vg equal to the ground voltage. 
     Please refer to  FIG.  3    below.  FIG.  3    is a schematic diagram of an electrostatic discharge protection circuit according to another embodiment of the disclosure. An electrostatic discharge protection circuit  300  includes a discharge switch  310 , a transistor M 1 , an inverter INV 1 , and a feedback circuit  320 . The discharge switch  310  is constructed by the transistor T 1 . A control end of the transistor T 1  receives the control voltage Vg and may be turned on or cut off according to the control voltage Vg. The feedback circuit  320  includes a voltage divider composed of resistors R 31  and R 32  and a transistor M 2 . The voltage divider composed of the resistors R 31  and R 32  is connected in series between the output end of the inverter INV 1  and the power rail PWL 2 . The resistors R 31  and R 32  are configured to divide the control voltage Vg at the output end of the inverter INV 1  and generate a divided voltage Vd. A first end of the transistor M 2  is coupled to the second end of the transistor M 1 , a second end of the transistor M 2  is coupled to the power rail PWL 2 , and a control end of the transistor M 2  is coupled to the resistors R 31  and R 32 , and receives the divided voltage Vd. 
     In the embodiment, in the normal working mode, the power rail PWL 1  may be configured to receive the operating power supply, and the power rail PWL 2  may be configured to receive the ground voltage. 
     Compared with the electrostatic discharge protection circuit  100  of the prior art in  FIG.  1   , the electrostatic discharge protection circuit  300  of the embodiment uses the transistor M 1  as the active load and replaces the resistor R 1  to provide different resistance values in different modes. The feedback circuit  320  is configured to replace the capacitor C 1  and may extend the length of time that the transistor T 1  is turned on and the discharge time of the electrostatic discharge current under the condition of occupying a smaller layout area. Accordingly, the electrostatic discharge protection circuit  300  of the disclosure can not only reduce the cost of the circuit, but also effectively improve the efficiency of electrostatic discharge protection, so as to have multiple advantages. 
     In terms of action details, reference may be made to schematic diagrams of equivalent circuits of the electrostatic discharge protection circuit in action according to the embodiment of the disclosure in  FIG.  4 A  and  FIG.  4 B . In  FIG.  4 A , the electrostatic discharge protection circuit  300  works in an electrostatic discharge protection mode. An electrostatic discharge voltage ESDV with positive pulse wave is applied onto the power rail PWL 1 . At this time, the transistor M 1  is cut off and provides a relatively high resistance value. The inverter INV 1  generates the control voltage Vg with a relatively high second voltage at the output end thereof according to a relatively low first voltage at the input end thereof. The control voltage Vg is provided to the control end of the transistor T 1  of the discharge switch  310 , so that the transistor T 1  is turned on. The turned-on transistor T 1  may provide a discharge path of an electrostatic discharge current EC and achieve the objective of protection. 
     At the same time, the voltage divider formed by the resistors R 31  and R 32  divides the voltage according to the control voltage Vg and generates the divided voltage Vd. The divided voltage Vd may be provided to the control end of the transistor M 2 , so that the transistor M 2  is turned on. Here, the turned-on transistor M 2  may provide a turn-on path between the input end of the inverter INV 1  and the power rail PWL 2 , so that the input end of the inverter INV 1  may be kept equal to the ground voltage. 
     By the way, the state of the feedback circuit  320  keeping the input end of the inverter INV 1  equal to the ground voltage may be released after the electrostatic discharge voltage ESDV drops to a specific voltage level. 
     In the embodiment, the resistance value of the resistor R 31  may be less than the resistance value of the resistor R 32 . A channel width-to-length ratio of the transistor M 1  may be less than a channel width-to-length ratio of the transistor M 2 . Also, the channel width-to-length ratios of the transistors M 1  and M 2  are both less than 1. 
     In  FIG.  4 B , the electrostatic discharge protection circuit  300  works in a normal working mode. An operating power supply VDD is applied onto the power rail PWL 1 , and the power rail PWL 2  receives the ground voltage. At this time, the transistor M 1  is turned on and provides a relatively low resistance value. The voltage at the input end of the inverter INV 1  is pulled up to the second voltage (equal to the operating power supply VDD), and the output end of the inverter INV 1  generates the control voltage Vg that is the first voltage (equal to the ground voltage). 
     The control voltage Vg is provided to the control end of the transistor T 1 , so that the transistor T 1  is cut off. In this way, the power rails PWL 1  and PWL 2  are not turned on relative to each other, and the subsequent circuit may keep normal operation. 
     Please refer to  FIG.  5   .  FIG.  5    is a current waveform diagram of the electrostatic discharge protection circuit according to an embodiment of the disclosure and the conventional electrostatic discharge circuit in the electrostatic discharge mode. The horizontal axis in  FIG.  5    is time in units of nanoseconds (ns). The vertical axis is current in unit of ampere (A). A curve  520  is, for example, the current waveform curve of the electrostatic discharge protection circuit  300  of the embodiment of the disclosure when executing an electrostatic discharge action. A curve  510  is the current waveform curve of the conventional electrostatic discharge protection circuit  100  when executing an electrostatic discharge action. Obviously, the discharge time of the electrostatic discharge current that may be provided by the electrostatic discharge protection circuit  300  of the embodiment of the disclosure is significantly longer than the discharge time of the electrostatic discharge current provided by the conventional electrostatic discharge protection circuit  100 . Therefore, the electrostatic discharge energy that may be discharged by the electrostatic discharge protection circuit  300  of the embodiment of the disclosure is far greater than the electrostatic discharge energy that may be discharged by the conventional electrostatic discharge protection circuit  100 . It can be known that the electrostatic discharge protection level that may be achieved by the electrostatic discharge protection circuit  300  of the embodiment of the disclosure can be further improved. 
     It is worth mentioning that the electrostatic discharge protection circuit  300  of the embodiment of the disclosure may have a lower leakage current than the conventional electrostatic discharge protection circuit  100  in the normal working mode. Since the conventional electrostatic discharge protection circuit  100  has the resistor R 1  and the capacitor C 1  connected in series between the power rails PWL 1  and PWL 2 , a relatively large leakage current is generated. In the electrostatic discharge protection circuit  300  of the embodiment of the disclosure, there is no voltage difference between two ends of the voltage divider (the resistors R 31  and R 32 ) in the normal working mode, and in the case where the transistors M 2  and T 1  are both cut off, the generated leakage current can be reduced. 
     In summary, the electrostatic discharge protection circuit of the disclosure can extend the discharge time of the electrostatic discharge current and greatly increase the ability of electrostatic discharge protection under the premise of reducing the layout area of the circuit through the cooperation of the feedback circuit and the transistor.