ESD protection circuit

An ESD protection circuit, connected to an input pad, comprising:

• a substrate of a first conductivity type, the substrate comprising a first well of a second conductivity type and a second well of the second conductivity type;
• a diode device, formed in the substrate and having a first end of the first conductivity type and a second end of second conductivity type, wherein the first end is connected to the input pad;
• a first snapback device, formed in the substrate and comprising a first doped region of the second conductivity type, a first gate and a second doped region of the second conductivity type, wherein the first doped region is coupled to the second end, and the first gate is coupled to the second doped region;
• a ring structure, formed in the substrate, the ring structure comprising a third doped region of the second conductivity type located between the diode device and the second doped region;
• and
• a control circuit, connected to the third doped region for providing an operational voltage to the ring structure in a normal operation;
• wherein the second doped region is located between the first doped region and the third doped region, the second well is located between the first well and the second doped region, the diode device is formed in the first well of the second conductivity type, and the third doped region is formed upon the second well of the second conductivity type.