Apparatus and methods for electrical overstress protection

An overstress protection circuit for a semiconductor chip, the overstress protection circuit comprising:

• an impedance element having a first end electrically connected to a pad and a second end electrically connected to a signal node;
• a controllable clamp electrically connected between the second end of the impedance element and a first supply node;
• an overstress sensing circuit electrically connected between the first end of the impedance element and the first supply node, the overstress sensing circuit configured to activate the controllable clamp to conduct a current from the second end of the impedance element to the first supply node in response to detecting an electrical overstress event at the first end of the impedance element;
• an overshoot limiting circuit electrically connected between the signal node and a second supply node, the overstress protection circuit arranged so as to divert charge associated with the electrical overstress event away from the signal node to thereby provide overstress protection;
• and
• a first reverse protection circuit electrically connected between the first end of the impedance element and the first supply node, and a second reverse protection circuit electrically connected between the second end of the impedance element and the first supply node.

A method of providing overstress protection, the method comprising:

• providing a voltage drop from a pad to a signal node in response to an electrical overstress event at the pad using an impedance element, the impedance element having a first end electrically connected to the pad and a second end electrically connected to the signal node;
• detecting the electrical overstress event at the first end of the impedance element using an overstress sensing circuit connected between the first end of the impedance element and a first supply node;
• activating a controllable clamp to conduct a current between the second end of the impedance element and the first supply node in response to detecting the electrical overstress event at the first end of the impedance element using the overstress sensing circuit;
• protecting the signal node from overshoot before the controllable clamp is activated using an overshoot limiting circuit;
• and
• protecting an internal circuit connected to the signal node using a first reverse protection circuit electrically connected between the first end of the impedance element and the first supply node, and a second reverse protection circuit electrically connected between the second end of the impedance element and the first supply node.