Output buffer circuit and method of operation

An output buffer circuit comprising:

• a pad output;
• a pull up transistor having a gate electrode, the pull up transistor having a channel region coupled between the pad output and a voltage supply, the pull up transistor having a bulk connection;
• a pull down transistor coupled between the pad output and a ground terminal;
• a first switch having an input and an output, the output of the first switch being coupled to the gate electrode of the pull up transistor;
• an inverter having an input coupled to the input of the first switch and an output;
• and
• a clamping transistor having an input coupled to the output of the inverter and a channel region coupled between the bulk connection of the pull up transistor and the gate electrode of the pull up transistor.

An output buffer circuit comprising:

• a pad output;
• a first pull up transistor having a gate electrode, the first pull up transistor having a channel region coupled between the pad output and a first voltage supply, the first pull up transistor having a bulk connection;
• a pull down transistor coupled between the pad output and a ground terminal;
• a first switch having a control electrode, an input, and an output, the output of the first switch being coupled to the gate electrode of the first pull up transistor;
• and
• a device coupled between the pad output and the control electrode of the first switch to detect a selected voltage at the pad and to switch off the first switch and turn off the first pull up transistor when the selected voltage is detected.

A method for driving an output voltage onto an output pad using an output driver circuit, the method comprising the steps of:

• (a) using a first output buffer in the output driver circuit to drive a voltage on the output pad from a first voltage to a second voltage;
• (b) detecting the voltage on the output pad to determine when the second voltage is obtained on the output pad;
• and
• (c) switching, once the second voltage is detected in step (b), the output driver circuit to use a second output buffer to drive the output pad from the second voltage to a third voltage, the first output buffer and the second output buffer being powered with different voltage supplies.