Direct drive cascoded switching circuit

A cascoded switch system, comprising:

• a cascoded switch comprising a JFET and a MOSFET coupled in series between a drain terminal and a source terminal of the cascoded switch;
• and
• a cascode-drive circuit comprising:
• a drain-sense circuit configured to compare a drain voltage at the drain terminal against a drain threshold;
• a logic gate configured to:
• assert a cascode-on signal in response to either of an on-state of an input signal or the drain voltage being less than the drain threshold;
• and
• assert a cascode-off signal in response to an off-state of the input signal and the drain voltage being greater than the drain threshold;
• a JFET drive circuit configured to:
• drive the JFET in a JFET on-state in response to the cascode-on signal;
• and
• drive the JFET with a negative voltage in response to the cascode-off signal;
• a comparator configured to compare a JFET gate voltage against a JFET-gate threshold;
• and
• a MOSFET drive circuit configured to:
• drive the MOSFET in a MOSFET on-state in response to the cascode-on signal;
• and
• drive the MOSFET in the MOSFET on-state in response to an assertion of the cascode-off signal and the comparator indicating that the JFET gate voltage is sufficient to drive the JFET in a JFET off-state;
• and
• drive the MOSFET in an MOSFET off-state in response to the assertion of cascode-off signal and the comparator indicating that the JFET gate voltage is not sufficient to drive the JFET in the JFET off-state.

A method, comprising:

• comparing a drain voltage at a drain of a cascoded switch against a drain threshold;
• receiving a input signal;
• asserting a cascode-on signal in response to either of an on-state of the input signal or the drain voltage being less than the drain threshold;
• driving a JFET of the cascoded switch in a JFET on-state in response to the cascode-on signal;
• driving a MOSFET of the cascoded switch in a MOSFET on-state in response to the cascode-on signal;
• asserting a cascode-off signal in response to an off-state of the input signal and the drain voltage being greater than the drain threshold;
• driving the JFET of the cascoded switch with a negative drive voltage in response to the cascode-off signal;
• comparing a JFET gate voltage to a threshold to generate a comparison signal;
• and
• driving the MOSFET in response to the cascode-off signal and the comparison signal.