Open drain driver, and a switch comprising the open drain driver

Abstract

An open drain driver ( 7 ) selectively switches a MOSFET switch (MN 1 ) which is passively held in the conducting state into the non-conducting state. The MOSFET switch (MN 1 ) switches an AC analogue input signal on a main input terminal ( 3 ) to a main output terminal ( 4 ) and the gate of the MOSFET switch (MN 1 ) is AC coupled by a capacitor (C 1 ) to the drain thereof. The open drain driver ( 7 ) comprises a first MOSFET (MN 2 ) and a second MOSFET (MN 3 ) through which the gate of the MOSFET switch (MN 1 ) is pulled to ground (Vss). The gate of the first MOSFET (MN 2 ) is coupled to the supply voltage (VDD) for maintaining the first MOSFET (MN 2 ) in the open state. A control signal is applied to the gate of the second MOSFET (MN 3 ) for selectively operating the open drain driver ( 7 ) in the conducting state for operating the MOSFET switch (MN 1 ) in the non-conducting state. When the second MOSFET (MN 3 ) is in the non-conducting state, the first MOSFET (MN 2 ) remains in the conducting state until the voltage on a coupling node ( 9 ) between the first and second MOSFETs (MN 2 ,MN 3 ) equals the difference between its gate voltage and its threshold voltage, at which stage, any over-voltages applied to the gate of the MOSFET switch (MN 1 ) are divided between the first and second MOSFETs (MN 2 ,MN 3 ). A coupling diode (D 1 ) coupling the coupling node ( 9 ) to the supply voltage (VDD) clamps the voltage on the coupling node ( 9 ) at the supply voltage (VDD) plus the conducting voltage of the diode (D 1 ), in the event of the voltage on the coupling node ( 9 ) rising after the first MOSFET (MN 2 ) has gone into the non-conducting state. The coupling node ( 9 ) may be capacitively coupled to the supply voltage (VDD) by a coupling capacitor instead of or as well as the diode (D 1 ) for limiting the voltage on the coupling node ( 9 ).

Description

technical field

[0001] The present invention relates to an open drain driver, and a switching circuit comprising a transistor switch operating under the control of the open drain driver. The invention also relates to a method for providing an open-drain driver with improved overvoltage breakdown protection. Background technique

[0002] Open drain drivers are known and have many applications. For example, open-drain drivers are commonly used to drive low signals from related circuits onto a common bus to reach distant external circuits. The advantage of using an open-drain driver in this application is that when the open-drain driver is not needed to drive a low signal onto the common bus, the open-drain driver is in a high-impedance state, thereby separating its associated circuitry from the common bus. isolation. In the case where multiple circuits share a bus, the rated power supply voltage of the circuit will vary depending on the type of circuit. Some circuits can o...

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