Power off delay circuit and method, and audio system with power off delay

Abstract

A power off delay circuit includes a switch connected between an external power input terminal and an internal power supply terminal, a capacitor connected to the internal power supply terminal, and a hysteresis comparator to switch the switch according to the voltages of the external power input terminal and the internal power supply terminal. During on-time of the switch, the external power input terminal is connected to the internal power supply terminal and the capacitor can be charged by the external power source. When the switch is off, the capacitor provides electric power for an internal circuit. Application of the power off delay circuit to an audio system may eliminate the turn-off pops of the audio system.

1 . An audio system, comprising:
 an audio line;   a drive transistor connected to the audio line; and   a power off delay circuit connected to the drive transistor, the power off delay circuit having an external power input terminal, an internal power supply terminal, and a capacitor connected to the internal power supply terminal, wherein the capacitor is charged during the internal power supply terminal has a voltage lower than that of the external power input terminal, and provides a current to the drive transistor to pull down a voltage of the audio line at power off.   
     
     
         2 . The audio system of  claim 1 , wherein the power off delay circuit comprises:
 a switch connected between the external power input terminal and the internal power supply terminal; and   a hysteresis comparator having a first input terminal connected to the external power input terminal, a second input terminal connected to the internal power supply terminal, and an output terminal connected to the switch for providing a control signal to switch the switch,   wherein the switch is on in a first state to connect the external power input terminal to the internal power supply terminal, and is off in a second state to disconnect the external power input terminal from the internal power supply terminal.   
     
     
         3 . The audio system of  claim 2 , wherein the switch comprises a MOSFET connected between the external power input terminal and the internal power supply terminal, controlled by the control signal. 
     
     
         4 . The audio system of  claim 2 , wherein the switch comprises:
 a PMOSFET formed on a well, connected between the external power input terminal and the internal power supply terminal, controlled by the control signal; and   a voltage switching circuit connected to the well to switch a voltage of the well.   
     
     
         5 . The audio system of  claim 4 , wherein the voltage switching circuit comprises:
 a second PMOSFET formed on the well, connected between the external power input terminal and the well, operative to connect the external power input terminal to the well in the first state; and   a resistor connected between the internal power supply terminal and the well, for applying a voltage from the internal power supply terminal to the well in the second state.   
     
     
         6 . The audio system of  claim 5 , wherein the resistor comprises a well resistor established by the well. 
     
     
         7 . The audio system of  claim 2 , wherein the hysteresis comparator comprises an initial-state setting resistor connected to the output terminal of the hysteresis comparator, for setting an initial logic state of the control signal. 
     
     
         8 . The audio system of  claim 2 , wherein the hysteresis comparator comprises:
 a first input transistor having a gate connected to the external power input terminal;   a second input transistor;   a hysteresis resistor connected between the second input terminal of the hysteresis comparator and a gate of the second input transistor; and   a hysteresis current source connected in series to the hysteresis resistor;   wherein the hysteresis resistor establishes a voltage drop to determine a hysteresis band of the hysteresis comparator.   
     
     
         9 . The audio system of  claim 1 , wherein the capacitor has a capacitance defining a delay time of the power off delay circuit.