Overvoltage protection with power sink
View Patent ↗A line card includes a subscriber line interface circuit, an interface line, and a first overvoltage protector. The subscriber line interface circuit has a first input terminal operable to receive power at a first reference voltage. The interface line is coupled to the subscriber line interface circuit for interfacing with a subscriber line external to the line card. The first overvoltage protector is coupled to the interface line and referenced to the first reference voltage to provide overvoltage protection responsive to a voltage on the interface line exceeding the first reference voltage by a first predetermined threshold.
1. A line card, comprising:
a subscriber line interface circuit having a first input terminal operable to receive power at a first reference voltage;
an interface line coupled to the subscriber line interface circuit for interfacing with a subscriber line external to the line card;
a first overvoltage protector coupled to the interface line and referenced to the first reference voltage to provide overvoltage protection responsive to a voltage on the interface line exceeding the first reference voltage by a first predetermined threshold; and
a power sink coupled to the interface line and arranged to provide a bypass path to ground for current up to a predetermined current limit responsive to a voltage on the interface line exceeding the first reference voltage prior to exceeding the first predetermined threshold, said power sink comprising;
a power transistor coupled between the interface line and ground;
a limiting resistor in series with the power transistor; and
a bias circuit coupled to a sate of the power transistor,
wherein said predetermined current limit is a function of the value of said limiting resistor.
2. The line card of claim 1 , wherein the gate is referenced to the first reference voltage.
3. The line card of claim 2 , wherein the bias circuit further comprises:
a bias diode coupled to receive the first reference voltage; and
a bias resistor coupled to the bias diode; wherein the gate of the power transistor is coupled to a node defined between the bias diode and the bias resistor.
4. The line card of claim 1 , wherein the power sink comprises:
an enable transistor coupled to the gate of the power transistor and having a gate referenced to the first reference voltage.
5. The line card of claim 4 , wherein the bias circuit further comprises:
at least one bias diode coupled to receive the first reference voltage; and
a bias resistor coupled to the bias diode, wherein a gate of the enable transistor is coupled to a node defined between the bias diode and the bias resistor.
6. The line card of claim 2 , wherein the power transistor comprises a plurality of power transistors in parallel.
7. The line card of card of claim 1 , further comprising at least one protection resistor coupled to the interface line.
8. The line card of claim 1 , wherein the first reference voltage comprises a positive reference voltage.
9. The line card of claim 1 , wherein the first overvoltage protector includes a first gate input terminal coupled to receive the first reference voltage, further comprising a first gate capacitor coupled to the first gate input terminal.
10. The line card of claim 1 , wherein the first overvoltage protector is operable to provide overvoltage protection by grounding the interface line.
11. A line card, comprising:
a plurality of subscriber line interface circuits each having a first input terminal operable to receive power at a first reference voltage;
a plurality of interface lines each coupled to one of the subscriber line interface circuits for interfacing with subscriber lines external to the line card;
a first overvoltage protector coupled to the interface lines and referenced to the first reference voltage to provide overvoltage protection responsive to a voltage on at least one of the interface lines exceeding the first reference voltage; and
a power sink coupled to the interface lines and arranged to provide a bypass path to ground for current up to a predetermined current limit responsive to a voltage on at least one of the interface lines exceeding the first reference voltage prior to exceeding the first predetermined threshold, said power sink comprising:
a power transistor coupled between the interface line and ground;
a limiting resistor in series with the power transistor; and
a bias circuit coupled to a gate of the power transistor,
wherein said predetermined current limit is a function of the value of said limiting resistor.
12. The line card of claim 11 , wherein the gate is referenced to the first reference voltage.
13. The line card of claim 12 , wherein the bias circuit further comprises:
a bias diode coupled to receive the first reference voltage; and
a bias resistor coupled to the bias diode; wherein the gate of the power transistor is coupled to a node defined between the bias diode and the bias resistor.
14. The line card of claim 11 , wherein the power sink comprises:
an enable transistor coupled to the gate of the power transistor and having a gate referenced to the first reference voltage.
15. The line card of claim 14 , wherein the bias circuit further comprises:
at least one bias diode coupled to receive the first reference voltage; and
a bias resistor coupled to the bias diode, wherein a gate of the enable transistor is coupled to a node defined between the bias diode and the bias resistor.
16. The line card of claim 12 , wherein the power transistor comprises a plurality of power transistors in parallel.
17. The line card of card of claim 11 , further comprising at least one protection resistor coupled to each of the interface lines.
18. The line card of claim 11 , wherein the first reference voltage comprises a positive reference voltage.
19. The line card of claim 11 , wherein the first overvoltage protector includes a first gate input terminal coupled to receive the first reference voltage, further comprising a first gate capacitor coupled to the first gate input terminal.
20. The line card of claim 11 , wherein the first overvoltage protector is operable to provide overvoltage protection by grounding the interface lines.
21. The line card of claim 11 , wherein the subscriber line interface circuits each have a second input terminal operable to receive power at a second reference voltage, further comprising:
a plurality of second overvoltage protectors referenced to the second reference voltage, wherein each of the second overvoltage protectors is coupled to an associated one of the interface lines and is operable to provide overvoltage protection responsive to a voltage on the associated interface line exceeding the second reference voltage.
22. The line card of claim 21 , wherein the first voltage source comprises a positive voltage source, and the second voltage source comprises a negative voltage source.
23. The line card of claim 21 , wherein the first overvoltage protector includes a first gate input terminal coupled to receive the first reference voltage and the second overvoltage protectors each include a second gate input terminal coupled to receive the second reference voltage, further comprising:
a first gate capacitor coupled to the first gate input terminal; and
a plurality of second gate capacitors, each coupled to one of the second gate input terminals.
24. The line card of claim 21 , wherein the first overvoltage protector is operable to provide overvoltage protection by grounding the interface lines responsive to a voltage on at least one of the interface lines exceeding the first reference voltage, and the second overvoltage protectors are each operable to provide overvoltage protection by grounding its associated interface line responsive to a voltage on the associated interface lines exceeding the second reference voltage.
25. A protection circuit, comprising:
a first terminal operable to receive a first reference voltage;
a second terminal operable to receive a second reference voltage;
an interface line for interfacing with an external subscriber line;
a first overvoltage protector coupled to the interface line and referenced to the first reference voltage to provide overvoltage protection responsive to a voltage on the interface line exceeding the first reference voltage by a first predetermined threshold; and
a power sink coupled to the interface line and arranged to provide a bypass path to ground for current up to a predetermined current limit responsive to a voltage on the interface line exceeding the first reference voltage prior to exceeding the first predetermined threshold, said power sink comprising:
a power transistor coupled between the interface line and ground;
a limiting resistor in series with the power transistor; and
a bias circuit coupled to a gate of the power transistor,
wherein said predetermined current limit is a function of the value of said limiting resistor.
26. A method for protecting a line card including an interface line for interfacing with an external subscriber line, comprising:
monitoring a voltage on the subscriber line;
coupling the subscriber line to ground responsive to determining that the voltage on the subscriber line exceeds a first reference voltage by a first predetermined threshold; and
sinking power from the interface line to ground responsive to a voltage on the interface line exceeding the first reference voltage prior to exceeding the first predetermined threshold using a power sink, the power sink comprising:
a power transistor coupled between the interface line and ground;
a limiting resistor in series with the power transistor; and
a bias circuit coupled to agate of the power transistor,
wherein said predetermined current limit is a function of the value of said limiting resistor.
27. The method of claim 26 , wherein the first reference voltage comprises a positive reference voltage.