Phase-locked loops with electrical overstress protection circuitry

1. A phase-locked loop on an integrated circuit die, comprising:

a phase frequency detector that receives a reference clock signal;

a variable oscillator that is controlled by the phase frequency detector and that outputs a feedback clock signal to the phase frequency detector;

a control block that receives the reference clock signal and the feedback clock signal and that selectively deactivates the phase frequency detector;

a voltage bias line; and

a switch that is coupled between the voltage bias line and the variable oscillator and that is only turned on while the phase frequency detector is deactivated.

2. The phase-locked loop of claim 1 , wherein the control block selectively deactivates the phase frequency detector in response to detecting a clock loss event.

3. The phase-locked loop of claim 1 , further comprising:

a charge pump that is coupled between the phase frequency detector and the variable oscillator and that is placed in a tristate mode when the phase frequency detector is deactivated.

4. The phase-locked loop of claim 3 , wherein the charge pump includes a pull-up switch and a pull-down switch that are both turned off while the phase frequency detector is deactivated.

5. The phase-locked loop of claim 1 , further comprising:

a source follower circuit coupled between the phase frequency detector and the variable oscillator.

6. The phase-locked loop of claim 5 , wherein the source follower circuit includes a source follower transistor with a gate terminal on which a given voltage is provided, and wherein the given voltage is driven to a predetermined bias voltage level when the phase frequency detector is deactivated.

7. The phase-locked loop of claim 1 , wherein the control block is further configured to receive a sampling clock signal in addition to receiving the reference clock signal and the feedback clock signal.

8. A method for operating a phase-locked loop on an integrated circuit, comprising:

with a phase frequency detector within the phase-locked loop, receiving a reference clock signal;

with a variable oscillator within the phase-locked loop, outputting a feedback clock signal to the phase frequency detector;

with a control block within the phase-locked loop, receiving the reference clock signal and the feedback clock signal and selectively deactivating the phase frequency detector in response to detecting that either the reference clock signal has stopped toggling or the feedback clock signal has stopped toggling;

receiving a sampling clock signal that is different than the reference clock signal and the feedback clock signal at the control block; and

using the sampling clock signal to sample the reference clock signal and the feedback clock signal.

9. The method of claim 8 , further comprising:

using the control block to monitor for a clock loss event; and

in response to detecting the clock loss event, waiting for a filter latency period before deactivating the phase frequency detector.

10. The method of claim 8 , wherein the phase-locked loop also includes a charge pump, the method further comprising:

placing the charge pump in a tristate mode when the phase frequency detector is deactivated.

11. The method of claim 10 , wherein the phase-locked loop also includes a source follower transistor, the method further comprising:

using the charge pump to actively drive the source follower transistor while the phase frequency detector is activated; and

supplying a predetermined bias voltage to the source follower transistor while the phase frequency detector is deactivated and while the charge pump is in the tristate mode.

12. The method of claim 11 , further comprising:

using the source follower transistor to directly control the frequency at which variable oscillator oscillates.

13. Phase-locked loop circuitry, comprising:

a phase frequency detector;

a variable oscillator coupled to the phase frequency detector in a loop;

a source follower transistor interposed in the loop; and

a control block that selectively applies a predetermined voltage level to the source follower transistor in response to detecting a clock loss event.

14. The phase-locked loop circuitry of claim 13 , wherein the phase frequency detector receives a reference clock and a feedback clock, and wherein the control block includes a first clock loss detector circuit that determines when the reference clock has stopped toggling and a second clock loss detector circuit that determines when the feedback clock has stopped toggling.

15. The phase-locked loop circuitry of claim 14 , wherein the control block further includes a logic OR gate that receives output signals from the first and second clock loss detector circuits.

16. The phase-locked loop circuitry of claim 14 , wherein the first clock loss detector circuit includes a single flip-flop for generating first reset pulses corresponding to rising edges in the reference clock and a chain of flip-flops for generating second reset pulses corresponding to rising edges in a sampling clock that is different than the reference clock and the feedback clock.

17. The phase-locked loop circuitry of claim 16 , wherein the first clock loss detector circuit further includes a multiplexer having inputs connected to different locations along the chain of flip-flops.