Duty cycle correction circuit
View Patent ↗A duty cycle correction circuit for correcting the duty cycle of a clock signal generated by a clock generator includes a complementary buffer chain, level shifter circuits and a self-bias circuit. A clock signal with a distorted duty cycle and its complement are provided to the level shifter circuits. The level shifter circuits reduce the magnitude of voltage of the clock signal and the complement and generate level shifted signals. The level shifted signals are provided to a differential amplifier that generates a control signal indicating the magnitude of distortion in the duty cycle. The control signal is used to correct the duty cycle of the clock signal. The self-bias circuit is used to bias the differential amplifier.
1. A duty cycle correction circuit for correcting the duty cycle of an input clock signal, comprising:
a pulse-width control block for generating an adjusted clock signal, wherein the pulse-width control block receives the input clock signal and a control signal, wherein pulse-width of the input clock signal is adjusted based on the control signal to generate the adjusted clock signal;
a first buffer chain for generating a first clock signal using the adjusted clock signal, wherein the first buffer chain comprises an even count of inverters and a pass transistor; and
a second buffer chain for generating a second clock signal using the adjusted clock signal, wherein the second buffer chain comprises an odd count of inverters;
a first duty cycle-to-voltage converter, connected to the first buffer chain, for generating a first duty cycle signal representing the duty cycle of the first clock signal;
a second duty cycle-to-voltage converter, connected to the second buffer chain, for generating a second duty cycle signal representing the duty cycle of the second clock signal;
a first level shifter, connected to the first duty cycle-to-voltage converter, for level shifting the first duty cycle signal by a predetermined value to generate a first level shifted signal;
a second level shifter, connected to the second duty cycle-to-voltage converter, for level shifting the second duty cycle signal by the predetermined value to generate a second level shifted signal;
a differential amplifier, connected to the first and second level shifters for receiving the first and second level shifted signals and generating the control signal; and
a self-bias generator, connected to the first and second level shifters for receiving the first and second level shifted signals, and generating a self-bias signal used to bias the differential amplifier.
2. The duty cycle correction circuit of claim 1 , wherein the pulse width control block comprises a pseudo-inverter and an odd count of inverters sequentially connected, wherein the pseudo-inverter comprises a p-type metal oxide semiconductor (PMOS) transistor and an n-type metal oxide semiconductor (NMOS) transistor, wherein the PMOS transistor receives the control signal at a gate thereof and the NMOS transistor receives the input clock signal at a gate thereof, and wherein a drain of the PMOS transistor is connected to a drain of the NMOS transistor.
3. The duty cycle correction circuit of claim 1 , wherein the input clock signal is generated by a phase locked loop clock generator.
4. The duty cycle correction circuit of claim 1 , wherein the first level shifter comprises a first NMOS transistor and a first resistor connected to a source of the first NMOS transistor, the second level shifter comprises a second NMOS transistor and a second resistor connected to a source of the second NMOS transistor, wherein the first duty cycle signal is provided to a gate of the first NMOS transistor, and the second duty cycle signal is provided to a gate of the second NMOS transistor.
5. The duty cycle correction circuit of claim 4 , wherein the first level shifted signal is obtained at the source of the first NMOS transistor, and the second level shifted signal is obtained at the source of the second NMOS transistor.
6. The duty cycle correction circuit of claim 1 , wherein the first level shifter comprises a first PMOS transistor and a first resistor connected to a source of the first PMOS transistor, wherein the second level shifter comprises a second PMOS transistor and a second resistor connected to a source of the second PMOS transistor, and wherein the first duty cycle signal is provided to a gate of the first PMOS transistor, and the second duty cycle signal is provided to a gate of the second PMOS.
7. The duty cycle correction circuit of claim 6 , wherein the first level shifted signal is obtained at the source of the first PMOS transistor, and the second level shifted signal is obtained at the source of the second PMOS transistor.
8. The duty cycle correction circuit of claim 1 , wherein the self-bias generator comprises first and second PMOS transistors, wherein a drain of the first PMOS transistor is connected to a drain of the second PMOS transistor, a source of the first PMOS transistor is connected through a first resistor to a supply voltage, a source of the second PMOS transistor is connected through a second resistor to the supply voltage, a gate of the first PMOS transistor receives the first level shifted signal, and a gate of the second PMOS transistor receives the second level shifted signal.
9. The duty cycle correction circuit of claim 8 , wherein the self-bias generator further comprises a current mirror circuit connected to drains of the first PMOS and second PMOS transistors and to a bias terminal of the differential amplifier.
10. The duty cycle correction circuit of claim 1 , further comprising a capacitor connected at an output stage of the differential amplifier.
11. The duty cycle correction circuit of claim 1 , wherein the first and second duty cycle to voltage converters comprise RC circuits.