Level shifter with low duty cycle variation

A level shifter that shifts a voltage level of an input signal, where the input signal oscillates between a voltage level of a first supply voltage and ground. The level shifter includes a bias circuit, and first and second transistors. The bias circuit provides a bias voltage to the first transistor based on the first supply voltage. The second transistor is connected in series with the first transistor, and the series combination is connected between voltage supplies that provide second and third supply voltages. The second transistor receives the input signal at its gate, and a level-shifted version of the input signal is output at a node between the first and second transistors. The level-shifted signal oscillates between the voltage levels of the second and third supply voltages.

BACKGROUND

The present invention relates generally to integrated circuits, and, more particularly, to a level shifter circuit.

Level shifters are used in integrated circuits to shift voltage levels of signals that cross voltage domains that operate at different voltage levels. Phase-locked loop (PLL) circuits commonly use level shifters to shift the voltage level of an oscillating signal generated by a voltage-controlled oscillator (VCO) of the PLL. The frequency of the oscillating signal is controlled by an input control voltage. The VCO includes a voltage-to-current converter and a current-controlled oscillator (CCO). The voltage-to-current converter converts the input control voltage into a current input, and the CCO modulates the frequency of the oscillating signal based on the current input.

A ring oscillator is the most commonly used CCO and includes an odd number of cascade-connected inverters, for example, three cascade-connected inverters. Such a ring oscillator generates three oscillating signals that have a predetermined phase difference between them. Each oscillating signal oscillates from zero (i.e., ground voltage level) to a voltage level that is equal to a ring voltage of the CCO. The VCO includes a level shifter that is connected to the CCO to shift the voltage level of an oscillating signal to a supply voltage level of the level shifter.

A known technique for shifting the voltage level of an oscillating signal requires the oscillating signal and another oscillating signal from the CCO as inputs to the level shifter. This technique requires the two input signals to be exactly 180° out of phase with each other for the level shifter to maintain the duty cycle of the level-shifted oscillating signal equal to that of the input oscillating signals. However, since the CCO contains an odd number of cascade-connected inverters, the phase difference between any two oscillating signals is not 180°, and hence, this technique does not provide an effective solution. Further, in order to match capacitive loading at each phase of the CCO, a level shifter is required for each oscillating signal, leading to an increase in power consumption and circuit area.

Another known technique is to use a bias circuit along with the level shifter to adjust the operating points (i.e., the quiescent points) of transistors of the level shifter using the ring voltage. However, this technique requires a bias circuit for each oscillating signal, which increases power consumption and circuit area.

It would be advantageous to have a level shifter that does not increase circuit area and power consumption, and maintains the duty cycle of the level-shifted oscillating signal substantially equal to that of the oscillating signal input to the level shifter.

DETAILED DESCRIPTION

In an embodiment, the present invention provides a level shifter that generates an output voltage based on an input voltage, where the input voltage oscillates between a voltage level of a first supply voltage and a ground voltage level. A first transistor has a gate that receives the first supply voltage, and a source that receives a second supply voltage. A second transistor has a drain connected to its gate and a drain of the first transistor, and a source that receives a third supply voltage. A third transistor has a source that receives the third supply voltage, and a gate connected to the gate of the second transistor. A fourth transistor has a gate that receives the input voltage, a source that receives the second supply voltage, and a drain connected to a drain of the third transistor for generating the output voltage. The output voltage oscillates between voltage levels of the second and third supply voltages.

In another embodiment, the present invention provides a voltage-controlled oscillator (VCO). The VCO includes a voltage-to-current converter, a current-controlled oscillator (CCO), and a level shifter. The voltage-to-current converter receives an input voltage and generates an output current. The CCO receives the output current and generates a plurality of oscillating signals that have a predetermined phase difference therebetween. A ring voltage is output from a node between the voltage-to-current converter and the CCO. A first transistor has a gate that receives the ring voltage, and a source that receives a first supply voltage. A second transistor has a drain connected to its gate and a drain of the first transistor, and a source that receives a second supply voltage. A third transistor has a gate connected to the gate of the second transistor, and a source that receives the second supply voltage. A fourth transistor has a gate connected to the CCO for receiving a first oscillating signal of the plurality of oscillating signals, a source that receives the first supply voltage, and a drain connected to a drain of the third transistor for generating the output voltage. The first oscillating signal oscillates between a voltage level of the ring voltage and ground, and the output voltage oscillates between voltage levels of the first and second supply voltages.

In yet another embodiment, the present invention provides a phase-locked loop (PLL) that includes a VCO having a voltage-to-current converter, a CCO, and a level shifter. The voltage-to-current converter receives an input voltage and generates an output current. The CCO receives the output current and generates a plurality of oscillating signals therefrom that have a predetermined phase difference therebetween. A ring voltage is output from a node between the voltage-to-current converter and the CCO. A first transistor has a gate that receives the ring voltage, and a source that receives a first supply voltage. A second transistor has a drain connected to its gate and a drain of the first transistor, and a source that receives a second supply voltage. A third transistor has a gate connected to the gate of the second transistor, and a source that receives the second supply voltage. A fourth transistor has a gate connected to the CCO for receiving a first oscillating signal of the plurality of oscillating signals, a source that receives the first supply voltage, and a drain connected to a drain of the third transistor for generating the output voltage. The first oscillating signal oscillates between a voltage level of the ring voltage and ground, and the output voltage oscillates between voltage levels of the first and second supply voltages.

Various embodiments of the present invention provide a level shifter that includes first through fourth transistors. The first transistor has a gate that receives a first supply voltage and a source that receives a second supply voltage. The second transistor has a drain connected to its gate and a drain of the first transistor, and a source that receives a third supply voltage. The third transistor has a source that receives the third supply voltage, and a gate that is connected to the gate of the second transistor. The fourth transistor has a gate that receives an input voltage, a source that receives the second supply voltage, and a drain that is connected to a drain of the third transistor for generating an output voltage. Thus, the second and third transistors form a current mirror, and the third and fourth transistors form an inverter stage. The input voltage oscillates between a voltage level of the first supply voltage and ground (i.e., a ground voltage level), and the output voltage oscillates between voltage levels of the second and third supply voltages.

The level-shifted output signal has substantially the same duty cycle as that of the oscillating signal input to the level shifter. Further, when used in a PLL, the same bias circuit can be used for each oscillating signal of the PLL, thereby reducing power consumption and the circuit area.

Referring now toFIG. 1, a schematic block diagram of a phase-locked loop (PLL)100in accordance with an embodiment of the present invention is shown. The PLL100includes a voltage-controlled oscillator (VCO)102that includes a voltage-to-current converter104, a current-controlled oscillator (CCO)106, and a level shifter108. The level shifter108includes a low-pass filter110and first through fourth transistors112-118. In the presently preferred embodiment, the first and fourth transistors112and118are n-channel metal-oxide semiconductor (NMOS) transistors, the second and third transistors114and116are p-channel metal-oxide semiconductor (PMOS) transistors, and the low-pass filter110is a resistance-capacitance (RC) filter.

The voltage-to-current converter104receives a first supply voltage (AVDD) and an input voltage (VIN), and generates an output current (IOUT).

The CCO106is connected between the voltage-to-current converter104and ground, and receives the output current (IOUT). In the presently preferred embodiment, the CCO106is a ring oscillator and includes first through third cascade-connected inverters120a-120cthat generate first through third oscillating signals (VOSC_1-VOSC_3), respectively (collectively referred to as the oscillating signals (VOSC)) based on the output current (IOUT). A ring voltage (VRING) is output from a node between the voltage-to-current converter104and the CCO106. The oscillating signals (VOSC) have a predetermined phase difference between them and oscillate between a voltage level of the ring voltage (VRING) and a ground voltage level (i.e., zero).

A source of the first transistor112receives a second supply voltage (VSS) and a gate thereof is connected to the node between the voltage-to-current converter104and the CCO106to receive the ring voltage (VRING). A drain of the second transistor114is connected to its gate and a drain of the first transistor112, and a source thereof receives a third supply voltage (VDD). A gate of the third transistor116is connected to the gate of the second transistor114by way of a resistor122and a source thereof receives the third supply voltage (VDD). A capacitor124is connected between the gate of the third transistor116and a voltage supply (not shown) that provides the third supply voltage (VDD). A gate of the fourth transistor118is connected to the CCO106to receive the first oscillating signal (VOSC_1) and a source thereof receives the second supply voltage (VSS). A drain of the fourth transistor118is connected to the drain of the third transistor116to generate an output voltage (VOUT). Since the supply voltages received by the level shifter108are the second and third supply voltages (VSSand VDD), the output voltage (VOUT) oscillates between voltage levels of the second and third supply voltages (VSSand VDD). In one embodiment, the second supply voltage (VSS) is ground, i.e., the voltage level of the second supply voltage (VSS) is equal to the ground voltage level. In another embodiment, the voltage level of the second supply voltage (VSS) is different from the ground voltage level. The resistor122and the capacitor124form the low-pass filter110, and the first and second transistors112and114and the low-pass filter110form a bias circuit126.

In the presently preferred embodiment, the level shifter108includes series-connected fourth through sixth inverters128a-128cthat buffer the output voltage (VOUT), and THEREBY OUTPUT A BUFFERED OUTPUT VOLTAGE (VBUF_OUT). Each inverter128is a complementary metal-oxide semiconductor (CMOS) inverter. For each oscillating signal (VOSC), a four-stage inverter, similar to the four-stage inverter formed by the third and fourth transistors116and118and the inverters128, is required to buffer the oscillating signal (VOSC). Moreover, the same bias circuit126can be used with each four-stage inverter. Thus, the level shifter108requires less circuit area and power. The use of a four-stage inverter for each oscillating signal (VOSC) of the CCO106ensures equalization of capacitive loading at each phase of the CCO106. It will be understood by those with skill in the art that any number of the inverters128can be used to buffer an oscillating signal (VOSC).

An increase in the output current (IOUT) increases the ring voltage (VRING), which in turn increases the frequency and the amplitude of the oscillating signals (VOSC) and the current through the first and second transistors112and114. An increase in the amplitude of the oscillating signals (VOSC) results in an increase in the current through the fourth transistor118as well as in the current through the first and second transistors112and114and results in a decrease in the bias voltage (VBIAS) received by the third transistor116at its gate. Similarly, a decrease in the output current (IOUT) results in a decrease in the frequency and amplitude of the oscillating signals (VOSC) and the current through the first and second transistors112and114, which in turn increase the bias voltage (VBIAS). Thus, when the current through the fourth transistor118increases, the current through the third transistor116also increases, i.e., when the fourth transistor118becomes stronger, the third transistor116also becomes stronger. Similarly, when the fourth transistor118becomes weaker, the third transistor116also becomes weaker.

Thus, the charging and discharging rates of an output node of the level shifter108formed by the connection of the drains of the third and fourth transistors116and118vary based on the ring voltage (VRING), and hence, the duty cycle of the output voltage (VOUT) is substantially equal to that of the first oscillating signal (VOSC_1) even at high frequencies of the first oscillating signal (VOSC_1).