Schmitt trigger with threshold voltage close to rail voltage

Voltage level shifting in a switching output stage is presented. The circuit may include a switching output stage configured to receive an analog input signal and provide a responsive digital output signal, the switching output stage having a first switching device coupled to a first supply voltage and a second switching device coupled to a second supply voltage, the first switching device and the second switching device being coupled to a common output node. The apparatus may also include a voltage level shifter circuit coupled to a switching control node of the second switching device, the voltage level shifter configured to shift a voltage level at the switching control node of the second switching device relative to the analog input signal, wherein the digital output signal at the common output node transitions as the input signal reaches a predetermined threshold value.

FIELD

This disclosure relates generally to a Schmitt trigger circuit, more specifically, a Schmitt trigger circuit that has one of its threshold voltage very close to rail voltages

BACKGROUND

Switching amplifiers are common for many different applications, such as pulse generators. One application for a switching amplifier is in the audio field. Often a switching amplifier is used as an Analog-to-Digital Converter (ADC), or as a digital signal driver for an acoustic device, such as a speaker. One type of switching amplifier is commonly referred to as a class-D amplifier. Although various embodiments described herein may be discussed with relation to a class-D amplifier, one of ordinary skill will recognize that the embodiments may be equally useful with other switching amplifier configurations.

A class-D amplifier is an electronic amplifier where internal power devices, such as Metal-On-Silicon Field-Effect-Transistors (MOSFETs), are operated as binary switches. The MOSFETs are often driven to be either fully on or fully off. Ideally, zero time is spent transitioning between those two states. Class D amplifiers work by generating a variable duty cycle square wave of which the low-frequency portion of the spectrum nearly resembles the desired output signal, and of which the high-frequency portion serves no purpose other than to make the waveform binary so it can be amplified by switching the power devices.

The switching amplifier often includes a PMOS transistor and an NMOS transistor. The PMOS and the NMOS may switch on and off respectively to provide the switching output. Switching from one MOS to another needs to be done carefully so that direct current from PMOS to NMOS should be minimized, and the non-overlap time during which both PMOS and NMOS are off should also be minimized. It is also well known that electromagnetic interference (EMI) related to class D amplifier can be alleviated by slowing down the output slew rate.

To accurately control the PMOS and NMOS in a class D amplifier in a manner described above, it is desirable to use the amplifier output as feedback to control the slew rate. Specifically, it is desirable to drive the gate of the power transistors with a weak strength during the transition of output, but once output finishes its transition, it is desired to drive the gate of the power transistors with full force so that the overshoot, undershoot and energy loss all can be minimized. As used herein, a “weak” strength MOSFET may include a device that is small in physical dimensions relative to its complimentary device, or a device with lower charge-carrier mobility characteristics relative to its complimentary device. Prior art of using just a skewed inverter or conventional Schmitt trigger often cannot accurately control or robustly set the threshold voltage very close to the rail voltage, which is where the switching truly finishes.

SUMMARY

Embodiments of systems, apparatuses, and methods for implementing a Schmitt trigger with threshold voltage close to rail voltage are presented. In an embodiment, the apparatus includes a switching output stage configured to receive an analog input signal and provide a responsive digital output signal, the switching output stage having a first switching device coupled to a first supply voltage and a second switching device coupled to a second supply voltage, the first switching device and the second switching device being coupled to a common output node. The apparatus also includes a voltage level shifter circuit coupled to a switching control node of the second switching device, the voltage level shifter configured to shift a voltage level at the switching control node of the second switching device relative to the input signal, wherein the digital output signal at the common output node transitions as the input signal reaches a predetermined threshold value that is very close to rail voltage. Additionally, the apparatus may include a third switching device and a fourth switching device for implementing hysteresis in the apparatus. Beneficially, such embodiments may shift the threshold of this Schmitt trigger toward the rail voltage. When the output of a class D amplifier is connected to the input of the embodiments of this Schmitt trigger, the exact moment when the class D amplifier finishes switching can be sensed. Therefore, overshoot and/or undershoot of the class D output can be reduced and the performance of the class D amplifier can be improved.

In an embodiment, the first supply voltage is higher than the second supply voltage. In such an embodiment, the voltage level shifter may shift the input signal up by a predetermined shift value. Thus, the predetermined threshold value is shifted toward the second supply voltage.

In another embodiment, the first supply voltage is lower than the second supply voltage. In such an embodiment, the voltage level shifter is configured to shift the input signal down by a predetermined shift value, and thus the predetermined threshold voltage is shifted toward the second supply voltage.

In one embodiment, the second switching device is stronger than the first switching device. As used herein a “strong” strength MOSFET may include a device that is large in physical dimensions relative to its complimentary device, or a device with higher charge-carrier mobility characteristics relative to its complimentary device. In another embodiment, a current supply drives the source node of the first switching device.

In an embodiment, the voltage level shifter includes a first diode and a second diode, wherein an anode of the first diode is coupled to the input signal and a cathode of the first diode is coupled to the switching control node of the second switching device, and wherein an anode of the second diode is coupled to the switching control node of the second switching device and a cathode of the second diode is coupled to the input signal.

In another embodiment, the voltage level shifter includes a first level shifter transistor, wherein a drain node and a gate node of the first level shifter transistor are coupled to the input signal, and a source node of the level shifter transistor is coupled to the switching control node of the second switching device, and a second level shifter transistor, wherein a source node of the second level shifter transistor is coupled to the input signal, and a drain node and a gate node of the second level shifter transistor are coupled to the switching control node of the second switching device.

A method is also presented. In an embodiment, the method includes receiving, in a switching output stage, an analog input signal. The method may also include generating a responsive digital output signal to a common output node of the switching output stage, the switching output stage having a first switching device coupled to a first supply voltage and a second switching device coupled to a second supply voltage, the first switching device and the second switching device being coupled to the common output node. Additionally, the method may include shifting a voltage level at a switching control node of the second switching device relative to the input signal with a voltage level shifter circuit coupled to the switching control node of the second switching device, wherein the digital output signal at the common output node transitions as the input signal reaches a predetermined threshold value that is close to rail voltage.

A system is also presented. In an embodiment, the system includes an acoustical driver configured to convert an electrical signal into acoustical energy. Additionally, the system may include a signal driver coupled to the acoustical driver. The signal driver may include a switching output stage configured to receive an analog input signal and provide a responsive digital output signal, the switching output stage having a first switching device coupled to a first supply voltage and a second switching device coupled to a second supply voltage, the first switching device and the second switching device being coupled to a common output node. Additionally, the signal driver may include a voltage level shifter circuit coupled to a switching control node of the second switching device, the voltage level shifter configured to shift a voltage level at the switching control node of the second switching device relative to the input signal, wherein the digital output signal at the common output node transitions as the input signal reaches a predetermined threshold value.

DETAILED DESCRIPTION

The present embodiments provide for implementing a Schmitt trigger with threshold voltage close to rail voltage. Embodiments of systems, apparatuses, and methods are described. Such embodiments may shift an input signal level on one of the switching devices such that the switching device turns on closer to the voltage rail coupled to the switching device. For example, if the threshold of the Schmitt trigger is desired to be close to ground, then embodiment of this Schmitt trigger may include a weak PMOS transistor and a strong NMOS transistor, then the gate of the NMOS transistor may be coupled to the output of the level shifting circuit which shifts the input voltage up such that the NMOS transistor only switches off when the input signal is relatively close to ground. Since the NMOS transistor is much stronger than PMOS transistor, output will only toggle when the NMOS is fully turned off. Therefore, the threshold voltage in terms of input when output starts to toggle is shifted down. Beneficially, such embodiments may shift the threshold toward the rail voltage. When the output of a class D amplifier is connected to the input of the embodiments of this Schmitt trigger, the exact moment when the class D amplifier finishes switching can be sensed. Therefore, overshoot and/or undershoot of the class D output can be reduced and the performance of the class D amplifier can be improved.

FIG. 1is a schematic block diagram illustrating one embodiment of a system100for implementing a Schmitt trigger with threshold voltage close to rail voltage. In an embodiment, the system100includes a Schmitt trigger device102. The system100may also include a class D output stage104. In an example embodiment, the class D output stage104may include a first transistor and a second transistor coupled to a high and low voltage rail+Vdd. Additionally, each transistor may be driven by a pre-driver106. The class D output stage104may receive an input signal from a pulse width modulator108which receives an input signal110. The Schmitt trigger circuit102may capture the moment when the output class D output stage104finishes switching from one rail voltage to the other rail voltage. The pre-driver106may use the output of the Schmitt trigger102to adjust the slew rate for the gate of the transistors in the class D output stage104. One of ordinary skill will recognize alternative embodiments of a system100that may be used in association with the present embodiments.

FIG. 2is a schematic block diagram illustrating one embodiment of a Schmitt trigger102with threshold voltage close to rail voltage. Circuit200may comprise an embodiment of the Schmitt trigger device102described inFIG. 1. In an embodiment, the circuit200includes a switching output stage202. The switching output stage202may include a first switching device204and a second switching device206, each sharing a common output node208. The second switching device206may be much stronger than the first switching device204. In an embodiment, the second switching device206may be coupled to the voltage level shifter circuit210. The voltage level shifter circuit210may be configured to shift a voltage level at a switching control node of the second switching device206relative to an input signal214. For example, the voltage level shifter circuit210may shift the input signal214by a voltage level that is very close to the threshold voltage of the second switching device206. In such an embodiment, the voltage level shifter circuit210may cause the digital output signal at the common output node208to transition as the input signal reaches a predetermined threshold, which is very close to ground. The third switching device212may provide hysteresis in the circuit200. Further examples of the structure and functions of the voltage level shifter circuit210are described inFIGS. 3-5.

FIG. 3is a schematic block diagram illustrating another embodiment of a circuit300for implementing a Schmitt trigger with threshold voltage close to rail voltage. In one embodiment, the circuit300may be used as Schmitt trigger device102. In an embodiment, the circuit300includes a switching output stage202and a voltage level shifter circuit210as described inFIG. 2. In a further embodiment of the circuit300, the switching output stage202includes a first voltage rail302, and a second voltage rail310. In the illustrated embodiment, the first voltage rail is Vdd, which may be a positive voltage supply, and the second voltage rail310may be ground. In a particular embodiment, the first voltage rail302may be at a higher magnitude potential level than the second voltage rail310.

In an embodiment, the first switching device204for circuit300may be a first transistor device304. Similarly, the second switching device206for circuit300may be a second transistor device306. Additionally, the third switching device214for circuit300may be a third transistor device314. In a further embodiment, the first transistor304and the second transistor306may be a Complimentary MOSFET pair, where the first transistor304is a PMOS device and the second transistor306is an NMOS device. In particular, the second transistor306may be much stronger than the first transistor device304. In such an embodiment, the common output node308may be coupled to the drain node of each of the first transistor304and the second transistor306. Additionally, an input signal may be received on input line312and coupled to a control node, such as the gate node, of each of the first transistor304and the third transistor314. The gate node of the second transistor306may be coupled to the voltage level shifter circuit210.

The voltage level shifter circuit210may be coupled to the common output node308as well as the input line312. In a further embodiment, the voltage level shifter circuit210may be coupled between the input node312and the control or gate node of the second transistor306, which may be an NMOS transistor in some embodiments. According to one embodiment, the voltage level shifter circuit210may shift the voltage level up at the gate node of the second transistor306such that the transistor is configured to switch off when the voltage at the input signal312is closer to ground than would be the case without the voltage shifting. For example, if the second transistor306has a threshold voltage (Vth) of 0.6 V, then the voltage level shifter circuit210may shift the input signal312up by around 0.6 V. In some embodiments, the voltage level shifter circuit210may shift the voltage of the input signal312up by about 0.1 V, 0.2 V, 0.3 V, 0.4 V, or 0.5 V. One of ordinary skill will recognize that the voltage level shifter circuit may shift the input signal more or less within the range of 0V-0.6 V, depending upon system configurations or performance requirements. In a further embodiment, the circuit300may include a third transistor314and a fourth transistor316configured to provide hysteresis. When the input312transitions from high to low, the third transistor314may be turned off when the input signal312reaches just below the NMOS threshold voltage, while the second transistor306still remains on. Therefore, the threshold voltage of the circuit300is decided by the level shifter210and the second transistor306. Once the common output308changes from low to high, the fourth transistor316will turn on and strongly shut off the second transistor306. When the input312transitions start to change from low to high, initially output308and the fourth transistor316will keep the second transistor fully shut off, and the common output will be held high until the input reaches the NMOS threshold voltage to turn on the third transistor312. Therefore, the threshold for circuit300is very close to ground when the input transitions from high to low and the threshold is close to the NMOS threshold when input transitions from low to high.

FIG. 4is a schematic block diagram illustrating another embodiment of a circuit400for implementing a Schmitt trigger with threshold voltage close to rail voltage. In one embodiment, the circuit400may be used as Schmitt trigger device102as shown inFIG. 1. In an embodiment, the circuit400includes the switching output stage202as described inFIG. 3. The circuit400may also include a voltage level shifter circuit210. In the depicted embodiment, the voltage level shifter circuit210may include a diode or transistor402and a second diode or transistor404arranged in a back-to-back or source-to-drain configuration.

For example, a drain node and a gate node of the first level shifter transistor402are coupled to the gate node of the second transistor306, and a source node of the level shifter transistor402is coupled to the input signal312. A second level shifter transistor404is arranged such that a gate node and a drain node of the second level shifter transistor404is coupled to the input signal312, and a source node of the transistor of second level shifter404is coupled to the switching control node (e.g., gate node) of the second transistor306.

FIG. 5is a schematic block diagram illustrating another embodiment of a circuit500for implementing a Schmitt trigger with threshold voltage close to rail voltage. In one embodiment, the circuit500may be used as Schmitt trigger device102. The circuit500ofFIG. 5is similar in structure and function to the circuit400ofFIG. 4; however, the first switching transistor304is coupled to a current source502configured to drive a small constant current I1to the source node of the first transistor304so that the driving force of the first transistor304is limited by the small current provided by current source502.

It should be understood that the various operations described herein, particularly in connection withFIG. 6, may be implemented by processing circuitry or other hardware components. The order in which each operation of a given method is performed may be changed, and various elements of the systems illustrated herein may be added, reordered, combined, omitted, modified, etc. It is intended that this disclosure embrace all such modifications and changes and, accordingly, the above description should be regarded in an illustrative rather than a restrictive sense.

FIG. 6is a flowchart diagram illustrating one embodiment of a method600for voltage level shifting in a switching output stage of a signal amplifier. In an embodiment, the method600includes receiving an analog input signal at the input node308as shown at block602. The switching output stage202may generate a responsive digital output signal to a common output node208of the switching output stage202, as shown at block604. In an embodiment, the switching output stage202has a first switching device204coupled to a first supply voltage302and a second switching device206coupled to a second supply voltage310. The first switching device204and the second switching device206may be coupled to the common output node208. As shown at block606, the voltage level shifter circuit210may shift a voltage level at a switching control node of the second switching device206relative to the input signal308. In an embodiment, the digital output signal at the common output node208transitions as the input signal312reaches a predetermined threshold value.

Although this disclosure makes reference to specific embodiments, certain modifications and changes can be made to those embodiments. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of this disclosure. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims.