This application claims the priority of Korean Patent Application No. 2002-49294, filed Aug. 20, 2002, which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an oscillator generating an oscillation signal, and more particularly, to an RC oscillator using a Schmitt trigger circuit.
2. Description of the Related Art
An oscillator is an electronic circuit that generates an oscillation signal having a constant oscillation frequency. It is essential for an oscillator to have a reliable oscillation frequency. That is, the oscillator must maintain a constant oscillation frequency regardless of external conditions such as temperature and power supply voltage. One problem with conventional oscillators is that they are not independent of external conditions.
FIGS. 1 through 4 are circuit diagrams of conventional oscillators. FIG. 1 shows a ring oscillator with a structure in which a plurality of inverters 110 (three inverters 110 in FIG. 1) are connected in series. A ring oscillator has a simple structure, but the transconductance value of a delay device, i.e., an inverter 110, can vary with temperature or power supply voltage, wherein an oscillation frequency also varies.
An oscillator shown in FIG. 2 includes a delay cell controlled by a bias current. Such an oscillator can be referred to as a current-starved delay oscillator. In the oscillator shown in FIG. 2, a bias current unit 220 controls the current flowing in three delay cells 210 implemented as inverters. A delay time of each delay cell 210 is determined in accordance with the current flowing in the delay cell 210, thus if the current is constant, an oscillation frequency is also kept constant. However, this oscillator has a problem in that its oscillation frequency changes when the current changes due to a change in a power supply voltage VDD.
FIG. 3 shows a conventional Schmitt trigger oscillator. The oscillator shown in FIG. 3 alternately charges and discharges a capacitor CC by alternately turning on a first switch SW1 and a second switch SW2 according to the voltage level of a signal output from a Schmitt trigger 310. When a power supply voltage VDD changes, a charging speed and a discharging speed also change, thereby changing the oscillation frequency. In addition, when the transition voltages VIH and VIL of the Schmitt trigger 310 change due to, for example, temperature, the oscillation frequency also changes.
To suppress a problem of an oscillation frequency changing according to external conditions such as power supply voltage and temperature, RC oscillators, which are relatively unaffected to the external conditions, can be used.
An oscillator shown in FIG. 4 is a conventional RC oscillator, which includes an odd number (five) of inverters IV1 through IV5, a capacitor Cf, and a resistance device Rf. The oscillation frequency of the RC oscillator shown in FIG. 4 is mainly determined by the resistance value of the resistance device Rf and the capacitance of the capacitor Cf. Accordingly, if the resistance value and capacitance are unaffected by temperature and power supply voltage, the oscillation frequency of an oscillation signal CLK can be kept constant.
Since the resistance device Rf is typically connected to the outside by a user, the resistance value can be kept constant. The designed capacitance of the capacitor Cf may be affected by, for example, the manufacturing process. To compensate for a change in capacitance, a plurality of capacitors can be connected in parallel, and some capacitors are disconnected by cutting fuses when necessary during use. In other words, a plurality of capacitors are provided, and if the capacitance during use is greater than an initially designed capacitance, specified capacitors are disconnected by cutting their fuses. However, the control of capacitance using fuses only reduces capacitance by disconnecting some capacitors. Accordingly, the control method using fuses can increase an oscillation frequency but cannot decrease it.
In addition, the conventional RC oscillator has a problem of high-voltage stress applied to the inverter IV1 connected to a first node N1. This is because of an effect in which charges are pumped by the capacitor Cf since the first and second nodes N1 and N2 are connected through the capacitor Cf.
Referring to FIG. 5, which shows voltage waveforms in the RC oscillator shown in FIG. 4, high-voltage stress is applied to the inverter IV1 at time instants t1, t2, t3, and t4 when second and third node voltages V2 and V3 change. A high voltage, which is higher than a power supply voltage VDD by a predetermined reference voltage VTR, is applied at the time instances t1 and t3. If high-voltage stress is continuously applied to the inverter IV1, the life or reliability of the RC oscillator is decreased.
To solve the above-described problems, it is an object of the present invention to provide an oscillator that generates an oscillation signal having a constant oscillation frequency, which is independent of external conditions such as power supply voltage and temperature.
It is another object of the present invention to provide an RC oscillator in which an oscillation frequency can be controlled during and after manufacture and in which high-voltage stress is not applied, thereby increasing reliability.
To achieve the above objects of the present invention, in one aspect, an RC oscillator is provided comprising a transition voltage generator circuit, which generates a high transition voltage and a low transition voltage, the high and low transition voltages being proportional to power supply voltage. The RC oscillator further comprises a Schmitt trigger circuit, which generates an output voltage having a first level when an input voltage becomes greater than the high transition voltage and having a second level when the input voltage becomes less than the low transition voltage. The RC oscillator further comprises an RC delay circuit having a resistance and comprising a capacitor, the RC delay circuit generating the input voltage in response to the output voltage, and a quantizer, which quantizes the input voltage to output a square-wave oscillation signal.
Preferably, the transition voltage generator circuit divides the power supply voltage, thereby generating the high transition voltage and the low transition voltage, wherein the low transition voltage is lower than the high transition voltage.
Preferably, the Schmitt trigger circuit comprises a voltage comparator, which compares the input voltage with a transition voltage to generate the output voltage; and a controller, which provides one of the high and low transition voltages as the transition voltage in response to the output voltage.
Preferably, the RC delay circuit includes first and second resistors connected to an output node; a first transistor, which is formed between the power supply voltage and the first resistor, is gated in response to the output voltage; a second transistor, which is formed between the second resistance and a ground voltage, is gated in response to the output voltage; and a capacitor, which is formed between the output node and the ground voltage.
In another aspect, there is provided an RC oscillator comprising a Schmitt trigger circuit, which generates an output voltage having a first level when an input voltage becomes greater than a high transition voltage and having a second level when the input voltage becomes less than a low transition voltage. The RC oscillator further comprises an RC delay circuit, which generates the input voltage by charging or discharging at least one capacitor, which is formed between a predetermined output node and a ground voltage, in response to the output voltage, and a quantizer, which outputs an oscillation signal having a predetermined oscillation frequency in response to the input voltage. The levels of the respective high and low transition voltages are proportional to a power supply voltage.
Preferably, the RC oscillator also comprises a transition voltage generator circuit, which divides the power supply voltage, thereby generating the high transition voltage and the low transition voltage lower than the high transition voltage. Preferably, the transition voltage generator circuit comprises a plurality of resistance devices, which are connected in series between the power supply voltage and the ground voltage; and a fuse, which is connected to at least one among the plurality of resistance devices in parallel.
Preferably, the RC delay circuit comprises a first transistor, which is formed between the power supply voltage and a predetermined first node and is gated in response to the output voltage and a second transistor, which is formed between a predetermined second node and the ground voltage and is gated in response to the output voltage. The RC delay circuit further comprises the plurality of capacitors, which are formed between the output node and the ground voltage, a first external terminal for electrically connecting a predetermined first resistor between the first node and the output node, and a second external terminal for electrically connecting a predetermined second resistor between the second node and the output node.