Abstract:
There is disclosed a self-calibrating resistor-capacitor (RC) oscillator in which a resistor has a resistance value varied minimally by temperature change and process variation, a capacitor has a capacitance value selected adequately according to needs, and the resistor and capacitor are configurable as a one-chip by a complementary metal-oxide semiconductor (CMOS) process. The self-calibrating RC oscillator comprises a resistor part including a first resistor having a resistance value reduced with increase in temperature and a second resistor connected in series with the first resistor and having a resistance value increasing with increase in temperature.

Description:
CLAIM OF PRIORITY 
   This application claims the benefit of Korean Patent Application No. 2006-47722 filed on May 26, 2006 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a resistor-capacitor (RC) oscillator in which a resonance frequency is determined by a resistor and a capacitor, and more particularly, to a self-calibrating RC oscillator which is less dependent on change in external temperature and can be integrated into one chip. 
   2. Description of the Related Art 
   In 2000, the Institute of Electrical and Electronics Engineers (IEEE) launched a move to standardize a low-speed wireless private network application into IEEE 802.15.4. This standardized version requires a battery to run for at least 5 to 7 years, which thus necessitates designing of a circuit consuming minimal power. The most widely known technology for minimizing power consumption is to turn off a circuit when it does not need to be in operation. 
   Meanwhile, in a system where a power is turned off during deactivation for less power consumption, the power should be turned on and off through a shorter time to reduce power consumption of the system. Especially, this characteristic is notably important in an oscillator circuit for generating a reference frequency which is essentially used in a telecommunication system. 
   In general, a conventional crystal oscillator needs time for self-start to amplify noises generated internally to an operational level. Accordingly, the conventional crystal oscillator entails a long operation time and significant power consumption to start oscillating. In consequence, the crystal oscillator with such operation time and power consumption is ill-suitable for a system which has a power turned off during deactivation to minimize power consumption. 
   To overcome problems with the conventional crystal oscillator, a resistor-capacitor (RC) oscillator has been suggested. The RC oscillator is inexpensive, low in power consumption, short in starting time and easily controllable in a frequency. 
     FIG. 1  is a block diagram illustrating a configuration of a conventional RC oscillator. As shown in  FIG. 1 , the conventional RC oscillator includes a resistor part R, a capacitor part C, a bias circuit  11 , a lamp-hold circuit  12 , a comparator  12 , a divider  14 , a charge pump  15  and an oscillator part  16 . 
   In a brief explanation of the conventional RC oscillator, the bias circuit  11  generates a preset reference voltage V REF1 , and a ramp current I RAMP  to be proportional to the reference voltage V REF1  and inversely proportional to a resistance value of the resistor part R. 
   The ramp and hold circuit  12  charges the capacitor part C using the ramp current during a predetermined charging time. Here, the charging time is set by the divider  14  which divides a period of an oscillation signal outputted from the RC oscillator by a predetermined divide ratio. The charging time is proportional to the period of the oscillation signal of the RC oscillator. After the charging, the ramp and hold circuit  12  maintains a both-end voltage of the capacitor part C. 
   The comparator  13  compares the reference voltage with the both-end voltage of the capacitor part C and outputs a comparison result. The divider  14  controls the charge pump  15  to increase or decrease a control voltage of the oscillator part  16 . That is, in a case where the both-end voltage of the capacitor part C is greater than the reference voltage based on the comparison result, the comparator  13  increases a voltage V CP  supplied to the oscillator part  16  by the charge pump  15 , thereby increasing a frequency of the oscillation signal outputted from the oscillator part  16 . In contrast, in a case where the both-end voltage of the capacitor part C is smaller than the reference voltage based on the comparison result, the comparator  13  decreases the voltage V CP  supplied to the oscillator part  16  by the charge pump  15 , thereby decreasing the frequency of the oscillation signal outputted from the oscillator part  16 . 
   In this conventional RC oscillator, the oscillation signal has the frequency determined by the resistance value of the resistor part R and a capacitance value of the capacitor part C. Therefore, in order to obtain the oscillation signal with a desired frequency, it is of great importance to manufacture the RC oscillator in which the resistance value and capacitance value have a predetermined value in designing a circuit. 
   However, the resistor part R and the capacitor part C, when configured as a one-chip by a complementary metal-oxide semiconductor (CMOS) process, have the resistance value and capacitance value widely varied according to temperature change and process variation. For example, an RC time constant may be varied by about ±50%. In consequence, conventionally, the resistor and the capacitor are hardly integrated with other circuits into one chip. Yet, an external lumped resistor and capacitor having adequately selected values respectively are connected to the circuit ‘A’ configured as a one-chip. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to solve the foregoing problems of the prior art and therefore an aspect of the present invention is to provide a self-calibrating resistor-capacitor (RC) oscillator in which a resistor has a resistance value varied minimally by temperature change and process variation, a capacitor has a capacitance value selected adequately according to needs, and the resistor and capacitor are configurable as a one-chip by a complementary metal-oxide semiconductor (CMOS) process. 
   According to an aspect of the invention, the invention provides a self-calibrating resistor-capacitor (RC) oscillator for generating an oscillation signal including a resistor part including a first resistor having a resistance value reduced with increase in temperature and a second resistor connected in series with the first resistor and having a resistance value increasing with increase in temperature, the resistor part having a resistance value formed by the first and second resistors connected in series with each other; a bias circuit generating a preset first reference voltage, and a ramp current to be proportional to the first reference voltage and inversely proportional to the resistance value of the resistor part; a capacitor part having a capacitance value; a ramp and hold circuit supplying the ramp current to the capacitor part to charge the capacitor part during a charging time set according to a period of the oscillation signal, and maintaining a both-end voltage of the capacitor part after the charging; a first comparator comparing a size of the first reference voltage with a size of the both-end voltage of the capacitor part; a first charge pump for outputting a control voltage varying in response to a control signal; a control logic outputting the control signal for increasing and decreasing the control voltage outputted from the first charge pump in response to a comparison result of the first comparator, receiving the oscillation signal through feed-back, and setting the charging time during which the ramp and hold circuit supplies the ramp current to the capacitor part, in response to the received oscillation signal; and an oscillator part for outputting the oscillation signal having a frequency controlled in response to the control voltage of the first charge pump. 
   According to a preferred embodiment of the invention, the capacitor part includes a plurality of switches connected in parallel with one another to a terminal to which the ramp current is inputted; and a plurality of capacitors each having one end connected to a corresponding one of the switches and another end grounded, wherein the control logic controls on/off of the switches. 
   According to another preferred embodiment of the invention, the self-calibrating RC oscillator further comprises a crystal resonance part generating a reference frequency from a resonance signal generated from a crystal resonator; a phase frequency detector for comparing a frequency of the oscillation signal with the reference frequency; a second charge pump for outputting a comparison voltage controlled in response to a comparison result of the phase frequency detector; and a second comparator for comparing the comparison voltage outputted from the second charge pump with a preset second reference voltage to provide to the control logic, wherein the control logic controls on/off of the switches in response to a comparison result of the second comparator to determine the capacitance value of the capacitor part. 
   The self-calibrating RC oscillator may further include a storage for storing on/off status of the switches in response to the comparison result of the second comparator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is block diagram illustrating a configuration of a conventional RC oscillator; 
       FIG. 2  is a block diagram illustrating a configuration of an RC oscillator according to an embodiment of the invention; and 
       FIG. 3  is a graph illustrating temperature characteristics of a resistor applied to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the shape and dimensions may be exaggerated for clarity, and the same reference signs are used to designate the same or similar components throughout. 
     FIG. 2  is a block diagram illustrating a resistor-capacitor (RC) oscillator according to an exemplary embodiment of the invention. Referring to  FIG. 2 , the RC oscillator according to an embodiment of the invention includes a resistor part R, a capacitor part C, a bias circuit  21 , a ramp and hold circuit  22 , a first comparator  23 , a control logic  24 , a first charge pump  25  and an oscillator part  26 . 
   In addition to these constituents, the RC oscillator according to this embodiment may further include a crystal resonator  31 , a phase frequency detector  32 , a second charge pump  33 , a second comparator  34  and a storage  35 . 
   The resistor part R includes a first resistor R 1  having a resistance value reduced with increase in temperature and a second resistor R 2  connected in series with the first resistor R 1  and having a resistance value increasing with increase in temperature. It has been known in the art that a resistor having a resistance value increasing or decreasing with increase in temperature can be manufactured by a complementary metal-oxide semiconductor (CMOS) process. 
     FIG. 3  illustrates a graph illustrating resistance values of the first and second resistors R 1  and R 2 , varying with temperature. As shown in  FIG. 3(   a ), the first resistor R 1  has a resistance value reduced from about 76 kΩ to about 66 kΩ at a temperature ranging from −40 □ to 80 □. Also, as shown in  FIG. 3(   b ), the second resistor R 2  has a resistance value reduced from about 60 kΩ to about 85 kΩ at a temperature ranging from −40 □ to 80 □. According to this exemplary embodiment of the invention, as shown in  FIGS. 3(   a ) and ( b ), the resistors with two temperature-dependent characteristics are connected in series with each other. Accordingly, the two characteristics are cancelled out, thereby reducing a variation range of the resistance values. 
     FIG. 3(   c ) is a graph illustrating temperature dependence of a resistance value in a case where the first resistor R 1  and the second resistor R 2  are connected in series with each other. As shown in  FIG. 3(   c ), the resistance value in the structure having the two resistors connected in series is varied by about 70 kΩ to about 71.5 kΩ at a temperature ranging from −40 □ to 80 □. That is, even in case of a big change in temperature, the resistance value is varied by about 1.5 kΩ. Therefore, by connecting the first resistor R 1  and the second resistor R 2  in series with each other as described above, the RC oscillator may be notably reduced in variations of an oscillation frequency, caused by temperature-induced variation in the resistance value. 
   Referring back to  FIG. 2 , the bias circuit  21  generates a preset first reference voltage V REF1 , and a ramp current I RAMP  to be proportional to the first reference voltage V REF1  and inversely proportional to the resistance value of the resistor part R. 
   The capacitor part C may be embodied as a capacitor array capable of selectively employing capacitance values to compensate for variation thereof which may occur during a manufacturing process. In this exemplary embodiment of the invention, preferably, the capacitor part C includes a plurality of switches S 1  to S 4  connected in parallel with one another to a terminal to which the ramp current I RAMP  is inputted and a plurality of capacitors C 1  to C 5  each having one end connected to a corresponding one of the switches S 1  to S 4  and another end grounded. 
   The switches can be switched on/off by a control logic  24 . That is, in this embodiment, the switches S 1  to S 4  are controllably switched on/off, thereby selectively employing capacitance values of the capacitors C 1  to C 5  connected in parallel with one another. Particularly, when the capacitor C 1  having a reference capacitance value does not employ a switch, the capacitors C 2  to C 5  having smaller capacitance values may employ a switch while selectively applying capacitance values, to compensate for variation in the capacitance values. 
   The ramp and hold circuit  22  supplies the ramp current I RAMP  to the capacitor part C to charge the capacitor part during a charging time set according to a period of the oscillation signal generated from the oscillator and maintains a both-end voltage of the capacitor part C after the charging. The charging time is set by the control logic  24  which divides the period of the oscillation signal outputted from the RC oscillator by a predetermined divide ratio. Here, the charging time is proportional to the period of the oscillation signal of the RC oscillator. 
   The first comparator  23  compares a size of the first reference voltage V REF1  with a size of the both-end voltage of the capacitor part C. The first comparator  23  may be configured as a simple inverter type formed of two transistors. In a case where the both-end voltage of the capacitor part C is greater than the first reference voltage V REF1 , the first comparator  23  converts an output from high to low. The control logic  24  controls the first charge pump  25  to increase or decrease a control voltage of the oscillator  26  in response to the output of the first comparator  23 . A more detailed explanation will be given hereunder about operations of the control logic  24  and the first charge pump  23  in response to a comparison result of the first comparator  23 . 
   The control logic  24  outputs a control signal in response to the comparison result of the first comparator  23 , receives the oscillation signal through feed-back and sets the charging time during which the ramp and hold circuit  22  supplies the ramp current I RAMP  to the capacitor part C. As described above, the charging time is proportional to the period of the oscillation signal outputted from the RC oscillator. 
   The first charge pump  25  outputs the control voltage varying in response to the control signal of the control logic  24 . The oscillator part  26  outputs the oscillation signal having a frequency controlled in response to the control voltage of the first charge pump  25 . 
   Hereinafter, operations of the first comparator  23 , the control logic  24 , the first charge pump  25  and the oscillator part  26  will be described. In a case where the both-end voltage of the capacitor part C is greater than the first reference voltage V REF1  based on the comparison result of the first comparator  23 , the control logic  24  provides the control signal for increasing the control voltage V CP  of the oscillator part  26  outputted from the first charge pump  25 , to the first charge pump  25 . Accordingly, the first charge pump  25  increases the control voltage V CP  supplied to oscillator part  26 , thereby increasing a frequency of the oscillation signal outputted from the oscillator part  26 . In contrast, in a case where the both-end voltage of the capacitor C is smaller than the first reference voltage V REF1  based on the comparison result of the first comparator  23 , the control logic  24  provides the control signal for decreasing the control voltage V CP  of the oscillator part  26  outputted from the first charge pump  25 , to the first charge pump  25 . Accordingly, the first charge pump  25  decreases the control voltage V CP  supplied to the oscillator part  26 , thereby decreasing the frequency of the oscillation signal outputted from the oscillator part  26 . 
   The self-calibrating RC oscillator according to an exemplary embodiment of the invention may further include a crystal resonance part  31 , a phase frequency detector  32 , a second charge pump  33 , and a second comparator  34 . The crystal resonance part  31  generates a reference frequency from a resonance signal generated from a crystal resonator. The phase frequency detector  32  compares a frequency of the oscillation signal with the reference frequency. The second charge pump  32  outputs a comparison voltage controlled in response to a comparison result of the phase frequency detector  32 . The second comparator compares the comparison voltage outputted from the second charge pump  32  with a preset second reference voltage to provide to the control logic  24 . 
   With these additional constituents, when the RC oscillator initially operates, the oscillator part  26  outputs the oscillation signal controlled by the crystal resonance part  31  which generates a relatively precise frequency. Then, the capacitor part C has the capacitance value determined to output the oscillation signal controlled by the crystal resonance part  31 . Thus, the control logic  24  controls on/off of the switches S 1  to S 4  of the capacitor part C. This configuration is effective in determining the capacitance value of the capacitor part C when the RC oscillator starts operating. After the capacitance value of the capacitor part C is determined, the crystal resonance part  31 , the phase frequency detector  32 , the second charge pump  33  and the second comparator  34  may be all turned off to reduce unnecessary power consumption. 
   As set forth above, according to exemplary embodiments of the invention, a first resistor having a resistance value decreasing with increase in temperature is connected in series with a second resistor having a resistance value increasing with increase in temperature. This ensures an RC oscillator to be significantly reduced in variations of an oscillation frequency, caused by temperature-induced variation in a resistance value. 
   While the present invention has been shown and described in connection with the preferred embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.