Patent Publication Number: US-2005141589-A1

Title: Temperature sensing oscillator circuit

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to a temperature sensing oscillator circuit for generating a pulse signal whose period is changed depending on temperature by sensing temperature, and more specifically, to a temperature sensing oscillator circuit for regulating a period of a pulse signal outputted from an oscillator by varying an operating power of the oscillator depending on temperature change.  
      2. Description of the Prior Art  
      It is important how long mobile products such as a cellular phone or a laptop computer can be successively operated with given batteries. As a result, memory devices mounted on these products are required to have small power consumption. A low power DRAM and a Pseudo SRAM have been widely used for those memory devices.  
      In order to reduce power consumption in the memory devices, a self-refresh cycle is properly regulated depending on temperature change to reduce the amount of current required at a self-refresh mode to the maximum extent.  
       FIG. 1  is a circuit diagram of a conventional temperature sensing oscillating circuit for automatically regulating a self-refresh cycle depending on temperature change in a self-refresh circuit.  
      The temperature sensing oscillator circuit of  FIG. 1  comprises a voltage comparison unit  11 , an inversion delay unit  12 , a control unit  13  and a temperature sensing unit  14 .  
      The voltage comparison unit  11  compares an output voltage from the temperature sensing unit  14  with a reference voltage, and outputs a signal having a high level or a low level corresponding to the result of the comparison. The inversion delay unit  12  inverts and delays an output signal from the voltage comparison unit  11  to secure a predetermined pulse width of a refresh signal TEMPOSC. The control unit  13  controls generation of the refresh signal TEMPOSC depending on an output signal from the inversion delay unit  12 , an output signal TOSCRSTB having a predetermined cycle regardless of temperature change, and a temperature sensing operating signal TEMPON. The temperature sensing unit  14  outputs a voltage varied by the temperature change to the voltage comparison unit  11 .  
      In the temperature sensing oscillator circuit of FIG.  1 , current flowing in serially connected NMOS transistors D 1  and D 2  of the temperature sensing unit  14  is differentiated depending on temperature change. The voltage comparison unit  11  receives a voltage varied by the NMOS transistors D 1  and D 2 , and compares the voltage with the reference voltage to generates the refresh signal TEMPOSC with a variable cycle depending on temperature change.  
      In the temperature sensing oscillator circuit of  FIG. 1 , the cycle of the refresh signal TEMPOSC increases in a exponential form if temperature becomes lower so that the refresh signal TEMPOSC is not oscillated at less than a predetermined temperature.  
       FIG. 2  is a simulation diagram illustrating generation of a refresh signal with change of temperature at −15° C., 15° C., 45° C., 70° C. and 85° C. while a voltage VDD is applied to a terminal to receive the pulse signal TOSCRSTB in  FIG. 1 .  
      In  FIG. 2 , since the refresh signal TEMPOSC is not oscillated at less than 15° C., the circuit is not normally operated.  
      In order to solve the above-described problem, the control unit  11  receives the pulse signal TOSCRSTB having a predetermined period which is not affected by temperature change, and the refresh signal TEMPOSC is generated in response to the pulse signal TOSCRSTB at low temperature where the temperature sensing oscillator circuit of  FIG. 1  does not operate. In this way, the conventional temperature sensing oscillator circuit requires an additional circuit to generate a pulse signal having a predetermined period which is not affected by temperature change.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to simplify configuration of a temperature sensing oscillator circuit and to improve characteristics of a refresh signal depending on temperature change.  
      In an embodiment, a temperature sensing oscillator circuit comprises a temperature sensing pulse generating unit and a pulse width regulating unit. The temperature sensing pulse generating unit changes an operating power supplied to an oscillator depending on temperature change to generate a pulse signal having a period varied depending on the temperature change. The pulse width regulating unit regulates a pulse width of a pulse signal outputted from the temperature sensing pulse generating unit to correspond to that of a refresh signal.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
       FIG. 1  is a circuit diagram of a conventional temperature sensing oscillating circuit for automatically regulating a self-refresh cycle depending on temperature change in a self-refresh circuit;  
       FIG. 2  is a simulation diagram illustrating generation of a refresh signal while temperature of the circuit of  FIG. 1  is changed;  
       FIG. 3  is a circuit diagram of a temperature sensing oscillator circuit according to an embodiment of the present invention;  
       FIG. 4  is a diagram illustrating pulse waveforms of a pulse width regulating unit of  FIG. 3 ;  
       FIG. 5  is a circuit diagram illustrating another example of a temperature sensing oscillator circuit according to an embodiment of the present invention;  
       FIG. 6  is a simulation diagram illustrating generation of a refresh signal while temperature of the circuit of  FIG. 5  is changed.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention will be described in detail with reference to the accompanying drawings.  
       FIG. 3  is a circuit diagram of a temperature sensing oscillator circuit according to an embodiment of the present invention.  
      The temperature sensing oscillator circuit of  FIG. 3  comprises a temperature sensing pulse generating unit  20 , a pulse width regulating unit  30  and a buffer unit  40 .  
      The temperature sensing pulse generating unit  20  outputs a pulse signal having a cycle varied by changing an operating power depending on temperature change. The temperature sensing pulse generating unit  20  comprises a variable power supply unit  22  and a ring oscillator  24 .  
      The variable power supply unit  22  variably supplies a operating power of the ring oscillator  24  depending on temperature change. The variable power supply unit  22  comprises a diode-connected PMOS transistor P 1  connected between a power voltage terminal VDD and the ring oscillator  24 . In other words, in an embodiment, an internal power generated from the inside of the chip is not directly supplied to the ring oscillator  24  but through the diode-connected PMOS transistor P 1 . Here, the diode-connected PMOS transistor P 1  supplies a variable operating power to the ring oscillator  24  depending on temperature change because a threshold voltage value of the diode-connected PMOS transistor P 1  is differentiated by temperature change.  
      The ring oscillator  24  receives the operating power from the variable power supply unit  22 , and outputs a pulse signal in response to a temperature sensing operating signal TEMPON. The ring oscillator  24  comprises an inverter chain including serially connected inverters IV 1 ˜IV 4  and a NAND gate ND 1 . The NAND gate ND 1  performs a NAND operation on an output signal from the inverter chain and the temperature sensing operating signal TEMPON. An output signal from the NAND gate ND 1  is applied to the pulse width regulating unit  30  and an inverter chain input terminal of the ring oscillator  24 . That is, the ring oscillator  24  outputs an oscillated output signal from the inverter chain to the pulse width regulating unit  30  when the temperature sensing operating signal TEMPON is activated to a high level. The temperature sensing operating signal TEMPON to selectively operating the temperature sensing function is constantly kept on in an embodiment of the present invention.  
      The pulse width regulating unit  30  regulates a pulse width of an output signal from the ring oscillator  24  to correspond to that of a refresh signal TEMPOSC. The pulse width regulating unit  30  comprises an inverter IV 5 , a delay unit  32  and a NOR gate NOR 1 . The inverter IV 5  inverts an output signal from the ring oscillator  24 . The delay unit  32  delays an output signal from the inverter IV 5 . The NOR gate NOR 1  performs a NOR operation on output signals from the delay unit  32  and the ring oscillator  24 .  FIG. 4  is a diagram illustrating pulse waveforms of the pulse width regulating unit  30 .  
      The buffer unit  40  buffers an output signal from the pulse width regulating unit  30 , and outputs the refresh signal TEMPOSC. The buffer unit  40  comprises inverters IV 6  and IV 7  connected serially. The above-described buffer unit  40  is included in consideration of loading to other circuit which uses an output signal from the pulse width regulating unit  30  not for generation of the refresh signal TEMPOSC but for another object.  
      Hereinafter, the operation of the above-described temperature sensing oscillator circuit is described.  
      If the temperature sensing operating signal TEMPON is turned on, the inverters IV 1 ˜IV 4  and the NAND gate ND 1  of the ring oscillator  24  receive the operating power from the variable power supply unit  22  to generate a pulse signal for the refresh signal TEMPOSC.  
      However, in common MOS transistors, a threshold voltage becomes higher if temperature becomes lower or lower if temperature becomes higher. As a result, the variable power supply unit  22  comprising the diode-connected PMOS transistor P 1  supplies a lower operating power to the ring oscillator  24  as the temperature becomes lower, and a higher operating power to the ring oscillator  24  if the temperature becomes higher.  
      As the operating power supplied from the variable power supply unit  22  becomes lower, the response speed of the inverters IV 1 ˜IV 4  and the NAND gate ND 1  in the ring oscillator  24  becomes slower gradually. As a result, the cycle of the pulse signal outputted from the ring oscillator  24  becomes longer as the temperature becomes lower.  
      On the other hand, as the operating power supplied from the variable power supply unit  22  becomes higher, the response speed of the inverters IV 1 ˜IV 4  and the NAND gate ND 1  in the ring oscillator  24  becomes faster gradually. As a result, the cycle of the pulse signal outputted from the ring oscillator  24  becomes shorter.  
      In the above-describe way, the temperature sensing pulse generating unit  20  generates a pulse signal having a variable cycle by regulating the operating power of the ring oscillator  24  depending on temperature change. Accordingly, a pulse signal stably oscillated at low temperature can be generated without a pulse signal TOSCRSTB.  
      The pulse signal outputted from the ring oscillator  24  is regulated to have a pulse width proper to the refresh signal TEMPOSC.  
       FIG. 5  is a circuit diagram illustrating another example of a temperature sensing oscillator circuit according to an embodiment of the present invention.  
      In the temperature sensing oscillator circuit of  FIG. 5 , the variable power supply unit  22  comprises a diode-connected NMOS transistor N 1  instead of the diode-connected PMOS transistor P 1 . Since the other configuration of  FIG. 5  is the same as that of  FIG. 3 , the same reference numbers are used and the detailed explanation is omitted.  
       FIG. 6  is a simulation diagram illustrating generation of the refresh signal TEMPOSC with change of temperature at −15° C., 15° C., 45° C., 70° C. and 85° C. in temperature sensing oscillator circuit of  FIG. 3  and  FIG. 5 .  
      As shown in  FIG. 6 , the refresh signal TEMPOSC is stably oscillated at less than 15° C in the temperature sensing oscillator according to an embodiment of the present invention unlike the conventional temperature sensing oscillator circuit of  FIG. 1 .  
      As discussed earlier, in a temperature sensing oscillator circuit according to an embodiment of the present invention, the configuration of the circuit is simplified by changing an operating power of a ring oscillator which is a pulse generating unit depending on temperature change, and a refresh signal is stably generated at low temperature without additional signals.  
      While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and described in detail herein. However, it should be understood that the invention is not limited to the particular forms disclosed. Rather, the invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined in the appended claims.