Abstract:
Provided is a memory device capable of automatically controlling a self refresh cycle by sensing an ambient temperature, rather than setting Extended Mode Register Set (EMRS) code. The memory device includes a temperature sensing unit for generating a first voltage independent of a temperature variation and a second voltage dependent upon a temperature variation, a comparing unit for comparing the first voltage with the second voltage to provide a comparison result signal, and a self refresh signal generating unit for receiving a self refresh entry signal and generating a self refresh signal of temperature compensated cycle under the control of the comparison result signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates to semiconductor design technologies, and more particularly, to a memory device with a self refresh cycle control function. 
   DESCRIPTION OF RELATED ART 
   In general, since Dynamic Random Access Memory (DRAM) cells are arranged in a dynamic way, data destruction happens due to leakage current. Therefore, it is required to read out the data in memory cells and again recharge them to an initial charge amount according to the read data before the cell data becomes lost owing to lack thereof to the extent that it is not sensed. This recharging process of memory cells is called refresh. Besides, a self refresh implies that a semiconductor memory device of DRAM performs the refresh in a fixed cycle for itself in order to maintain the data stored in memory cells in a standby state. 
   On the other hand, the feature of leakage current is that it increases twice when the temperature rises by 10° C. In other words, the hold time of cell data decreases to ½ when the temperature rises by 10° C., and decreases to 1/32 when it rises by 50° C. 
   As described above, the leakage current is intimately associated with the temperature; and thus, the refresh cycle is affected by the temperature serving as an important factor. Namely, the self refresh cycle should be short at a relatively high temperature. 
   Therefore, Temperature Compensated Self Refresh (TCSR) is necessary to regulate the self refresh cycle by detecting an ambient temperature. 
   Conventionally, a device in which a user programs a cycle variation according to a temperature variation has been used. That is, the conventional device varies the self refresh cycle according to an address number 7 in the course of setting Extended Mode Register Set (EMRS). 
   In the conventional TCSR cycle generation device, the self refresh cycle is varied depending on the temperature set by the user. If such conventional device is used at low temperature at which data preservation time is relatively long compared to a high temperature, the self refresh cycle is set to be longer. Conversely, if such conventional device is utilized at the high temperature, the self refresh cycle is set to be shorter by EMRS code setting to make the self refresh cycle frequent. By doing so, it is possible to secure the normal operation of DRAM. 
     FIG. 1  shows a block diagram of a conventional memory device with a self refresh cycle control function. 
   First, an oscillator  10  is enabled by a self refresh entry signal SREF to generate a basic self refresh signal srefreg. Upon receipt of the basic self refresh signal srefreg, a frequency divider  20  performs a frequency division thereof, determines a divided value in response to a selection signal HTSRRES from an EMRS block  30 , and provides a self refresh signal newsrefreg having a final self refresh cycle required. The EMRS code setting is decided based on the address number 7. 
   As mentioned above, in the conventional self refresh, the TCSR is carried out by setting the EMRS code by the specific address, as required in the related spec. However, if the temperature to be used is out of the spec, the DRAM operation cannot be secured, which limits the use thereof. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a memory device capable of automatically controlling a self refresh cycle by sensing an ambient temperature, rather than setting EMRS code. 
   In accordance with one aspect of the present invention, there is provided a memory device with a self refresh cycle control function, including: a temperature sensing unit for generating a first voltage independent of a temperature variation and a second voltage dependent upon a temperature variation; a comparing unit for comparing the first voltage with the second voltage to provide a comparison result signal; and a self refresh signal generating unit for receiving a self refresh entry signal and generating a self refresh signal of temperature compensated cycle under the control of the comparison result signal. 
   In accordance with another aspect of the present invention, there is provided a memory device with a self refresh cycle control function, including: a temperature sensing unit for generating a first voltage independent of a temperature variation and a second voltage dependent upon a temperature variation; a comparing unit for comparing the first voltage with the second voltage to provide a comparison result signal; an oscillating unit for receiving and oscillating a self refresh entry signal; and a frequency dividing unit for dividing an output of the oscillating unit and providing one of a plurality of divided values as a self refresh signal in response to the comparison result signal. 
   In accordance with still another aspect of the present invention, there is provided a memory device with a self refresh cycle control function, including: a temperature sensing unit for generating a first voltage independent of a temperature variation and a second voltage dependent upon a temperature variation; a comparing unit for comparing the first voltage with the second voltage to provide a comparison result signal; an oscillating unit for receiving and oscillating a self refresh entry signal, wherein an oscillating period is controlled based on the comparison result signal; and a frequency dividing unit for dividing an output of the oscillating unit to generate a temperature compensated self refresh signal. 
   The other objectives and advantages of the invention will be understood by the following description and will also be appreciated by the embodiments of the invention more clearly. Further, the objectives and advantages of the invention will readily be seen that they can be realized by the means and its combination specified in the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the instant invention will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows a block diagram of a conventional memory device with a self refresh cycle control function; 
       FIG. 2  is a block diagram illustrating a configuration of a memory device with a self refresh cycle control function in accordance with a first embodiment of the present invention; 
       FIG. 3  illustrates a detailed circuit diagram of the temperature sensing unit shown in  FIG. 2 ; 
       FIG. 4  illustrates a detailed circuit diagram of the comparing unit depicted in  FIG. 2 ; 
       FIG. 5  exemplifies a detailed circuit diagram of the oscillating unit of  FIG. 2 ; 
       FIG. 6  provides a detailed block diagram of the frequency dividing unit shown in  FIG. 2 ; 
       FIG. 7  is a block diagram illustrating another example of the frequency dividing unit depicted in  FIG. 2 ; 
       FIG. 8  exemplifies a block diagram of a memory device with a self refresh cycle control function in accordance with a second embodiment of the present invention; 
       FIG. 9  illustrates a detailed circuit diagram of the oscillating unit presented in  FIG. 8 ; 
       FIG. 10  exemplifies a detailed block diagram of the frequency dividing unit of  FIG. 8 ; and 
       FIG. 11  illustrates a block diagram of a memory device with a self refresh cycle control function in accordance with a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, preferred embodiments of the present invention will be set forth in detail with reference to the accompanying drawings so that the invention can be readily carried out by those in the art to which the invention pertains. 
     FIG. 2  is a block diagram illustrating a configuration of a memory device with a self refresh cycle control function in accordance with a first embodiment of the present invention. Referring to  FIG. 2 , the memory device of the present invention includes a temperature sensing unit  100 , a comparing unit  200 , an oscillating unit  300  and a frequency dividing unit  400 . 
   The temperature sensing unit  100  serves to generate a second voltage Vtemp dependent upon a temperature variation and a first voltage Vbias independent of the temperature variation. As described later referring to  FIG. 3 , the temperature sensing unit  100  applies a band gap reference circuit well known in the art. 
   At the comparing unit  200 , the second voltage Vtemp is compared with the first voltage Vbias to produce a comparison result signal compare. 
   In the meantime, the oscillating unit  300  is enabled by a self refresh entry signal SREF to provide a basic self refresh signal srefreg. 
   At the frequency dividing unit  400 , the basic self refresh signal srefreg is divided in response to the comparison result signal compare serving as a selection signal of divided value to thereby generate a final refresh signal newsrefreg. 
     FIG. 3  illustrates a detailed circuit diagram of the temperature sensing unit  100  shown in  FIG. 2 . Referring to  FIG. 3 , the temperature sensing unit  100  includes a band gap reference voltage generator  110  for generating a reference voltage Vref of level independent of a process variation and temperature variation based on a junction voltage characteristic of bipolar transistors (junction voltage between the emitters and bases of Q 1  and Q 2 ) and thermal voltage characteristic (VT=kT/q), a first voltage generator  120  for providing the first voltage Vbias by using the reference voltage Vref, and a second voltage generator  130  for producing the second voltage Vtemp based on the junction voltage characteristic of bipolar transistor (junction voltage between the emitter and base of Q 2 ). 
   More specifically, the band gap reference voltage generator  110  is provided with resistors R 2  and R 1  and a bipolar transistor Q 2  for diode which are connected in serial between an output node N 1  of the reference voltage Vref and a ground voltage terminal to constitute a first current path, a resistor R 3  and a bipolar transistor Q 1  for diode which are connected in series between the output node N 1  of the reference voltage Vref and the ground voltage terminal to form a second current path, an operational amplifier op_amp 1  whose positive input terminal is coupled with a connection node of the resistors R 2  and R 1  and negative input terminal is coupled with a connection node of the resistor R 3  and the bipolar transistor Q 1 , and a PMOS transistor MP 1  whose gate takes an output of the operational amplifier op_amp 1  and source-drain path is established between a power supply voltage terminal Vdd and the output node N 1 . 
   The first voltage generator  120  is provided with a voltage divider  121  for dividing an input power supply voltage to provide the first voltage Vbias and a divided voltage Vm, an operational amplifier op_amp 2  whose negative input terminal accepts the reference voltage Vref and positive input terminal receives the divided voltage Vm, and a current source  122  for supplying the power supply voltage to the voltage divider  121  in response to an output of the operational amplifier op_amp 2 . The current source  122  is composed of a PMOS transistor MP 2  whose gate takes an output of the operational amplifier op_amp 2  and source-drain path is connected between the power supply voltage terminal Vdd and the voltage divider  121 . And the voltage divider  121  is formed with a plurality of resistors connected in series and provides the first voltage Vbias and the divided voltage Vm at any one of connection nodes of the resistors. 
   The second voltage generator  130  is equipped with resistors R 4  and R 5  connected in series between an output node N 2  of the second voltage Vtemp and a ground voltage terminal Vss, an operational amplifier op_amp 3  whose positive input terminal is coupled with a connection node of the resistors R 4  and R 5  and negative input terminal is connected to an emitter of the bipolar transistor Q 2  of the band gap reference voltage generator  110 , and a PMOS transistor MP 3  whose gate takes an output of the operational amplifier op_amp 3  and source-drain path is connected between the power supply voltage terminal and the output node N 2 . 
   Specifically, provided at the band gap reference voltage generator  110  of the temperature sensing unit  100  is the reference voltage Vref independent of the temperature variation while being insensitive to the process condition and variation of driving voltage. At the operational amplifier op_amp 1 , a certain voltage is issued to turn on the MOS transistor MP 1 . In response to the certain voltage, the MOS transistor MP 1  is turned on; and thus, a current is supplied to the resistors R 2  and R 1  and R 3 , and in turn fixed voltages are applied to the two input terminals of the operational amplifier op_amp 1 . Due to such an operation, an output voltage of the operational amplifier op_amp 1  is regulated and a turning-on rate of the MOS transistor MP 1  is varied, thereby controlling an amount of current supplied to the resistors R 2  and R 3  through the MOS transistor MP 1 . This operation continues until same voltage levels are applied to the two input terminals of the operational amplifier op_amp 1 . When the same voltage levels are applied to the two input terminals of the operational amplifier op_amp 1 , the reference voltage Vref of fixed level is inputted to a common node of the resistors R 2  and R 3 . The reference voltage Vref generated is then provided to the first voltage generator  120 , and may be represented by:
 
Vref=Vbe1+( R 2/ R 1)* VT *ln( NR 2/ R 3)  Eq. (1)
 
   In Eq. (1) denoting the reference voltage Vref, because a base-emitter voltage Vbel of the bipolar transistor has a negative coefficient of about −2 mv and VT has a positive coefficient with respect to temperature, the constant voltage Vref independent of temperature can be obtained by regulating a value of (R 2 /R 1 )*ln(NR 2 /N 3 ) and absolutizing the two coefficients. 
   Meanwhile, the second voltage generator  130  amplifies the voltage applied to the emitter of the bipolar transistor Q 2  to produce the second voltage Vtemp. The base-emitter voltage Vbel of the bipolar transistor Q 2  has a negative value of about −2.1 mV/C with respect to increase in temperature, as described above. This may be used as the second voltage Vtemp as it is; but, in such a case, a varied amount of voltage sensed according to temperature variation is very small, thereby rendering its sensing difficult. Therefore, the voltage to the emitter of the bipolar transistor Q 2  is amplified by the resistors R 4  and R 5  ratio and then outputted. At this time, the second voltage Vtemp has a lower level as the temperature becomes higher. 
   On the other hand, inputted to the first voltage generator  120  is the reference voltage Vref that is insensitive to the process condition and variation of driving voltage. The reference voltage Vref is divided at the voltage divider  121  to generate the first voltage Vbias and the divided voltage Vm. According to the spec of semiconductor memory, the standard temperature is 85° C.; and thus, if the voltage divider  121  is configured to provide the level of the second voltage Vtemp at 85° C. as the first voltage Vbias, the second voltage Vtemp and the first voltage Vbias have the same value when the temperature is 85° C. 
     FIG. 4  illustrates a detailed circuit diagram of the comparing unit  200  depicted in  FIG. 2 . Referring to  FIG. 4 , the comparing unit  200  is provided with an operational amplifier op_amp 4  whose negative input terminal is connected to the second voltage Vtemp and positive input terminal is connected to the first voltage Vbias, a first inverter INV 1  for receiving an output of the operational amplifier op_amp 4  as its input, and a second inverter INV 2  for taking an output of the first inverter INV 1  to provide a comparison result signal compare. 
   To be more specific, if the temperature is lower than 85° C., the level of the second voltage Vtemp would be higher than that of the first voltage Vbias; and therefore, an output of the comparing unit  200 , the comparison result signal compare would be logic low. On the contrary, if the temperature is higher than 85° C., the level of the second voltage Vtemp would be lower than that of the first voltage Vbias; and thus, the comparison result signal compare would be logic high. 
     FIG. 5  exemplifies a detailed circuit diagram of the oscillating unit  300  shown in  FIG. 2 . Referring to  FIG. 5 , the oscillating unit  300  is composed of an odd number of inverters INV 300 _ 1  to INV 300 _n connected in series, and a plurality of capacitors CPl to CPm, one terminal of each capacitor being connected to the ground and the other terminal being connected to at least one common node of an output terminal and an input terminal of each of the odd number of inverters. In this arrangement, the oscillating unit  300  receives a self refresh entry signal SREF and generates a signal srefreg having toggled basic refresh cycle. 
     FIG. 6  provides a detailed block diagram of the frequency dividing unit  400  shown in  FIG. 2 . Referring to  FIG. 6 , the frequency dividing unit  400  is equipped with a first divider  410  for dividing the basic self refresh signal srefreg, a second divider  420  for dividing an output signal of the first divider  410 , and a selector  430  for selecting one of output signals of the first and second dividers in response to the comparison result signal compare serving as a selection signal, and providing the selected output as a self refresh signal newsrefreg of finally temperature compensated cycle. 
   More concretely, the first divider  410  may be implemented with a ½ frequency divider, and the second divider  420  may be embodied by a single ½ frequency divider or a single ½ n  frequency divider, n being a natural number. 
     FIG. 7  is a block diagram illustrating another embodiment of the frequency dividing unit  400  shown in  FIG. 2 . As shown in  FIG. 7 , the frequency dividing unit  420   a  is configured to generate a plurality of divided signals having different divided values; and includes a plurality of ½ frequency dividers  420 _ 1  to  420 _n connected in series, and fuses  425 _ 1  to  425 _n for selecting and providing any one of outputs of the plurality of unit frequency dividers  420 _ 1  to  420 _n by fuse blowing. In addition, metal option may be employed instead of the fuses. By considering that the amount of leakage current is varied and thus refresh cycle may be diverse due to external environments, an optimal refresh signal is selected by test and then only a corresponding fuse is turned on. 
     FIG. 8  exemplifies a block diagram of a memory device with a self refresh cycle control function in accordance with a second embodiment of the present invention. Referring to  FIG. 8 , the memory device with the self refresh cycle control function includes a temperature sensing unit  500  for generating a second voltage Vtemp dependent upon a temperature variation and a first voltage Vbias independent of a temperature variation, a comparing unit  600  for comparing the second voltage Vtemp with the first voltage Vbias to provide a comparison result signal compare, an oscillating unit  700  for using the comparison result signal compare as its own capacitor enable signal and producing a self refresh signal c_srefreg corresponding to the temperature variation, and a frequency dividing unit  800  for accepting and dividing the self refresh signal c_srefreg to provide a refresh signal newsrefreg of finally temperature compensated cycle. 
   The temperature sensing unit  500  and the comparing unit  600  are substantially identical to those presented in the first embodiment as described above. Therefore, their details are omitted here; and other elements, the oscillating unit  700  and the frequency dividing unit  800  will be described below. 
     FIG. 9  illustrates a detailed circuit diagram of the oscillating unit  700  shown in  FIG. 8 . Referring to  FIG. 9 , the oscillating unit  700  is provided with an odd number of inverters INV 700 _ 1  to INV 700 _n connected in series, and a plurality of capacitors CP 700 _ 1  to CP 700 _n, in which one terminal of each capacitor receives the comparison result signal compare as a capacitor enable signal and the other terminal is connected between output terminals of each of the odd number of inverters INV 700 _ 1  to INV 700 _n. Specifically, the oscillating unit  700  uses the comparison result signal compare as the capacitor enable signal and turns on/off the plurality of capacitors CP 700 _ 1  to CP 700 _n, to thereby generate a self refresh signal c_srefreg having temperature compensated cycle. In such a configuration, if the temperature is lower than 85° C., the comparison result signal compare becomes logic low and thus the capacitors are enabled to produce a signal c_srefreg having long self refresh cycle; and if the temperature is higher than 85° C., the comparison result signal compare becomes logic high and thus specific capacitors are disabled to provide a signal c_srefreg having short self refresh cycle. 
     FIG. 10  exemplifies a detailed block diagram of the frequency dividing unit  800  shown in  FIG. 8 . The frequency dividing unit  800  in accordance with the second embodiment of the present invention may be comprised of a single ½ frequency divider or a single ½ n  frequency divider, n being a natural number. 
     FIG. 11  illustrates a block diagram of a memory device with a self refresh cycle control function in accordance with a third embodiment of the present invention. 
   As illustrated in  FIG. 11 , it is implemented in such a way that both an oscillating unit  1300  and a frequency dividing unit  1400  are controlled by a comparison result signal compare. 
   In the third embodiment of the present invention, the oscillating unit  1300  may be structured as shown in  FIG. 9  and the frequency dividing unit  1400  may be configured as shown in  FIG. 6  or  7 . Of course, the temperature sensing unit  1100  may be constructed as shown in  FIG. 3  and the comparing unit  1200  may be configured as shown in  FIG. 4 . 
   As described above, the present invention is not by EMRS code setting but provided with a temperature sensing unit that senses temperature by applying a band gap reference circuit. Accordingly, the present invention can provide a memory device with a self refresh cycle control function which is more convenient for users to use by controlling a self refresh cycle by means of sensing a specific temperature through the use of the temperature sensing unit. 
   The present application contains subject matter related to Korean patent application Nos. 2005-90913 and 2006-49122, filed with the Korean Intellectual Property Office Sep. 29, 2005 and May 31, 2006, the entire contents of which are incorporated herein by reference. 
   While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.