Patent Abstract:
A device and a method for refreshing the voltage of a circuit line that provides the capability of bringing the circuit line to a ground voltage or to a first voltage. The method provides storing the circuit line voltage in a capacitor; and controlling, by means of the stored voltage, a switch connecting the circuit line to a second voltage of absolute value greater than the first voltage, whereby the circuit line is set to the second voltage if, during the step of storing, the circuit line was at the first voltage.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to DRAMs, and more specifically to a method and a device enabling increasing the refreshment voltage of the cells in a DRAM. 
     2. Discussion of the Related Art 
     A DRAM includes memory cells in which a logic information “1” or “0” can be stored. Each memory cell includes a capacitor in which a predetermined voltage chosen from among two values is stored according to whether a 1 or a 0 is memorized in the cell. The capacitor of a memory cell can never be perfectly isolated, and the voltage on the capacitor is not steady and declines along time. After a predetermined duration, called the retention period, the voltage stored in the capacitor of a memory cell may thus be too small to be readable. To avoid loss of the information stored in each capacitor, the voltage stored in each capacitor is periodically refreshed. For this purpose, a read device periodically compares the voltage stored in each capacitor with a reference voltage, after which it charges each capacitor to one or the other of the predetermined voltages according to whether the compared voltage is greater or smaller than the reference voltage. 
     FIG. 1 schematically shows a conventional DRAM structure arranged in rows and columns. A single memory cell Mi of the memory is shown. Memory cell Mi includes a capacitor Ci having a first terminal connected to a reference voltage Vp. A second terminal of capacitor Ci is connected to a bit line BL via a switch Si. The second terminal of capacitor Ci forms an input/output terminal of memory cell Mi. The control terminal of switch Si forms a terminal of selection of memory cell Mi, and receives a selection signal WLi. Bit line BL is connected to an input terminal of a read device  2  via a switch  4 . Device  2  includes two identical inverters  6  and  8  assembled in antiparallel. Input I 6  of inverter  6 , connected to the output of inverter  8 , forms the input terminal of device  2 . The output of inverter  6  is connected to input I 8  of inverter  8 . A high supply terminal of inverters  6  and  8  is connected to a positive supply voltage Vdd via a switch  10 . Switch  10  receives a control signal RESTORE. A low supply terminal of inverters  6  and  8  is connected to a ground voltage GND via a switch  12 . Switch  12  receives a control signal SENSE. The input of inverter  8  is connected to a reference bit line BLref via a switch  14 . Switches  4  and  14  receive a same control signal PASS. Reference bit line BLref is provided to exhibit a stray capacitance identical to that of bit line BL. A reference memory cell Mref, having a structure identical to that of memory cell M, is connected to reference bit line BLref. Cell Mref includes a capacitor Cref connected to bit line BLref via a switch Sref. Capacitor Cref is identical to capacitor Ci. The terminal of selection of memory cell Mref receives a control signal WLref. A precharge circuit  16 , controlled by a signal PRA, is connected to terminals I 6  and I 8 . Precharge circuits, not shown, controlled by signal PRA, are connected to lines BL and BLref and to the input/output terminal of memory cell Mref. Control signals WLi, WLref, PASS, RESTORE, and PRA are generated by control means not shown. 
     Bit lines BL and BLref are connected to a refreshment device  18 . Device  18  includes P-channel MOS transistors  20  and  22 , having their respective drains connected to lines BL and BLref. The sources of transistors  20  and  22  are interconnected. The gate of transistor  20  is connected to the drain of transistor  22  and the gate of transistor  22  is connected to the drain of transistor  20 . A P-channel transistor  24  has its source connected to a supply voltage Vcc greater than voltage Vdd and its drain connected to the sources of transistors  20  and  22 . The gate of transistor  24  receives a control signal noBOOST. 
     FIGS. 2A through 2H illustrates the variation along time of the voltages of bit lines BL and BLref, and of signals WLi, WLref, SENSE, RESTORE, PASS, noBOOST, and PRA in a step of refreshment of memory cell Mi. At an initial time t 0 , signals WLi and WLref are at 0 and capacitors Ci and Cref of memory cells Mi and Mref are isolated from lines BL and BLref. Signal PASS is at 0 and terminals I 6  and I 8  are isolated from lines BL and BLref. Signals SENSE and RESTORE are at 0 and inverters  6  and  8  are deactivated. Signal PRA is at 1 and block  16  forces the voltages of terminals I 6  and I 8  to a voltage Vdd/2. Similarly, precharge circuits, not shown, force bit lines BL and BLref to voltage Vdd/2, and the input/output terminal of cell Mref to a reference voltage which is considered, for simplification, to be equal to Vdd/2. 
     At a time t 1 , signal PRA is brought to 0. The precharge circuits are then deactivated. 
     At a time t 2 , signals WLi, WLref, and PASS are brought to 1. Capacitors Ci and Cref are then respectively connected to terminals I 6  and I 8 . Bit line BL and terminal I 6  each exhibit a mainly capacitive predetermined impedance. From time t 2 , the charges stored in capacitor Ci distribute between capacitor Ci and the stray capacitances of line BL, of terminal I 6 , and of the gate of transistor  22 . FIG. 2 illustrates an example in which a voltage Vdd/2+ΔV is stored in capacitor Ci at a time t 2 . After time t 2 , the charges stored in capacitor Ci distribute between capacitor Ci and the stray capacitances of bit line BL, of terminal I 6 , and of the gate of transistor  22 . Because of the charge transferred to the stray capacitances, the terminal I 6  is thus brought to a voltage Vdd/2+δV smaller than voltage Vdd/2+ΔV. Terminal I 8 , connected to line BLref and to capacitor Cref, remains at voltage Vdd/2. 
     At a time t 3 , signal SENSE is brought to 1 to turn switch  12  on. The low supply terminals of inverters  6  and  8  are then connected to voltage GND. As a response to voltage Vdd/2+δV of terminal I 6 , inverter  6  forces terminal I 8  and line BLref to voltage GND. 
     At a time t 4 , signal RESTORE is brought to 1 to turn switch  10  on. Inverters  6  and  8  are then supplied by voltage Vdd, and inverter  8  forces terminal I 6  and line BL to voltage Vdd. Memory cell Mi is then recharged to voltage Vdd. Technological progress and the increase in memory circuit integration especially causes a reduction in the size of the transistors (not shown) forming inverters  6  and  8 , and a decrease in supply voltage Vdd of these transistors. Now, a memory cell refreshed with too small a voltage Vdd is rapidly discharged, that is, it soon becomes unable to provide a sufficient voltage Vdd/2+ΔV to control inverter  6  at time t 3 . Device  18  is provided to pull up the refreshment voltage of memory cell Mi. 
     At a time t 5 , signal noBOOST is brought to 0 to turn switch  24  on. Their transistors  20  and  22  must be matched so that their characteristics are identical and remain so, for example, in case of a variation in the operating temperature. In the example shown, the gate-source voltage of transistor  20  is more negative than the gate-source voltage of transistor  22  and transistor  20  becomes more conductive than transistor  22 . As a result, from time t 5 , line BL is quickly brought to voltage Vcc, which results in turning transistor  22  off. Line BLref thus remains at voltage GND. Memory cell Mi is then recharged to voltage Vcc, and the refreshment operation is over. 
     At a time t 6 , signal noBOOST is brought to 1 to make transistor  24  non-conductive and deactivate device  18 . At time t 6 , signal PASS is brought to 0, to turn off switches  4  and  14  and to isolate terminals I 6  and I 8  from lines BL and BLref, respectively. At time t 6 , signals SENSE and RESTORE are brought to 0 to turn off switches  10  and  12  and to deactivate inverters  6  and  8 . At time t 6 , signals WLi and WLref are brought to 0 to isolate capacitors Ci and Cref from lines BL and BLref. 
     At a time t 7 , precharge signal PRA is brought to 1 to control the precharge of terminals I 6  and I 8 , of lines BL and BLref, and of capacitor Cref, to prepare a next refreshment operation. 
     In the illustrated example, memory cell Mi stores before time t 2  a voltage Vdd/2+ΔV (logic “1”) greater than voltage Vdd/2 stored in reference memory cell Mref. In the case where memory cell Mi stores a voltage (logic “0”) smaller than the voltage stored in cell Mref, device  2  brings line BL to voltage GND at time t 3  and line BLref to voltage Vdd at time t 4 . At time t 5 , device  18  then brings bit line BLref to voltage Vcc and maintains line BL at voltage GND. 
     A read operation on memory cells Mi includes the refreshment operation just described. The result of the read operation is for example indicated by the state of terminal I 6  at time t 5 . For a write operation on cell Mi, a means not shown forces the state of terminal I 6  before activating device  18 , whatever the voltage stored in capacitor Ci. 
     Matched transistors  20  and  22  must have a large gate length to be able to undergo high voltages, and a large gate width, to be able to rapidly switch when switched, at time t 6 . The gate connections of transistors  20  and  22  must have the same lengths for transistors  20  and  22  to switch under the same conditions. In practice, the implantation of matched transistors  20  and  22  is particularly difficult and a significant surface area is reserved in the extension of each pair of memory cell columns to adequately arrange these transistors. A mismatching is likely to cause a switching failure and a read error. 
     Further, voltage Vdd/2+δV provided to inverter  6  from voltage Vdd/2+ΔV stored in the memory cell is all the smaller as the gate capacitance of transistor  22  is strong. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a device for refreshing bit lines of a DRAM, which enables faultless refreshment, without requiring the presence of two perfectly matched transistors. 
     An embodiment of the present invention provides a method for refreshing the voltage of a circuit line capable of being brought to a ground voltage or to a first voltage, including the successive steps of: 
     storing a line voltage in a capacitor; and 
     controlling, by means of the stored voltage, a switch connecting the line to a second voltage of absolute value greater than the first voltage, whereby the line is set to the second voltage if, during the step of storing, the line was at the first voltage. 
     According to an embodiment of the present invention, the circuit is a DRAM, the line being connected to at least one memory cell of the DRAM, and being likely to be brought to the ground voltage or to the first voltage by a read device of the memory cell. 
     An embodiment of the present invention is also directed at a circuit for refreshing the voltage of a circuit line initially brought to a ground voltage or to a first voltage, including: 
     a first switch connecting the line to a second voltage having an absolute value greater than the first voltage; 
     a capacitor having a first terminal connected to the control terminal of the first switch; 
     a second switch connecting the line to the first terminal of the capacitor; 
     a third switch connecting a second terminal of the capacitor to the line; 
     a fourth switch connecting the second terminal of the capacitor to the ground voltage; and 
     a control means for, first, turning on the second and fourth switches and turning off the third switch and, second, turning on the third switch and turning off the second and fourth switches. 
     According to an embodiment of the present invention, the first switch is a first N-channel MOS transistor having its drain and its source respectively connected to the second voltage and to the line, and having its gate connected to the first terminal of the capacitor; 
     the second switch is a second N-channel MOS transistor having its drain connected to the first terminal of the capacitor and having its source connected to the line; 
     the third switch is a third N-channel MOS transistor having its drain connected to the second terminal of the capacitor and having its source connected to the line. 
     According to an embodiment of the present invention, the fourth switch includes a fourth N-channel MOS transistor having its drain connected to the second terminal of the capacitor and having its source connected to the ground voltage. 
     According to an embodiment of the present invention, the line is connected to a plurality of memory cells of the DRAM, and is likely to be brought to the ground voltage or to the first voltage by a device for reading from the memory cell. 
     The foregoing features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1, previously described, schematically shows a device for reading from a DRAM, provided with a conventional refreshment means; 
     FIGS. 2A through 2H, previously described, illustrates the operation of the device of FIG. 1; 
     FIG. 3 schematically shows a device for reading from a DRAM provided with a refreshment device according to the present invention; and 
     FIGS. 4A through 4F illustrates the operation of the device of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention provides a refreshment device including two distinct refreshment circuits respectively connected to the bit line and to the reference bit line. The refreshment circuits operate independently from each other and they can be implanted at any location of the circuit. The same reference numbers and labels designate the same elements in FIGS. 1 and 3. Only those elements necessary to the understanding of the present invention have been shown. 
     FIG. 3 schematically shows a read device  2  connected to a memory cell Mi of a DRAM by a switch  4  and by a bit line BL. A refreshment circuit  26  according to the present invention is connected to bit line BL by a terminal  27 . Device  2  is also connected to a reference memory cell Mref by a switch  14  and a bit line BLref, a refreshment circuit  26 ′ identical to circuit  26  being connected to line BLref by a terminal  27 ′. In the example shown, switches  4  and  14  are N-channel transistors having their gates maintained at a predetermined voltage V PASS  which will be described hereafter. Device  26  includes a capacitor C having a first terminal A connected to the drain of an N-channel transistor T 1 . The source of transistor T 1  is connected to terminal  27 . A second terminal B of capacitor C is connected to the drain of a second N-channel transistor T 2 . The source of transistor T 2  is connected to terminal  27 . Terminal B is also connected to the drain of an N-channel transistor T 3 . The source of transistor T 3  is connected to ground voltage GND. An N-channel transistor T 4  has its source connected to terminal  27  and its drain connected to voltage Vcc. The gate of transistor T 4  is connected to terminal A. The gates of transistors T 1 , T 2 , T 3  respectively receive control signals COM 1 , COM 2 , and COM 3  generated by a control means  28 . The structure of means  28 , within the abilities of those skilled in the art, is not detailed. 
     FIGS. 4A through 4F illustrate the variation along time of the voltages of bit lines BL and BLref, of terminals A and B and of control signals COM 1 , COM 2 , COM 3 , PRA, WLi, and WLref upon refreshment of memory cell Mi by read device  2  and refreshment circuit  26 . Control signals SENSE and RESTORE of read device  2  have not been shown. The time scale is given as an indication only. In practice, the illustrated signals may have an aspect different from the curves of FIGS. 4A to  4 F. 
     At a time t 0 , before the beginning of the refreshment, signal COM 2  is at 0 and transistor T 2  is non-conductive, FIG.  4 D. Signals COM 1  and COM 3  are at 1 and transistors T 1  and T 3  are conductive, FIG.  4 C. Terminal A is connected to bit line BL and terminal B is connected to ground GND. The gate and the source of transistor T 4  are short-circuited. Signal PRA of FIG. 4E is at 1 so that precharge circuit  16  as well as precharge circuits not shown force terminals I 6  and I 8 , and bit lines BL and BLref to voltage Vdd/2 as shown in FIG.  4 A. Signal PRA also controls a precharge circuit not shown to force capacitor Cref of reference memory cell Mref to voltage Vdd/2. Signals WLi and WLref are at 0 as shown in FIG. 4F, so that switches Si and Sref are off and that capacitors Ci and Cref are isolated from bit lines BL and BLref. Voltage V PASS  is chosen so that transistors  4  and  14  are on as long as their drain voltage does not exceed a predetermined threshold, to avoid a voltage rejection from terminals  27 ,  27 ′ to read device  2 . In the illustrated example, V PASS  can be substantially equal to Vdd increased by the threshold voltage of transistors  4  or  14 . 
     At successive times t 1 , t 2 , t 3 , and t 4 , signals PRA, WLi, and WLref, as well as signals SENSE and RESTORE (not shown) for controlling the read device  2  are controlled to stop the precharge, compare the voltage stored in cell Mi to the voltage stored in cell Mref, then bring line BL to voltage Vdd or to voltage GND according to whether cell Mi contains a voltage greater or smaller than the voltage stored in cell Mref. In the illustrated example, line BL is brought to voltage Vdd. 
     At a time t 5 ′, signals COM 1  and COM 3  of FIG. 4C are brought to 0 to turn off switches T 1  and T 3 . Capacitor C remains charged and memorizes the voltage of line BL (Vdd in the illustrated example). 
     At a time t 6 ′, signal COM 2  of FIG. 4D is brought to 1 to turn on transistor T 2 . The voltage across capacitor C is thus applied between the gate and the source of transistor T 4 . If the voltage across capacitor C is equal to 0, transistor T 4  remains off and line BL remains at voltage GND. If the voltage across capacitor C is substantially equal to Vdd (as in the illustrated example), the voltage of terminals B and A respectively increase towards voltages Vdd and 2 Vdd. Transistor T 4  is turned on at a time t 7 ′, when the gate/source voltage of transistor T 4  exceeds its threshold voltage V T4 . Bit line BL and terminal B are then brought to voltage Vcc. The memory cell is then recharged to voltage Vcc, and the refreshment operation is over. 
     At a time t 8 ′, signals WLi and WLref of FIG. 4F are controlled to isolate capacitors Ci and Cref. 
     At a time t 9 ′, signal COM 2  is brought to 0 to turn off transistor T 2 . 
     At a time t 10 ′, signal PRA is controlled to activate the precharge and signals COM 1  and COM 3  are brought to 1 to turn on T 1  and T 3 . 
     A read operation on the memory cell is identical to the refreshment operation just described. A write operation on cell Mi is performed conventionally by forcing the voltage of terminal I 6  to bring the bit line to voltage Vdd, then by activating refreshment device  26 . 
     An advantage of the present invention is that circuits  26  and  26 ′ operate independently from each other, and that they can accordingly be implanted independently in the memory circuit. Especially, circuits  26  and  26 ′ may be arranged in spaces left free by the implantation of other elements of the memory circuit. This enables forming a memory circuit substantially smaller than a memory circuit using conventional refreshment devices such as circuit  18  of FIG. 1, although the sum of the sizes of circuits  26  and  26 ′ are of the same order as the size of circuit  18 . 
     Further, circuit  26  of FIG. 3 introduces substantially no stray capacitance on line BL, which eases the reading of the voltage stored in memory cell Mi and is an additional advantage of the present invention. 
     It has been considered, up to now, that bit line BL is connected to a column of memory cells Mi and that reference bit line BLref is connected to a reference memory cell Mref. In practice, bit lines BL and BLref are identical. Bit line BL is connected to a reference memory cell Mref′ and bit line BLref is connected to a memory cell column Mi′. Upon refreshment or reading of a memory cell Mi connected to bit line BL, memory cell Mref connected to bit line BLref is activated, as described previously, and circuit  26  refreshes cell Mi. Upon refreshment or reading of a memory cell Mi′ connected to bit line BLref, reference memory cell Mref′ connected to bit line BL is activated, and circuit  26 ′ refreshes cell Mi′. 
     The present invention has been described with a single pair of bit lines BL, BLref and a single pair of refreshment circuits  26 ,  26 ′. However, in practice, a memory circuit will include a large number of pairs of bit lines and of pairs of refreshment circuits. 
     The present invention has been described in a case where a single pair of bit lines BL, BLref is connected to a read device  2 . However, in practice, several pairs of bit lines may be selectively connected to a same read device  2  by an appropriate control of switches  4  and  14  associated with each bit line pair. 
     For simplicity, the present invention has been described without taking into account the voltage drops introduced by transistors T 1  and T 2  and switch  4  when on. In practice, transistors T 1  and T 2  and switch  4  introduce voltage drops substantially equal to their threshold voltages. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. For example, the present invention has been described in relation with positive voltages Vdd and Vcc, but those skilled in the art will easily adapt the present invention to negative voltages, especially by replacing the described N-channel MOS transistors with P-channel MOS transistors. In such a case, it is possible to precharge bit lines BL and BLref to voltage GND between two refreshment operations. It is then possible for circuit  26  to include no transistor T 3  in charge of bringing terminal B to voltage GND, if transistor T 2  is turned on during this precharge. 
     The present invention has been described in relation with, before each refreshment or read operation, a precharge of the input/output terminal of reference memory cell Mref to a voltage Vdd/2. In practice, the input/output terminal of the reference memory cell will be precharged to a predetermined voltage depending on the memory circuit. For example, this predetermined voltage may be substantially equal to half of voltage Vdd/2+ΔV stored in the memory cells memorizing a “1” upon their reading before refreshment. 
     The operation of the refreshment circuit according to the present invention has been described in relation with a specific sequencing of the control signals shown in FIGS. 4C through 4F, but those skilled in the art will easily adapt the present invention to any other sequencing of the control signals enabling similar operation of the refreshment circuit. 
     The present invention has been described in the context of a use in a DRAM circuit, but those skilled in the art will readily adapt the present invention to a use in any circuit requiring devices for pulling up a low voltage to a higher voltage, especially analog-to-digital and digital-to-analog conversion circuits. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.

Technology Classification (CPC): 6