Patent Document

FIELD OF THE INVENTION  
         [0001]    The present invention relates to a high voltage switching device. The high voltage switching device may be applied to non-volatile memories, such as a FAMOS type non-volatile memory as well as EPROM, EEPROM and Flash EPROM-type non-volatile memories.  
           [0002]    The above mentioned memories include connection lines connected to rows of memory cells. A high voltage or a low voltage must be provided to these rows of memory cells. The present invention intends to accelerates the discharge rate of the connection lines from the high voltage to the low voltage. More particularly, the invention is concerned with output lines controlled by the switching device that are capacitive and are capable of conducting current.  
         BACKGROUND OF THE INVENTION  
         [0003]    In an exemplary application to a FAMOS type non-volatile memory, connection lines will typically be connected to rows of memory cells that contact the sources of FAMOS transistors within each memory cell. In this type of application, the switching device, in addition to the usual translator type switch, must have an additional switching transistor. First, the switching transistor initially helps the switch to discharge the output line(s) from the high voltage level to a low voltage level, which is typically the logic supply voltage Vcc. Second, the switching transistor must hold the low voltage level on these lines. Thus, in the latter case, this switching transistor provides the current needed to hold the voltage level on the line.  
           [0004]    In the exemplary application to the rows of the FAMOS memory cells, the rows must be held at the logic supply voltage level Vcc during the read operation. In an exemplary application to Flash EPROMs, these output lines will be the rows connected to the control gates of the floating gate transistors. The switching transistor then holds these lines at the logic supply voltage level during a read operation, and at a high voltage level during a programming operation. In an application of this kind, a device for drawing the voltage to zero must be provided to draw the unselected rows to zero.  
           [0005]    [0005]FIG. 1 shows a prior art MOS technology switching device applied to a row of FAMOS type memory cells. The device is made, in the example, with N-type and P-type MOS transistors. This switching device conventionally, as explained above, comprises a translator type switch and an additional switching transistor.  
           [0006]    The switching device has two switching arms B 1  and B 2 . The two switching arms have a similar structure. The switching arm B 1  has a load transistor M 10  connected to the high voltage HV, a switching transistor M 13  connected to ground Gnd and receiving at its gate a logic control signal CTRL, and a cascode stage connected between these two transistors. In the example, the cascode stage has two cascode transistors M 11  and M 12 . In practice, it has at least one of them. The connection point N 10  between the load transistor M 10  and the cascode transistor M 11  of an arm is applied as a signal to activate the load transistor of the other arm.  
           [0007]    Similarly, the arm B 2  comprises a load transistor M 20  connected to the high voltage HV, a switching transistor M 23  connected to ground Gnd and receiving at its gate the reverse control signal CTRL, and a cascode stage connected between these two transistors M 20  and M 23 . In the example, the cascode stage has two cascode transistors M 21  and M 22 , biased by reference voltages V REFP  and V REFN  given by a generation circuit that is not shown. The transistors M 20  and M 21  are P-type MOS transistors, and the MOS transistors M 22  and M 23  are N-type MOS transistors. The connection point N 20  between the load transistor M 20  and the cascode transistor M 21  of an arm is applied as a signal to activate the load transistor M 10  of the other arm.  
           [0008]    In the example, it is the node N 10  of the first arm B 1  that gives the output signal of the switch. This node N 10  is therefore connected to an output line L 1 , which is a capacitive line. In a well known way, and without being necessary to provide a detailed description on the operation of the switch, the node N 10  has a level corresponding to the high voltage HV when the control signal CTRL is at the logic level  1  and while the load transistor M 13  is off. The node N 10  has a logic supply voltage level Vcc when the control signal CTRL is at the logic level  0  and while the load transistor M 13  is off.  
           [0009]    In the example, the output line L 1  controlled by the switch corresponds to a row of FAMOS cells, which contacts all the sources of the FAMOS transistor in the row of cells. FIG. 1 shows only one of these cells, which comprises a FAMOS transistor and an associated selection transistor. The cell sources are connected to a corresponding bit line through which the cell state is read while the row is taken to the logic supply voltage level Vcc. Furthermore, cell programming is obtained while the associated row is taken to the high voltage level  1 .  
           [0010]    In the read mode, if the cell is programmed, the row potential tends to drop. It is therefore necessary to plan for the switching device to maintain the row at the read potential, namely the logic supply voltage level Vcc. This function is fulfilled by the additional switching transistor M 30 , connected between the logic supply voltage level Vcc and the output line L 1 .  
           [0011]    Furthermore, when the line L 1  is at the high voltage and the switch trips into the other state, this switching transistor provides knowledge that the node N 10  has gone to the logic supply voltage level Vcc. This provides the discharge current of the capacitive line L 1 , and brings this line from the high voltage level to the lower logic supply voltage level Vcc. In practice, this transistor is controlled by the connection node between the two cascode transistors of the arm providing the output signal of the switch. In the example, it is the node N 11  of the arm B 1 .  
           [0012]    If the switch comprises only one cascode transistor per arm, the connection node used is the one between the single cascode transistor and the switching transistor of the arm giving the output signal. When the node N 10  is at the high voltage level, the node N 11  is also at the high voltage level HV. When the node N 10  is at the logic supply voltage level Vcc, the node N 11  is drawn to zero.  
           [0013]    The additional switching transistor M 30  is subjected to repeated electrical stresses in the device. If it is assumed that the node N 10  and therefore the output line L 1  are at the high voltage level HV, the gate of the switching transistor M 30  connected to the node N 11  is also at the high voltage level. This turns the switching transistor M 30  off. If the switch trips, the node N 10  goes to Vcc. The switching transistor M 30  absorbs the discharge current that takes the capacitive line L 1  from the high voltage level HV to the low voltage level Vcc.  
           [0014]    However, this discharge is slow due to the capacitance of the line L 1 . The node N 11  of the switch swiftly goes to a zero voltage level. Thus, there is a very high potential difference between the gate and the electrode of the transistor M 30  connected to the line L 1  during a part of the discharge time of the line L 1 .  
         SUMMARY OF THE INVENTION  
         [0015]    An object of the invention is to resolve the problem of electrical stresses on the switching transistor M 30 . The invention is based upon keeping the potential difference between the gate of the transistor M 30  and the output line constant. In a high voltage level switching device with a translator type switch, a low voltage level switching transistor is provided. This transistor is activated by an output signal of the switch. This output signal is controlled by a circuit used to set up a control loop between the drop in the gate voltage level of the switching transistor, and the drop in the voltage level of the output line controlled by the switch.  
           [0016]    The invention therefore relates to a MOS technology switching device comprising a translator type switch to switch a high voltage level to at least one capacitive type output line, and a transistor for switching a low voltage level to the output line. The switching transistor may be controlled by a signal given by the switch. The signal level may be at the high voltage level to turn the transistor off when the switch applies the high voltage level at the output line. Alternately, the signal level may be at ground level to turn the transistor on and take the output line to the low voltage level.  
           [0017]    The switching device furthermore comprises a control circuit to control the drop in the voltage level of the control signal of the switching transistor from the high voltage level to the ground voltage level. The control circuit may be looped with the voltage drop level on the output line from the high voltage level to the low voltage level.  
           [0018]    In one embodiment, the connection node between the load transistor and the cascode transistor of an arm is connected to the output line. In another embodiment, the switch comprises an additional load transistor connected between the high voltage and the output line, and is controlled by the same control signal as the load transistor of one of the switching arms.  
           [0019]    In yet another embodiment, the switch furthermore comprises a transistor mounted as a resistor between the two switching arms. The connection point of the transistor in both switching arms may be the connection point between the load transistor and the cascode transistor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    Other features and advantages of the invention shall be described in detail in the following description, given by way of an indication that in no way restricts the scope of the invention, with reference to the appended drawings of which:  
         [0021]    [0021]FIG. 1 is a schematic diagram of a device for switching a high voltage to a capacitive output line according to the prior art;  
         [0022]    [0022]FIG. 2 is a schematic diagram of one embodiment of a switching device according to the invention; and  
         [0023]    [0023]FIG. 3 is a schematic diagram of another embodiment of the switching device according to the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    [0024]FIG. 2 shows a MOS technology switching device according to a first embodiment of the invention. The elements common to FIGS. 1 and 2 have the same references. All the transistors are MOS transistors.  
         [0025]    According to the invention, in addition to the elements described above, the switching device comprises a circuit  4  to control the gate voltage of the transistor M 30 . The control circuit  4  comprises a follower transistor M 40  connected between the cascode transistors M 11  and M 12  of the arm B 1 , which provides the output signal that controls the output line L 1 . Had there been only one cascode transistor, M 11  for example, the follower transistor M 40  would be connected in series between the cascode transistor M 11  and the switching transistor M 13  of the arm B 1 .  
         [0026]    With the switching device comprising at least one cascode transistor in each arm, the follower transistor in the arm B 1  associated with the output line Ll is connected in series with the cascode transistor M 11  of the arm B 1 . The follower transistor is between the cascode transistor M 11  and the switching transistor M 13  of the arm B 1 .  
         [0027]    The transistor M 40  is controlled by a voltage translator circuit  40  for applying a voltage Vbias 2  to the gate of the transistor M 40 . The voltage Vbias 2  may be obtained by translating the logic supply voltage level Vcc to the line L 1  voltage level. The line voltage level is referenced VL and the threshold voltage of a MOS transistor is referenced VT.  
         [0028]    The starting point is where VL=HV and the node N 10  may be at the same voltage level. At this time, the gate voltage level of the transistor M 40 , Vbias 2 , equals VL−Vcc, that is HV−Vcc. Since the switching transistor is not on, the node N 11  is at HV like N 10 . The switch trips and the signal CTRL goes from logic a 1 to a 0 and the switching transistor M 13  turns on. The follower effect is operational, and the level of the voltage at the node  11  is imposed by the gate of the transistor M 40 , which is equal to Vbias 2 +VT.  
         [0029]    Thus, while the line Li has not had the time to discharge, the voltage level at the line L 1  is still at HV. But, the level at the gate of the transistor M 30  does not switch over to zero, as in the prior art. Rather it goes to HV−Vcc. The difference in potential between the gate and the output line is equal to Vcc−VT. Consequently, the transistor M 30  is not subjected to stress and the line L 1  is gradually discharged, mainly through the transistor M 30 .  
         [0030]    As the line voltage VL diminishes, the gate voltage diminishes too. Thus, the potential difference between the gate of the transistor M 30  and the line L 1  to which it is connected remains constant. When the line L 1  reaches the voltage level Vcc, the voltage level at the gate becomes equal to VT. The role of the transistor M 30  is inverted and becomes a current generator for the line L 1 , to hold the voltage level of the line L 1  at the logic supply voltage level.  
         [0031]    It will be noted that the switching device according to the invention is applicable for switching a high voltage level or a low voltage level V 1  to the output line L 1 , which cannot have the logic supply voltage level, but also to any bias voltage level defined as a function of the application. In this case, the control circuit  4 , according to the invention, applies HV−V 1  to the gate of the transistor M 30  (when the switching transistor M 13  is on) to maintain the potential difference between the gate of the transistor M 30  and the output line L 1  at the voltage level V 1 −VT.  
         [0032]    The voltage translator circuit has two arms. The first arm has a first transistor M 41  whose gate and drain are connected together at a bias node N 40  and a bias voltage level Vbias 1  given by a first assembly of diode-mounted transistors connected in series between the supply voltage Vcc (V 1 ) and the bias node N 40 . In the example, this is illustrated by a single transistor M 42 . The second arm has a second transistor M 43  mounted as a current mirror with respect to the first transistor. Its gate is connected to the gate of the transistor M 41  to control an identical current in both arms. The transistors M 41  and M 43  are identical.  
         [0033]    A second assembly of diode-mounted transistors is connected in series between the output line L 1  and the second transistor M 43 . The connection node N 41  between the second assembly and the transistor M 43  gives the gate control signal of the follower transistor M 40 . The second series assembly comprises, in the example, a first transistor M 44  identical to the transistor M 42  of the other arm, and a transistor M 45  whose dimensions are defined to obtain a voltage level drop between the line Ll and the node N 41  to the desired value in the translation, namely Vcc (V 1 ). This is determined by knowing, because of the current mirror structure, that the drop in the voltage level at the terminals of the transistor M 44  is equal to the drop in the voltage level at the terminals of the transistor M 42 , namely Vcc−Vbias 1 .  
         [0034]    If the first assembly linked to the first arm of the voltage translator circuit comprises more than one transistor, for example, two transistors, the second assembly has the same number of transistors in addition to at least one additional transistor to make the desired adjustment in the voltage level drop. Thus, a voltage level Vbias 2  is obtained at the gate of the transistor M 40 . The voltage level Vbias 2  equals the line L 1  voltage level VL minus the logic supply voltage level Vcc (or at least V 1 ).  
         [0035]    In FIG. 2, the switch comprises an additional transistor M 50 , connected between the nodes N 10  and N 20 . The transistor M 50  is controlled at its gate by a reference voltage level given by a reference source (not shown) so that it is on. It then works like a resistor, enabling the diagonal bias of the cascode stage. At a given time and as a function of the logic control signal CTRL, the load transistor of one arm and the cascode and switching transistors of the other arm are on.  
         [0036]    The transistor M 50  lets through the current along the diagonal going in the direction from the load transistor to the switching transistor, with these two transistors being on. For the transistor M 50  not to shunt excessive current, a reference voltage level on the order of the high voltage level minus a transistor threshold voltage level is applied to the transistor M 50  gate. The transistor mounted as a resistor between the two arms favors the biasing of the cascode stage transistors. This improves the function of the switch.  
         [0037]    [0037]FIG. 3 shows another embodiment of the invention. In this embodiment, the switch comprises an additional load transistor M 14  controlled by the same control signal as the load transistor of the arm controlling the output line, namely through the signal at the node N 11 . The transistor M 14  is connected between the high voltage HV and the output line L 1 . Thus, the output signal is given by the transistor M 14  and no longer by the node N 11 .  
         [0038]    This separates the pure switching function from the function of supplying a charging current at the high voltage level, namely the power function of the switch related to the capacitive nature of the output line L 1 . Thus, the tripping speed of the switch is improved. In this way, the load transistors M 10  and M 20  of the switching function are identical. The load transistor M 14  of the power function is far bigger.  
         [0039]    It is noted that, in this embodiment, the discharging of the output line L 1  from the voltage level HV to the lower voltage level Vcc (V 1 ) is entirely performed by the transistor M 30 . In the other embodiment, as illustrated in FIG. 2, the transistor M 13  of the output arm of the switch shared this function with the transistor M 30 .  
         [0040]    The high voltage level switching device, according to the invention, may be used in any application with a capacitive line capable of switching current. Apart from the application to the rows of FAMOS memory cells or flash EPROM cells, it may be applied to other types of non-volatile memory cells.

Technology Category: 5