Abstract:
A replenish circuit for a semiconductor memory device, including a bias current generating unit adapted to generate a bias current, a frequency controllable oscillator adapted to receive the bias current and to provide an oscillating output, and a pulse generator adapted to receive the oscillating output and to generate first and second pulses as a function of the oscillating output, the second pulse being embedded in the first pulse, the first pulse causing the bias current generating unit to be connected to a power supply, and the second pulse being fed to sample-and-hold circuitry adapted to sample the bias current and hold the value thereof during the first pulse.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to semiconductor devices requiring replenishment of internal nodes voltage in circuitry that generates internal high voltages.  
       BACKGROUND OF THE INVENTION  
       [0002]     In flash memory devices, such as ones based on NROM (nitride read only memory) technology, a successful read operation should be performed with minimum latency (also known as random access). The read operation involves applying a voltage to the NROM cell gate so that enough read current is flowing in the cell. The cell gate voltage level may be larger then the available VCC supply. For example (although not necessarily), the read voltage level may be around 4 V, whereas the VCC supply may be about 2.7 V. Accordingly, a charge pump circuit is typically used to generate an internal high voltage level to generate the read voltage level.  
         [0003]     Since the charge pump operation consumes a significant amount of power, it is undesirable or impossible to keep it running during stand-by mode. Instead, the read voltage is typically maintained and floated by means of a large capacitor in the internal power supply and the charge pump may be switched off. However, the charge on the capacitor tends to leak over time. In order to solve the leakage problem, the charge pump circuitry replenishes the large capacitor of the internal power supply at a pre-defined internal frequency.  
         [0004]     One method used in the prior art to replenish the internal power supply is by generating a high-power pulse that activates the internal circuitry. The high-power pulse may be generated by means of an oscillator (e.g., a free running ring oscillator), timer or voltage sensing circuit, for example. As is known in the art, this may require a bias, either in the form of an electric current or voltage or both generated by an external circuit. However, it is problematic to control the frequency and duty cycle of the high-power pulse generation so that the internal power supply is replenished with minimum power dissipation during standby. The capacitor leakage, being the reason for the replenishment, tends to grow exponentially with temperature. This fact imposes greater challenges for solving the problems mentioned above.  
         [0005]     Some applications require a variation of the replenishment period as a function of temperature and/or of power supply voltage (usually a shorter period for high temperature and for low voltage). This is commonly accomplished in the art by introducing a temperature and/or supply voltage coefficient to the generator bias, although other schemes are possible.  
         [0006]     The replenish pulse forming circuit and some circuits that are fed by the replenish pulse signal are usually designed in such a way as to source a minimum amount of electric current from the power supply between replenish pulses (referred to as the time when the pulse is de-asserted). Maintaining low electric current may extend battery operation life in portable applications, and may reduce heat energy dissipation, thereby extending the operating life and reliability of the system.  
         [0007]      FIG. 1  shows a typical block diagram for the prior art implementation described above.  
         [0008]     A current bias source  1  generates a DC current  2 . The current source  1  generates the current as a function of a power supply voltage  3  and IC (integrated circuit) temperature  4 . The bias current from current source  1  drives a ring oscillator  5 .  
         [0009]     Ring oscillator  5  may comprise three NMOS (n-channel metal oxide semiconductor) transistors  14 ,  15  and  16 , whose gates are all connected to one another. The sources of the NMOS transistors  14 ,  15  and  16  may be grounded. The drains of the NMOS transistors  14 ,  15  and  16  are connected as inputs to inverters  17 ,  18  and  19 , respectively (any odd number of inverter may be used). The inverters are connected in series with capacitors  20 ,  21  and  22 , respectively connected to the inverter outputs. The output of inverter  19  is input to inverter  17 . The gate of NMOS transistor  14  may be connected to the gate of another NMOS transistor  12 . The drain of the NMOS transistor  12  may be connected to its gate, and its source may be grounded.  
         [0010]     NMOS transistors  12 ,  14 ,  15  and  16  form a current mirror. In a current mirror one transistor is “diode connected”, which in the case of MOS transistors means the gate terminal is connected to the drain terminal (as in NMOS transistor  12 ) and one or more other transistors (NMOS transistors  14 ,  15  and  16 ) are connected with their gate terminals to the first transistor&#39;s gate while all source terminals are also connected together to the same voltage (such as ground in this example). In this manner, a gate-source voltage is generated between the connected gate terminals and the connected source terminals, and a secondary current is generated in the drain terminals of each of the NMOS transistors  14 ,  15  and  16 .  
         [0011]     The output pulse period of ring oscillator  5  shortens in response to a rise in bias current. The oscillator output period is multiplied using a frequency divider  6  and fed to a pulse generator  7 , which drives circuitry such as the internal power supply  8 , and charges a supply capacitor  10 . All of the components  1 - 8  may be fed DC current from an external power source  9 .  
         [0012]     However, the bias generating circuit used to generate the high-power pulse must generally remain in operation continuously in order to guarantee generation of succeeding trigger signals that trigger the pulses. The bias circuit typically consumes most of the total supply current at the time when the pulse is de-asserted.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention seeks to provide a novel replenishment system, such as for circuitry that generates internal high voltages, as is described more in detail hereinbelow. The invention may be used to shut down the bias current and/or voltage bias sources in the period of time between replenish pulses. This may significantly reduce the average standby current consumption necessary for bias generation, while leaving only the trigger generator (e.g., oscillator and the like) and leakage currents as current consumers in the period between replenish pulses.  
         [0014]     There is thus provided in accordance with an embodiment of the present invention a replenish circuit for a semiconductor memory device, including a bias current generating unit adapted to generate a bias current, a frequency controllable oscillator adapted to receive the bias current and to provide an oscillating output, and a pulse generator adapted to receive the oscillating output and to generate first and second pulses as a function of the oscillating output, the second pulse being embedded in the first pulse, the first pulse causing the bias current generating unit to be connected to a power supply, and the second pulse being fed to sample-and-hold circuitry adapted to sample the bias current or voltage and hold the value thereof during the first pulse.  
         [0015]     In accordance with an embodiment of the present invention the sample-and-hold circuitry may include a diode connected NMOS (n-channel metal oxide semiconductor) transistor with its drain and gate terminals connected to an input current, a capacitor connected to a gate terminal of the NMOS transistor, and another NMOS transistor (or more) whose gate terminal is connected to the gate terminal of the diode connected NMOS transistor, mirroring current to its drain terminal.  
         [0016]     Further in accordance with an embodiment of the present invention the sample-and-hold circuitry may include a switch placed between the drain and gate terminals of the diode connected NMOS transistor.  
         [0017]     Still further in accordance with an embodiment of the present invention a grounded capacitor may be connected to the gate terminals of the diode connected NMOS transistor and the other NMOS transistor.  
         [0018]     In accordance with an embodiment of the present invention a power reset module may be connected to the frequency controllable oscillator.  
         [0019]     Further in accordance with an embodiment of the present invention the frequency controllable oscillator may include a plurality of inverters, such as an odd number of inverters.  
         [0020]     Still further in accordance with an embodiment of the present invention the pulse generator is adapted to generate the first pulse from a positive edge signal.  
         [0021]     In accordance with an embodiment of the present invention the bias current generator unit may include a positive temperature coefficient current.  
         [0022]     Further in accordance with an embodiment of the present invention the bias current generator unit may include a band-gap reference circuit.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:  
         [0024]      FIG. 1  is a simplified block diagram of a typical prior art bias generating circuit for generating high-power pulses;  
         [0025]      FIG. 2  is a simplified block diagram of a bias generating circuit for generating high-power pulses with self-refreshment, in accordance with an embodiment of the present invention; and  
         [0026]      FIG. 3  is a simplified graphical illustration of waveforms for charging a sample-and-hold circuitry of the bias generating circuit of  FIG. 2  with an initial sample pulse in accordance with a reset signal, in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0027]     Reference is now made to  FIG. 2 , which illustrates a bias generating circuit for generating high-power pulses with self-refreshment, in accordance with an embodiment of the present invention.  
         [0028]     For the sake of simplicity, the invention is described and illustrated based on the circuitry of  FIG. 1 . However, it is emphasized that the invention is not limited to this embodiment and the invention may be carried out with other bias generating circuitry and other pulse generating circuitry.  
         [0029]     A bias current generating unit, referred to as a current bias source  1  generates a DC current  2 , and may be powered by an external power source  9 . As is described in detail hereinbelow, the current bias source  1  may be disconnected and consume only negligible leakage current from the power source  9  throughout the period between successive pulses.  
         [0030]     The current source  1  may generate the current as a function of a power supply voltage (V)  3  and IC temperature (T)  4 . The current source  1  may include, without limitation, a positive temperature coefficient current. The current source  1  may comprise, without limitation, a band-gap reference circuit. The bias current from current source  1  drives a trigger generator  30 , which controls the frequency of pulse generation of a pulse generator  7  by feeding thereto an oscillating output. Trigger generator  30  is also referred to as a frequency controllable oscillator  30 . Sample-and-hold circuitry  45  may be implemented with trigger generator  30 , as described hereinbelow.  
         [0031]     The output pulse period of the trigger generator  30  may shorten in response to a rise in bias current. The output period of the trigger generator  30  may be multiplied using a frequency divider  6  and fed to pulse generator  7 , which drives circuitry such as the internal power supply  8 , and charges a supply capacitor  10 . Components of the bias generating circuit may be fed DC current from the external power source  9 .  
         [0032]     The trigger generator  30  may comprise, without limitation, three NMOS transistors  14 ,  15  and  16 , whose gates are all connected to one another. The sources of the NMOS transistors  14 ,  15  and  16  may be grounded. The drains of the NMOS transistors  14 ,  15  and  16  are connected as inputs to inverters  17 ,  18  and  19 , respectively. The inverters are connected in series with capacitors  20 ,  21  and  22 , respectively connected to the inverter outputs. The output of inverter  19  is input to inverter  17 . The gate of NMOS transistor  14  may be connected to the gate of another NMOS transistor  12 . The drain of the NMOS transistor  12  may be connected to its gate, and its source may be grounded. The gate of NMOS transistor  14  may be connected to the gate of another NMOS transistor  12 . The drain of the NMOS transistor  12  may be connected to its gate, and its source may be grounded. NMOS transistors  12 ,  14 ,  15  and  16  form a current mirror.  
         [0033]     The sample-and-hold circuitry  45  may be constructed in a variety of manners. In accordance with one non-limiting embodiment of the present invention, a portion of the sample-and-hold circuitry  45  may comprise a switch  32  placed between the drain and gate of the NMOS transistor  12 , thereby selectively disconnecting the connection between the drain and gate of the NMOS transistor  12 . One of the terminals of switch  32  may be connected to the drain side of NMOS transistor  12 , and another terminal of switch  32  may be connected to a node  34  being the gate of NMOS transistors  12 , 14 , 15  &amp;  16 . A grounded capacitor  36  may be connected to node  34 . In addition, another switch  38  may be connected between the output of the inverter  19  (after capacitor  22 ) and the input of inverter  17 . Switch  38  may selectively stop the oscillator during reset. The bias source  1  is controlled via a NOR gate  42  and to switch  32  via a NOR gate  44 . The NOR gates  42  and  44  share a common input from the power reset module  40 .  
         [0034]     The pulse generator  7  may generate two activation pulses, wherein one pulse may be embedded in the other pulse. For example, pulse generator  7  may generate a first pulse  46  from a first signal (e.g., a positive edge signal), and may generate a second pulse  48  bounded in the first pulse  46 . The first pulse  46  is input to the NOR gate  42  and controls the bias activation, whereas the second pulse  48  is input to the NOR gate  44  and controls the sample and hold.  
         [0035]     It is noted that the invention is not limited to the above-described and illustrated sample-and-hold circuitry  45 , and the skilled artisan will readily recognize that other sample-and-hold circuitry may be used for bipolar and other semiconductor technologies.  
         [0036]     Whenever pulse generator  7  generates first pulse  46 , the current bias source  1  is reconnected to the power supply  9  as the case is with other refreshed circuits. Following a delay intended to allow the bias output sufficient time to stabilize, the second pulse  48  is generated by pulse generator  7  and fed to the sample-and-hold circuit  45 , the purpose of which is to sample the bias and hold its value throughout the rest of the self-refresh cycle.  
         [0037]     Capacitor  36  remains charged to the bias voltage and the secondary transistors (i.e., NMOS transistors  14 ,  15  and  16 ) receive the same gate-source voltage and generate the same drain currents as in the prior art circuitry of  FIG. 1 , even without being connected to the current bias source  1 .  
         [0038]     Capacitor  36  may require a periodic refresh of the stored charge in order to compensate for charge lost through leakage currents and in order for the bias source to implement variations in bias current due to variations in temperature and supply voltage, if necessary. In the case of a voltage bias, the bias may be routed through a switch or pass gate (not shown) followed by a parallel capacitor (not shown) which will hold the voltage whenever the switch is open. This scheme may not be suitable if the voltage bias must also supply current, because the capacitor may discharge rapidly.  
         [0039]     In the illustrated embodiment, it may be necessary for an initial sample pulse to charge the sample-and-hold circuitry  45 , otherwise the trigger generator  30  may never generate the first self-refresh and the sample-and-hold circuitry  45  may never sample the bias. This requirement can be easily implemented by choosing to activate the trigger generator  30  only after a given reset signal reset_b turns off, and using the same reset signal reset_b for the initial sampling of the capacitor  36 .  FIG. 3  graphically illustrates the waveforms for charging the sample-and-hold circuitry  45  with the initial sample pulse in accordance with the reset signal reset_b.  
         [0040]     It is appreciated that various features of the invention which are, for clarity, described in the contexts of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.