Low Power Voltage Clamp Circuit

This invention relates to the field of integrated circuit voltage clamps and more specifically to a voltage clamping circuit which is adapted for use with CMOS electrically erasable read-only memory (EEPROM) devices.

EEPROM devices are well known memory devices which are used for non-volatile storage of data. CMOS EEPROM devices are particularly adapted for use where power consumption is critical. For example, in battery powered applications, as the power consumption of a device decreases, its usefulness increases.

During operation, EEPROM devices generate a plurality of signals having a variety of voltage levels. One type of EEPROM device, referred to as a "thick-oxide" EEPROM incorporates a three layer polysilicon structure as the individual bit memory element. The three layers of polysilicon are separated from each other by a layer of oxide so that the second layer is electrically insulated from the other two. Data is retained on the second layer, which is sometimes referred to as the "floating gate." Data is stored on the floating gate by placing a high programming voltage (in the range of 25 volts) on a programming electrode so that electrons tunnel across the insulating oxide layer from the programming electrode and are trapped on the floating gate. Data is erased from the floating gate by placing a sufficient voltage across the floating gate to force the electrons on the floating gate to tunnel across a second oxide layer from the floating gate to an erase electrode.

Virtually all memory devices consist of a plurality of bit memory cells organized in an array of columns and rows. Individual cells are selected by decoder circuitry which generates the appropriate signals to address the proper row and column to access a desired cell or group of cells. The control lines associated with particular rows and columns of the memory are typically referred to as select lines.

In CMOS EEPROM devices, it is desirable to control the voltage on the select lines as closely as possible. Since the memory cells employ a "floating gate" for memory retention, it is essential that known voltages be present on the other portions of the cell so that the magnitude of the charge on the floating gate can be referenced to some known voltage. Therefore, in an EEPROM device, some means are required to regulate or limit the voltage on the memory cell select lines. This problem is complicated by the fact that EEPROM devices present largely capacitive loads and abnormal voltages may be generated due to voltage overshoot during the operation of the device.

This problem has been addressed in prior devices by coupling voltage clamping circuits to the select lines (among others) in EEPROM devices to regulate critical voltages for predictable EEPROM operation. Prior voltage clamping circuits have relied on complicated circuit topologies using comparators and other components which consume a relatively large amount of current. It can be shown that the amount of current required by a voltage clamping circuit is directly related to the number of devices used in the circuit as well as the method used to sense the voltage to be clamped. For example, in one prior voltage clamping technique, a string of series connected diodes, operating above device threshold, is used to clamp the regulated voltage. Thus, this clamping circuit consumes current by maintaining the string of diodes in a forward biased condition. The present invention overcomes high current consumption by providing a voltage sensing circuit with negative feedback which supplies current to the diode string in levels which correspond to sub-threshold operating regions of the voltage clamping diodes, thus providing a substantial reduction in current consumption over prior devices. In addition, the present voltage clamping circuit achieves this result with substantially fewer components than prior clamp circuits, thus providing a substantial reduction in current drain and a substantial improvement in reliability over prior voltage clamping circuits.

Clamping a negative voltge is also useful in CMOS EEPROM devices which require negative reference voltages for accurate cell programming. The present invention provides a voltage clamping circuit wherein a negative reference voltage may be generated without altering the negative power source. Since the generation of a negative voltage typically involves a voltage regulator which consumes current, it is desirable to minimize the current demands on the negative power source. The present invention therefore provides a stable negative reference voltage while requiring an extremely small amount of current from the negative power source. It also enables operation with supply voltages which produce large variations in the negative voltage generated by the on-chip V.sub.BB generator.

Briefly described, the present invention contemplates a method and means to clamp positive and/or negative voltages while minimizing current drain in the voltage clamping circuit. The present invention comprises a voltage clamping circuit including a voltage sensing and clamping means which comprises a chain of series connected diodes disposed between the output of the voltage clamping circuit and a ground reference terminal. As a voltage is developed at said output, current is generated in the series connected diodes which is mirrored in a control circuit means. Current generating means are coupled to said control circuit means and said voltage sensing means for generating a current inversely proportional to said mirrored current and for coupling this generated current to said voltage sensing means. This generated current in the voltage sensing means thereby regulates the voltage at the output terminal at a predetermined level corresponding to the sum of the threshold voltage drops generated across the series connected diodes in the voltage sensing means. The present voltage clamping means consumes only enough current to develop a threshold voltage across each of the diodes in the voltage sensing means.

Accordingly, it is an object of the present invention to provide a voltage clamping circuit which is adapted to clamp either positive or negative voltages.

It is another object of the present invention to substantially reduce current drain in a voltage clamping circuit.

It is still another object of the present invention to provide a voltge clamping circuit which provides improved performance in a voltage clamping circuit with substantially fewer components than prior voltage clamps.

The present invention may be fully understood through the description below and the accompanying drawing in which:

FIG. 1 is a schematic diagram of the preferred embodiment of the present invention.

FIG. 2 is a schematic diagram of a second embodiment of the present invention adapted for clamping positive and negative voltages.

FIG. 3 is a timing diagram which exemplifies the operation of the circuits of FIGS. 1 and 2.

The present invention provides a voltage clamp circuit having a stable and accurate clamping voltage and yet requires minimal current and circuit area. Referring now to FIG. 1, the preferred embodiment of the present invention is shown in schematic form. As used in the description below, V.sub.CC refers to a positive power supply voltage, V.sub.SS refers to a ground reference voltage and V.sub.BB refers to a negative power supply voltage. The circuit 100 includes a current generating means 102 including series connected transistors 104, 106, and 108. Transistor 104 is a p-type MOS enhancement mode transistor disposed with its source coupled to V.sub.CC terminal 110 and its gate coupled in common with the gate of enhancement mode transistor 108. Enhancement mode transistor 106 is disposed in series between transistors 104 and 108 with its source coupled to the drain of transistor 104 and its drain coupled to the source of transistor 108. The drain of transistor 108 is coupled to V.sub.SS terminal 112.

A voltage detecting or sensing and clamping means 115 includes a chain of series connected diodes comprising diodes 114, 116, 118 and 120. Specifically, diode 114 comprises an enhancement mode transistor disposed with its gate and source coupled in common wherein this common connection is coupled to the common drain/source connection of transistors 106 and 108. This common connection is also coupled to output terminal 128. Transistor 116 is disposed with its gate and source coupled in common wherein this common connection is coupled to the drain of transistor 106. Transistor 118 is disposed with its gate and source coupled together wherein this common connection is coupled to the drain of transistor 116. Transistor 120 is disposed with its gate and source coupled together wherein this common connection is coupled to the drain of transistor 118. The drain of transistor 120 is coupled to V.sub.SS terminal 112. Feedback is provided by transistors 120, 124 and 126. Enhancement mode transistor 124 is disposed with its gate coupled to the common drain/gate-source connection of transistors 118 and 120. Transistor 124 forms a current mirror with respect to transistor 120. The drain of transistor 124 is coupled to V.sub.SS terminal 112. Transistor 126 is disposed with its gate coupled to the common gate/source connection of transistor 114. The source of transistor 126 is coupled to V.sub.CC terminal 110 and the source of transistor 104. The drain of transistor 126 is coupled to the source of transistor 124. The gate of transistor 106 is coupled to the common drain/source connection of transistors 126 and 124, which is also identified as node 130.

In operation, in an initial state of circuit 100, the input signal coupled to circuit 100 at the gate of transistor 108 is in a "HIGH" state, e.g. it is held at the voltage of V.sub.CC. In this state, the voltage coupled to the output terminal 128 of circuit 100 is zero volts (the V.sub.SS voltage) because transistor 108 is turned "ON" by the input "HIGH" state. Since terminal 128 is coupled to the voltage detecting means 115, there is no voltage drop across diodes 114, 116, 118 . and 120, so no current flows in these diodes with zero volts on terminal 128, the gate of transistor 126 is also at zero volts, thereby turning this enhancement transistor "ON", which pulls node 130 to V.sub.CC volts and causes transistor 106 to be "ON". Since transistor 104 is held "OFF" by the "HIGH" state of the input signal, no current flows through transistor 106 in this initial state. Thus, the circuit 100 consumes virtually no current in this initial state. As the input signal coupled to the gate of transistor 108 changes from "HIGH to "LOW", circuit 100 functions to cause output node 128 to be clamped to a predetermined voltage. As the gate of transistor 108 is driven low to zero volts, transistor 108 is driven out of conduction. Transistor 104 turns "ON", and since transistor 106 is already "ON", the voltage on output node 128 rises toward V.sub.CC. In the embodiment shown, when the voltage at node 128 reaches four diode voltage drops above V.sub.SS, current I1 will begin to flow in the series diode chain 115. Current I1 is mirrored by transistors 120 and 124 which acts as a control circuit means with transistor 126 for generating a corresponding current I2. Since the voltage at the gate of transistor 126 is rising, transistor 126 begins to turn "OFF" and node 130 is pulled toward V.sub.SS. Since node 130 is fed back to the gate of transistor 106, this voltage forces transistor 106 out of conduction, thus limiting the current coupled through transistor 106 to the series diode chain 115. Thus, through negative feedback, the current passing through transistors 104 and 106 is only the current I1 in the diode chain 115 required for I1 to mirror in transistor 124 sufficient current I2 to maintain the gain of transistor 106 at a point where a desired clamping voltage is developed across the series connected diode chain 115. In the preferred practice of the present invention, with the input signal "LOW" the series connected diode chain operates in a threshold region wherein an extremely small amount of current is required. The clamping voltage level produced by circuit 100 is controlled by the number of diodes in the series diode chain 115. That is, increasing the number of diodes increases the clamping voltage of the circuit 100. Varying the gain of the feedback transistors also can be used to control the clamping level.

As previously mentioned, when the Input signal coupled to the circuit 100 is high, the circuit 100 is completely disabled and consumes zero current. When the Input signal coupled to the circuit 100 is low (the clamping mode) current I1 is typically in the range of 1-microamperes and I2 is typically in the range of 3-microamperes, depending on the gain of transistors 124 and 126. In the preferred practice of the present invention it is desirable for transistor 126 to have relatively low gain. In addition, the gate voltages of transistors 124 and 126 are separated by the voltage generated by the diode chain 115 less one threshold voltage. This enables better current control by transistors 124 and 126 and minimizes the resultant current I1, generated in the steady state clamping condition.

Note also that p-channel transistors may also be used to form diode chain 115, although diode 120 would still need to be n-type. Various modifications to the cicuit would be necessary as would be apparent to those skilled in the art if p-channel devices are used.

Referring now to FIG. 2, the circuit 200 includes means for clamping positive and/or negative voltages and provides the same current saving advantages as the circuit 100. The Input signal coupled to the circuit 200 is first fed through an inverter 222 to the gate of a p-channel enhancement mode transistor 204. The source of transistor 204 is coupled to VCC terminal 215 and the drain of transistor 204 is coupled to the source of an n-channel enhancement mode transistor 206. The drain of transistor 206 is connected to the source of enhancement mode transistor 208 and this node is also defined as the circuit 200 Output node or terminal 244. The drain of transistor 208 is connected to the source of enhancement mode transistor 210. The drain of transistor 210 is coupled to V.sub.BB terminal 217. The output of inverter 222 is also coupled the source of a p-channel enhancement transistor 225. The gate of transistor 225 is coupled to V.sub.SS terminal 219 and the drain of transistor 225 is coupled to the gate of transistor 210. A current source 227, is disposed between the gate of transistor 210 and VBB terminal 215.

The positive clamping portion of the circuit 200 includes series connected diode chain 213 comprising enhancement mode transistors 214, 216, 218 and 220. Specifically, the gate and source of transistor 214 are coupled together and the drain of transistor 214 is coupled to the gate and source of transistor 216. The drain of transistor 216 is coupled to the gate and source of transistor 218. The drain of transistor 218 is coupled to the gate and source of transistor 220. The drain of transistor 220 is coupled to V.sub.SS terminal 219. A p-type enhancement mode transistor 224 is disposed with its source coupled to V.sub.CC terminal 215 and its gate coupled to the source drain connection of transistor 214. An n-channel enhancement mode transistor 226 is disposed with its gate coupled to the gate/source connection of transistor 220, its source coupled to the drain of transistor 224 and its drain coupled to V.sub.SS terminal 219.

The negative clamping portion of the circuit 200 includes series connected diode chain 233 comprising p-type enhancement node transistors 230, 232, 234 and 236. Specifically, the gate and drain of transistor 230 are coupled together and the drain of transistor 230 is coupled to the source of transistor 232. The source of transistor 230 is coupled to VSS terminal 219. The gate and drain of transistor 232 are coupled to the source of transistor 234. The gate and drain of transistor 234 are coupled to the source of transistor 236. The gate and drain of transistor 236 are coupled together. An enhancement mode transistor 240 is disposed with its source coupled to VSS terminal 219 and its gate coupled to the gate/drain connection of transistor 230. A depletion mode transistor 238 is disposed with its gate coupled to the gate/drain connection of transistor 236, its source coupled to the drain of transistor 240 and its drain coupled to V.sub.BB terminal 217. The gates of transistors 224 and 238 are also coupled to output node 244. The gate of transistor 206 is coupled to the common drain/source connection of transistors 224 and 226. The gate of transistor 208 is coupled to the common drain/source connection of transistors 238 and 240.

In operation, the negative clamping portion of the circuit 200 is referenced to V.sub.BB and the positive clamping portion is referenced to V.sub.CC. As the input to inverter 222 goes low, its output is driven high thus turning "OFF" transistor 204 and turning "ON" transistors 225 and 210. This pulls output terminal 244 towards V.sub.BB until the series connected diode chain 233 begins to conduct current. Current flow in the series connected diode chain is mirrored by transistors 236 and 238 which generate current I3 which flows through transistor 240. As transistor 240 begins to conduct current, output node 244 is driven back toward V.sub.SS which begins to turn transistor 208 "OFF", thereby limiting current at output node 244. Transistor 225 is an isolation device gated by V.sub.SS which allows the gate of transistor 210 to switch to V.sub.BB thus disabling it whenever the input to the circuit 200 is high and the output of the circuit 200 is clamped at its negative level.

The operation of the positive clamping portion of the circuit 200 is identical to the operation of the circuit 100. As the input of inverter 222 goes high, its output goes "LOW", transistor 204 turns "ON" and the voltage at output node 244 rises toward V.sub.CC. In the embodiment shown, when the voltage at output terminal 244 rises to four diode drops above V.sub.SS, current begins to flow in the series diode chain 213. This current is mirrored by transistors 220 and 226 which generate current I4 into transistor 224 which controls the gate of transistor 206. Since the voltage at the gate of transistor 226 is rising, transistor 226 begins to turn off and the gate of transistor 206 is pulled toward V.sub.SS. This forces transistor 206 out of conduction and limits the current delivered to the series connected diode chain 213 thus clamping the voltage at terminal 244 at a predetermined level correspondingto the sum of threshold voltages of the series connected diodes.

Referring now to FIG. 3, there is a series of graphs depicting the operation of the circuit 100. The input to the circuit 100 is a signal which, as shown in graph A, switches between 0 volts and V.sub.CC. When the input signal to transistor 108 switches from high to low, the voltage at output node 128 is clamped to 4 diode threshold voltages above V.sub.SS as shown in graph B. When the circuit 100 is in regulation, the voltage at node 130 settles at approximately V.sub.CC minus a diode threshold voltage drop, as shown in graph C. Node 130 switches to V.sub.CC whenever the output of the circuit 100 is at V.sub.SS. Graph D shows the output of the circuit 200 as the input to the circuit switches between zero volts and V.sub.CC as shown in graph `A` and the circuit 200 Output is switched between a positive voltage relative to V.sub.CC and a negative voltage relative to V.sub.BB.

Accordingly, an improved voltage clamping circuit has been described. The present invention includes a current generating means which is responsive to a control circuit. The control circuit controls the output current of the current generaing means so that only enough current is generated to maintain a desired voltage at the output of the voltge clamping circuit. Accordingly, other uses and modifications will be apparent to a person of ordinary skill in the art without departing from the scope of the present invention.