A voltage step-up circuit with regulated output voltage

Abstract: A voltage step-up circuit with regulated output voltage, comprising a voltage divider and a current-extract circuit means (MSEN) connected between the output terminal (OUT) of the circuit and ground. A control circuit (C) connected to the divider drives the switching of said current-extract circuit means (MSEN).

Description: Field of the Invention This invention relates to voltage step-up circuits, in particular to a voltage step-up circuit which can be integrated monolithically in CMOS technology and used to drive the drain of a field-effect transistor of a memory cell during the programming phase. In this kind of application, the memory cell must be supplied a drain voltage which be constant as far as possible while the current drawn by the cell varies. This technical problem becomes specially important where a single voltage step-up circuit is employed for programming a group of memory cells, and the overall current draw variations increase. Prior Art The arrangements adopted heretofore to provide adequate stability for the output voltage of a voltage step-up circuit consist basically of providing a Zener diode, or a series of diodes or diode-connected transistors, between the output terminal of the voltage step-up circuit and a reference potential, usually a ground potential. However, where high output voltages are involved, say on the order of ten volts, the implementation of a Zener diode in CMOS technology becomes difficult, whereas the use of a diode chain -- which would have to include five or six diodes -- may bring about significant deviations from nominal values due to variations in the technological process parameters. The technical problem which underlies this invention is, therefore, to provide a voltage step-up circuit which can be integrated in CMOS technology and regulated even at high output voltages. Another requirement is that the voltage step-up circuit output should be regulated using simple circuit arrangements, so that the incorporation of several such voltage step-up circuits to memory cell devices would lead to no excessive expansion of the integration area. Summary of the Invention The technical problem is solved by a voltage step-up circuit of the type described and characterized in the appended claims to this specification. The features and advantages of a step-up circuit according to the invention will become apparent from the following detailed description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 shows a diagram, partly in block form, of a voltage step-up circuit with regulated output voltage according to the invention; and Figure 2 shows a preferred circuit diagram for a control block C, as shown in Figure 1, implemented in CMOS technology. Detailed Description The theoretical diagram of a voltage step-up circuit according to the invention, shown in Figure 1, comprises a block P marked symbolically to indicate conventional functional parts of a voltage step-up circuit, usually a charge pump circuit, having at least one output terminal OUT to which there would be connected a load to be supplied a constant high voltage. The diagram further includes, according to the invention, an output voltage regulator block, shown in dash outline, which comprises a closed loop control circuit. It further includes a capacitive divider connected between the output terminal OUT and ground, whereby the voltage from said functional parts which constitutes the output voltage of the step-up circuit is divided by a predetermined factor. Connected to the divider is a control circuit which is responsive to the voltage output from the divider and represented in Figure 1 by a triangular block C. This control circuit drives a transistor MSEN "on", but only upon the voltage exceeding a predetermined level. In such a case, the transistor MSEN, which is connected between the output terminal OUT and ground, would draw current from the charge pump until the voltage drops to said predetermined level, at which the voltage will be held in the steady state condition. In the starting condition, of course, it should be possible to discharge the divider capacitors. If the division ratio is β and the predetermined voltage level is VRIF, then the regulated output voltage becomes, V OUT = 1/β VRIF In the diagram shown, the divider is a capacitive type, such that power consumption can be minimized without excessive occupation of integration area. In fact, if a resistive divider were employed, a limitation of the current (to be supplied from the charge pump) drawn thereby to reasonable values would involve the use of very high-value resistors requiring, in conventional integration technologies, an amount of occupied integration area far from negligible. The use of a capacitive divider in an integrated circuit further affords a very accurate division ratio. In other applications, of course, the divider could be a different type (e.g. resistive). There is a number of different arrangements that can be used for implementing the control circuit C, of which two are specially simple and advantageous. A first arrangement consists of using a comparator circuit, preferably of the threshold type, which can be implemented by conventional techniques. The output terminal of the comparator is connected to the control terminal of the transistor MSEN, an input terminal of the comparator is connected to the divider and another is connected to a reference voltage which can be obtained using techniques well known to the skilled ones. A second arrangement, which may be preferable where the voltage step-up circuit is implemented in the CMOS technology, is illustrated by Figure 2. In this case, no comparator proper is used, and as can be evinced from the diagram in Figure 2, the control circuit is comprised of two cascaded stages, each comprising two mutually complementary transistors, MGEN, MUP and MIN1, MIN2. The first stage comprises a generator of a current having a predetermined constant value (MGEN driven by the voltage VRIF), which causes the node D to be charged toward a low voltage level when the transistor MUP is off or is delivering a lower current than MGEN (as would occur if the input node of the block C is at a sufficiently high level). When MUP is on and delivering a higher current than that from MGEN, the node D is brought to a voltage high (as would occur if the input node of the block C is not at a sufficiently high level). The node D drives the block second stage, consisting of a logic inverter whose output, representing the output of the block C, drives the transistor MSEN of the regulator block in the voltage step-up circuit. Thus, when the output voltage from the charge pump is below a predetermined value (dependent on the value of VRIF and the size of the transistors in the block "Sens"), the output voltage from the block C will be low and the transistor "off", whereas when it is above that value, the transistor MSEN will be "on" and absorb a part of the current from the charge pump, as desired for regulating purposes. The voltage generated by the step-up circuit will, therefore, be regulated at a value which is dependent on the division ratio of the divider and the values (including the voltage VRIF) of the block C components. The second arrangement is simpler than the first, circuit-wise. However, the voltage generated by the first arrangement can be highly accurate, if a precision voltage VRIF is available. The voltage generated by the second arrangement may be less accurate because of its more marked dependence on the parameters of the integrated circuit manufacturing process and the operating conditions (such as temperature and supply voltage). The voltage regulation vis-a-vis the variations of the current drawn by the load is good with either arrangements. It will be appreciated that modifications or integrations may be made unto the embodiments described hereinabove without departing from the protection scope of the appended claims. As an example, the two stages of the control circuit in the second arrangement could be implemented in another technology using bipolar transistors.

Claims: A voltage step-up circuit with regulated output voltage (Vout), characterized in that it comprises a voltage divider and a current-extract circuit means (MSEN), connected between an output terminal (OUT) of the voltage step-up circuit and a potential reference (ground), the switching of said current-extract circuit means being driven by a control circuit (C) connected, through an input terminal, to an intermediate node of said divider. A voltage step-up circuit according to Claim 1, characterized in that said divider comprises first and second capacitors connected serially together through said intermediate node. A voltage step-up circuit according to any of the preceding claims, characterized in that the current-extract circuit means comprises at least one transistor (MSEN). A voltage step-up circuit according to any of Claims 1, 2 and 3, characterized in that the control circuit (C) is a threshold comparator. A voltage step-up circuit according to any of Claims 1, 2 and 3, characterized in that the control circuit (C) comprises first, second, third and fourth transistors connected between first (VDD) and second terminals (ground) of a voltage supply generator and each having first and second terminals and a control terminal, the control terminals of the first (MUP) and second transistors (MGEN) being input terminals (IN,VRIF) of the control circuit, the control terminals of the third (MIN1) and fourth transistors (MIN2) being both connected to the first terminal of the first transistor and to the first terminal of the second transistor, and the first terminals of the third and fourth transistors being connected together into an output terminal (OUT) of the control circuit.

Inventors: CALLIGARO CRISTIANO, MALCOVATI PIERO, TORELLI GUIDO, CALLIGARO,CRISTIANO, MALCOVATI,PIERO, TORELLI,GUIDO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H02M3
IPC codes: G11C5, G05F1