CONTROL CIRCUIT FOR EEPROM

Abstract: A control circuit generating a write/erase high voltage pulse for an EEPROM is disclosed. The control circuit comprises a low frequency clock pulse oscillation circuit, a voltage booster circuit having at least 19 FET booster stages and a high voltage shaping switch circuit having an exponential rising pulse edge driven by the clock pulse.

Description: BACKGROUND OF THE INVENTION This invention relates to a control circuit for an EEPROM (Electrically Erasable Programmable Read Only Memory), more particularly to a control circuit generating a high voltage pulse enabling a rewrite function of EEPROM data with low voltage and low power consumption. There are various kinds of EEPROMs available not only for use as a single unit but for constituting an one-chip microcomputer in combination with a microprocessor, which can be constructed as a CMOS structure. A semiconductor integrated circuit in which the EEPROM is incorporated has a wide variety of applications such as replacement of a mechanical switch, an IC card having function which stores and adjusts operational procedures of a calculator or stores and refreshes data. (Ex. See NIKKEI ELECTRONICS, Oct. 21, 1985 P. 127) Since the conventional EEPROM, however, employs a 5V power supply and needs a data rewrite cycle, it is difficult to be incorporated into the semiconductor integrated circuit utilized in watches, electric calculators, cameras, toys or IC cards driven by a solar cell. Fig. 6 shows a block diagram of a control circuit for EEPROM with 5V single power supply operation generating a high voltage pulse for data write/erase/read. The power supply voltage of 5V is applied to a clock pulse generation circuit 1 so that a clock pulse train having a frequency of 5-10MH z and a peak wave value 5V can be achieved. When the clock pulse is applied to a voltage booster circuit 2, a step-up voltage of 23-25V can be output. Fig. 7 shown an example of the conventional voltage booster circuit having 10-13 booster stages which are adequate for obtaining a step-up voltage of 20-25V from a source voltage of 5V. The high voltage of 20-25V generated by the booster circuit 2 is applied to a regulator circuit 3 which stabilizes the high voltage to a constant level voltage. Fig. 8(A) shows a concrete circuit of the regulator circuit, the output voltage of which is 18-20V in direct current (D.C.). A wave shaping circuit 4 generates a high voltage pulse which is necessary to write/erase data of cells of the EEPROM. The high voltage pulse requires a peak wave value of 18-20V and a constant rising rate, for example, 16V/ms. When the rising rate is too sharp, the reliability of the cells of the EEPROM will be deteriorated. One embodiment of the concrete wave shaping circuit is shown in Fig. 8(B). There is provided a high voltage switch circuit 5 to switch the high voltage pulse generated by the wave shaping circuit 4 to a writing direction or to an erasing direction, interchangeably. Fig. 8(C) shows a concrete circuit of the high voltage switch circuit 5. The high voltage pulse is switched interchangeably by the high voltage switch circuit 5 in such a manner that the high voltage pulse is applied to a drain electrode of the memory cell in accordance with a writing signal W in case of writing, whereas it is applied to a gate electrode of the memory cell in accordance with an erasing signal E on erasing. A memory cell 6 is constituted by being arrayed a plurality of FETs with a double layer silicon gate structure comprising a source electrode , a drain electrode, a floating gate and a gate electrode. One embodiment of the concrete circuit is shown in Fig. 9. The writing function to the memory cell 6 is performed such that the high voltage pulse is applied between the drain and the gate with the gate electrode being fixed to zero volt. The erasing cycle is performed such that the high voltage pulse is applied between the gate and the drain with the drain electrode being fixed to zero volt. A control circuit according to the preamble of claim 1 is known from "IEEE Journal of Solid-State Circuits", Vol. SC 18, No. 5, October 1983, pages 532 to 538. The known circuit relates to a voltage booster and a high voltage shaping switch which are operated by a supply voltage of 5 V. SUMMARY OF THE INVENTION An object of the present invention is to provide a control circuit for an EEPROM which enables writing and erasing data to the EEPROM with low voltage of an approximate 1.5V and low current less than a few hundred µA. To accomplish the above object, there is provided a control circuit according to claim 1. Further, the high voltage switch circuit having an exponential rising pulse edge requires no special wave shaping circuit so that a simplified and low voltage operated circuit can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a control circuit for an EEPROM illustrating one embodiment according to the present invention. Fig. 2 is a circuit diagram of a booster circuit utilized in one embodiment of the invention. Fig. 3 is a circuit diagram of a high voltage shaping switch circuit utilized in one embodiment of the invention. Fig. 4 is a circuit diagram of a memory cell and a read-out circuit thereof utilized in one embodiment of the invention. Fig. 5 is a wave form chart of a high voltage pulse showing a rising edge of the pulse according to the invention. Fig. 6 is a block diagram of a conventional EEPROM. Fig. 7 is a circuit diagram of a conventional booster circuit. Fig. 8(A), Fig. 8(B) and Fig. 8(C) are circuit diagrams of a conventional regulator circuit, a shaping circuit and a high voltage switch circuit, respectively. Fig. 9 is a circuit diagram of a conventional memory cell and a read-out circuit thereof. DETAILED DESCRIPTION OF THE EMBODIMENT Referring to Fig. 1, the control circuit for the EEPROM according to the present invention comprises a low frequency pulse oscillation circuit 11, a voltage booster circuit 12, and a high voltage shaping switch circuit 13. The low frequency clock pulse oscillation circuit 11 is an oscillator generating an approximate 30KHz frequency driven by low voltage and low current. It is essential to lower the frequency of the clock pulse for reducing power consumption of the whole control circuit. The adequate frequency range for the clock pulse lies between 20KHz and 100KHz. The present embodiment shows an oscillator operating with a 1.5V source voltage and a few µA consumption current. The voltage booster circuit 12 is a booster circuit with low power consumption which operates with low power source driven by the oscillation circuit 11. Though a conventional booster circuit has approximately 10 stages, the present invention employs at least 19 stages, particularly in the present embodiment 24 stages, to operate with low power voltage of an approximate 1.5V. Fig. 2 shows a circuit diagram of the voltage booster circuit 12 in accordance with the present embodiment. Transistors Q1 - Q24 constitute the booster stages, each of which is constituted by zero threshold FETs. The zero threshold FET is a FET where the threshold voltage is controlled to be nearly zero volt (approximately 0 ± 0.4V). Utilization of the zero threshold voltage FET in the booster stage enables to elevate voltage efficiently without voltage loss. In this embodiment, an approximate 20V high voltage necessary for writing and erasing of the EEPROM can be obtained as an output voltage V pp about 20ms later after the clock is supplied under conditions of the power source voltage V DD being 1.5V, the booster stage being 24 and the clock frequency being 30KHz. In this voltage booster circuit, as long as a clock is applied thereto, the output voltage V pp of about 24V in D. C. can be obtained. The voltage booster circuit according to the present invention employs at least 19 stage zero threshold FETs and is driven by the low voltage power source V DD having an approximate 1.5V and the slow clock having an approximate 30KHz so that the output voltage V pp having an approximate 20V which is adequate for writing and erasing of the cell of the EEPROM can be obtained. Accordingly, a regulator circuit which is used in the conventional circuit is not necessarily required because the voltage booster circuit has the same function. The high voltage shaping switch circuit 13 is a circuit which forms a high voltage pulse needed for data writing and erasing of the memory cell 14 by using the approximate 20V achieved by the booster circuit 12. Fig. 3 shows an embodiment of the circuit. When a writing signal W is to be "1", a high voltage WR for writing can be formed from the D. C. voltage V pp by the functions of transistors Q₂₅, Q₂₆ and Q₂₇ and two capacitors. This high voltage pulse is of an exponential type which has a 1-5 ms rising edge due to the function of transistor and capacitor of 1-2pF when driven by the clock pulse of an approximate 30KHz. Fig. 5 shows a wave form chart illustrating the rising edge of the pulse. The exponential type pulse is particularly suitable with an approximate 1 ms rising edge. The memory cell 14 itself is the same type as the conventional one arrayed by FETs for data storage having source-drain electrodes, a floating gate and a normal gate. Now, the writing and the erasing functions will be explained referring to Fig. 4. In case of writing, after the switch circuit is activated by the writing signal W being "1", the high voltage pulse is applied to the drain of the memory cell FET 601 and the gate of the FET 601 is simultaneously grounded to zero by the transistor 607. Then, electrons stored in the floating gate 605 are transferred through the drain so that positive electric charges are accumulated in the floating gate. In case of erasing, after the switch circuit is activated by the erasing signal E being "1", the high voltage pulse is applied to the gate of the memory cell FET 601 and the gate of the FET 601 is simultaneously grounded to zero by the transistor 608. Then, electrons are injected to the floating gate causing negative charging up. In other words, the positive electric charges injected in writing cycle are erased and further negative charges are accumulated. Now, data read-out operation will be explained. First, a read-out signal RD is set to be "1". Then, an approximate 1.3V read-out voltage is applied through the zero threshold FET 611 to the gate of the memory cell FET. In the event that the memory cell is being written, namely, the floating gate is being positively charged, the memory cell FET 601 becomes conductive between the source and the drain so that the drain current flows through the FETs 610, 601 and 606. Then, an inverter 614 and a FET 613 read out date "1". In the event that the memory cell is being erased, namely, the floating gate is being negatively charged, the memory cell FET 601 does not become conductive so that the drain current does not flow through the FETs 610, 601 and 606. Then, the inverter 614 and the FET 613 read out date "0".

Claims: A control circuit generating a write/erase high voltage pulse for an EEPROM, the control circuit comprising a pulse oscillation circuit (11), a voltage booster circuit (12) having a plurality of booster stages each of which has a FET being driven by the clock pulse, and a high voltage shaping switch circuit (13) comprising a first transistor (Q₂₅) having a first terminal connected to an output terminal of the voltage booster circuit (12), a second terminal and a control terminal connected to a node (A), a second transistor (Q₂₇) having a first terminal connected to the node (A), a second terminal connected to the second terminal of the first transistor (Q₂₅) and a control terminal connected to the second terminal of the first transistor (Q₂₅), and a first capacitor (133) one terminal of which is connected with the second terminal of the first transistor (Q₂₅), characterized in that the clock pulse oscillation circuit (11) generates a low frequency clock pulse (), the high voltage shaping switch circuit (13) comprises a first NAND gate (131) having a first input terminal to receive the clock pulse (), a second input terminal to receive a writing/erasing signal (W) and an output terminal, a second NAND gate (132) having a first input terminal to receive inverse pulse of the clock pulse (), a second input terminal to receive the writing/erasing signal (W) and an output terminal, a third transistor (Q₂₆) having a first terminal connected to the output terminal of the voltage booster circuit (12), a second terminal supplying the write/erase high voltage pulse (WR) and a control terminal connected to the node (A), and a second capacitor (134) connected between the output terminal of the second NAND gate (132) and the node (A), wherein the other terminal of the first capacitor (133) is connected to the output terminal of the first NAND gate (131), and that the voltage booster circuit (12) comprises at least 19 stages. A control circuit according to claim 1 wherein the low frequency clock pulse is approximately 30 KHz. A control circuit according to claim 1 wherein the low voltage of the low power voltage source (V DD ) is approximately 1.5V. A control circuit according to claim 1 wherein the capacitance of the first and the second capacitors (133, 134) is approximately 1 to 2pF. A control circuit according to claim 1 wherein the FET has a threshold voltage selected to be nearly zero volt. A control circuit according to claim 1 wherein the high voltage shaping switch circuit (13) further comprises a fourth transistor (Q₂₈) connected between a low voltage power source (V DD ) and the node (A), the fourth transistor (Q₂₈) having a control terminal to receive the writing/erasing signal (W).

Inventors: TANAGAWA, KOUZI C/O OKI ELECTRIC INDUSTRY CO.,LTD.
Applicants: OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1988
Main classification: G11C16
IPC codes: G11C16