Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
1999-09-30
2002-09-10
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S538000
Reexamination Certificate
active
06448845
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of electronic devices, and in particular to voltage generators for memory devices.
2. Description of Related Art
The reliability, or longevity, of a semiconductor memory device has been found to be related to the stress imposed on the device by rapid voltage transitions, particularly rapid high voltage transitions used to write or erase the memory contents.
Electrically erasable (EE) memory devices are particularly well suited for techniques that control the application of stress-inducing voltage transitions in order to improve the longevity of the device. Typically, EE devices are used as programmable read-only memories, wherein the EE device is relatively infrequently programmed to contain a data set that is frequently read. Because the programming is relatively infrequent, the speed at which the programming occurs is not as critical as other parameters of the design, and in particular, less critical than the longevity of the device.
FIG. 1
illustrates an example voltage generator
100
commonly used for programming and erasing an electrically erasable memory device. The generator
100
is designed to provide an output voltage
165
that increases from zero volts to a high voltage reference voltage at a controlled rate. The value of the high voltage reference, typically in the 10 to 12 volt range, is determined by fabricating and testing samples of the device to determine an optimal value, based on process parameters and other factors. A reference voltage Vref
115
is provided for controlling the peak value of the output voltage
165
, typically from a band-gap voltage source, common in the art. The controller
190
effects a charge transfer from the source of the reference voltage
115
to a comparator
150
, via switches S
1
110
and S
2
120
, and capacitors C
1
130
and C
2
140
, using techniques common in the art. The controller
190
asserts switch control Sa
101
to effect a charging of capacitor C
1
130
to the reference voltage
115
, while de-asserting switch control Sb
102
to isolate capacitor C
1
130
from C
2
140
. Thereafter, the controller
190
de-asserts switch control Sa
101
and asserts switch control Sb
102
, thereby isolating capacitor C
1
130
from the reference voltage Vref
115
, and coupling the capacitors C
1
130
and C
2
140
together. If the voltage of capacitor C
2
140
at the time of coupling to capacitor C
1
130
is less than the voltage on the capacitor C
1
130
(which, at the time of coupling, is equal to the reference voltage
115
), capacitor C
1
130
transfers charge to capacitor C
2
140
, thereby raising the voltage level of capacitor C
2
140
. The ratio of the capacitance of capacitor C
1
130
and capacitor C
2
140
, and the difference in voltage between the capacitors
130
,
140
at the time of coupling, determine the amount of the voltage increase at each coupling. Using this charge transfer technique, common in the art, the voltage Vramp
145
on the capacitor C
2
140
increases asymptotically to the voltage reference
115
, the rate of increase being determined by the ratio of the capacitance of the capacitors
130
,
140
.
A voltage controlled high-voltage source
160
provides the high-voltage output
165
. The control voltage
155
that controls the high-voltage source
160
is provided by a closed-loop feedback system comprising a scaler
170
and the comparator
150
. The scaler
170
scales the high-voltage output
165
by a factor S, and this scaled voltage
175
is compared to the aforementioned voltage Vramp
145
. The feedback control signal
155
controls the high-voltage output
165
to track the Vramp
145
signal, at the scale factor S. That is, if the scale factor S is 5/8, the high-voltage output
165
is 8/5*Vramp
145
. Because Vramp
145
increases to Vref
115
, the high-voltage output
165
increases to 8/5*Vref
115
. After providing the increasing high-voltage output
165
to the device that utilizes this voltage source, such as an EE memory device, the controller
190
closes switch S
0
180
to deplete the charge on capacitor C
2
and reduce its voltage to zero, thereby reducing the output voltage
165
to zero. The above process is repeated as required, whenever the increasing output voltage
165
is required.
As mentioned above, the peak of the high-voltage output
165
is preferably trimmed to optimize the longevity of the device that receives this high-voltage output
165
. This trim is effected by modifying the scale factor S, typically by physically modifying the devices that form the scaler
170
. For example, a conventional scaler
170
is a capacitor divider circuit, and the trimming of the scaler is effected by increasing or decreasing the plate area of one or more of the capacitors forming the scaler
170
. This typically requires a change to at least one of the metal masks used to fabricate the device, and cannot be economically applied to customize the high-voltage output
165
of individual voltage generators
100
.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a high-voltage generator that can be trimmed without a mask change. It is a further object of this invention to provide a voltage generator that can be individually trimmed after fabrication. It is a further object of this invention to provide a high-voltage generator that can be optimized for writing and erasing electrically erasable programmable devices.
These objects, and others, are achieved by providing a programmable reference voltage that is used for controlling a high-voltage source. A programmable voltage divider is used to scale a fixed reference voltage to a scaled reference value that is used to control the generation of a high voltage source. A comparator provides a feedback signal that is based on a difference between the scaled reference voltage and a scaled output voltage. This feedback signal controls the voltage-controlled output voltage source, so as to track the scaled reference value. In a preferred embodiment, the scale factor associated with the output voltage remains constant, whereas the scale factor associated with the reference voltage is programmable. In alternative embodiments of this invention, the reference scaling factor defaults to a mid-range value, and a bias offset is provided to easily select an output voltage value for either programming or erasing the contents of a programmable memory device.
REFERENCES:
patent: 4040408 (1977-08-01), Kraus et al.
patent: 4709225 (1987-11-01), Welland et al.
patent: 4952821 (1990-08-01), Kokubun
patent: 5168174 (1992-12-01), Naso et al.
patent: 5319370 (1994-06-01), Signore et al.
patent: 5546042 (1996-08-01), Tedrow et al.
patent: 5793249 (1998-08-01), Chen et al.
patent: 6118706 (2000-09-01), Smayling et al.
IBM Technical Disclosure Bulletin, “Switched Capacitor Voltage Reference That Approaches the Power Supply Rail”; vol. 31, No. 10, Mar. 1989, pp. 273-275.
Cunningham Terry D.
Koninklijke Philips Electronics , N.V.
Tra Quan
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