Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Synchronizing
Reexamination Certificate
2001-01-04
2002-10-29
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Synchronizing
C327S198000
Reexamination Certificate
active
06472912
ABSTRACT:
TECHNICAL FIELD
The present invention generally pertains to the field of electronic circuits. More particularly, the present invention is related to a technique for detecting that a power supply is below a pre-determined level and generating an indicator signal.
BACKGROUND ART
In some applications such as the Real Time Clock for PC, it is necessary to detect the power supply level and generate an indicator signal when the power supply has dropped below a threshold level. This is because below the threshold level, circuit operations are not guaranteed.
One conventional method for a voltage level detection circuit uses bandgap reference comparison. Referring to
FIG. 1
, a reference voltage, V
ref
106
, the desired threshold level, is generated by bandgap reference
102
and is compared with the power supply
104
via differential amplifier
104
. When the supply
104
drops below V
ref
106
, V
X
108
will change. This circuit
100
can be accurate; however, it cannot be used as power on reset because V
ref
106
is not available when the power is just switched on. Also, bandgap reference voltage level that is accurate over a wide temperature range centers only around 1.2 Volts. Therefore, a relatively limited range of threshold voltages may be detected. Furthermore, the bandgap reference increases the number of components in the voltage level detection circuit.
A second conventional method is a voltage divider configuration. Referring to
FIG. 2
, in this configuration, Z
1
201
and Z
2
202
are two impedances (e.g., resistors). They are chosen to have a ratio such that when V
DD
204
is at a normal operating level V
X
208
is low because transistor M
1
206
is on. When V
DD
204
drops to the selected threshold, V
i
210
will be lower than the threshold of M
1
206
. At that point, V
x
208
will switch to high because transistor M
1
206
is off. This circuit
200
can be used as power on reset. However, since the threshold of M
1
206
varies with temperature and process, the selected threshold of V
DD
204
at which V
x
208
will change, therefore, also varies with temperature and process.
FIG. 3
illustrates how temperature and process variations may affect V
GS
versus I
DS
of a NMOS transistor. The graph
300
shows the variation to be over 400 mV between a high temperature (e.g., 125 degrees Celsius) fast corner device
302
and a low temperature (e.g., −40 degrees Celsius) slow corner device
308
. Comparing the high temperature slow corner device
306
and the low temperature fast corner device
304
with the other curves reveals that even one of the variations (process or temperature) may account for approximately 200 mV differences in the device threshold. Moreover, in an actual circuit, the change in V
i
could be as high as 1 Volt due to the amplification effect of the impedance ratio.
Thus, a need has arisen for a technique for detecting a power supply is at a low voltage level. A further need exists for such as system which also functions as a power on reset indicator. A still further need exists for a technique which may be implemented with a relatively simple circuit. An even further need exists for a technique which is capable of detecting a wide range of power supply thresholds. A still further need exists for a technique which is not highly sensitive to semiconductor process variations and ambient temperature variations.
SUMMARY
The present invention provides a circuit for detecting that the level of a power supply is at a low level. Additionally, the circuit also functions as a power on reset indicator. The present invention also provides for a technique which may be implemented with a relatively simple circuit. A circuit of the present invention is capable of detecting a wide range of power supply thresholds. Additionally, the technique is not highly sensitive to semiconductor process variations and ambient temperature variations.
A circuit for power supply detection and power on reset is disclosed. In one embodiment, the circuit comprises two separate component groups for producing separate currents which vary in response to the level of a voltage source. For example, each may have a non-linear DC relationship between the power supply and a current of the respective component group. Furthermore, the DC voltage to current relationship of two groups is different. The component groups may be, for example, current sources or sinks. The circuit has an output with two modes. The two component groups are operable to force the output into one mode when the voltage source is above a threshold and a second mode when the voltage source is below the threshold. Therefore, the circuit is operable to detect when the supply voltage has dropped below a threshold and to also serve as a power on reset. However, the circuit does not rely on the threshold voltage of a transistor to trigger the output. Consequently, it provides for better tolerance than conventional voltage divider circuits, as it is not highly sensitive to semiconductor processing and ambient temperature variations.
These and other advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.
REFERENCES:
patent: 4309627 (1982-01-01), Tabata
patent: 4716323 (1987-12-01), Wada et al.
patent: 5300822 (1994-04-01), Sugahara et al.
patent: 5302861 (1994-04-01), Jelinek
patent: 5528182 (1996-06-01), Yokosawa
patent: 5883532 (1999-03-01), Bowers
patent: 5959477 (1999-09-01), Chung
patent: 6118315 (2000-09-01), Guedj
patent: 6137324 (2000-10-01), Chung
patent: 6181173 (2001-01-01), Homoi
Chan Wai Cheong
Chiu Hon Kin
Cunningham Terry D.
National Semiconductor Corporation
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