Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Synchronizing
Reexamination Certificate
2000-05-01
2001-10-30
Kim, Jung Ho (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Synchronizing
C327S198000
Reexamination Certificate
active
06310497
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to power loss detection in a circuit. More particularly, the invention pertains to detecting loss of a main power supply in order to generate a control signal for switching to an auxiliary power supply.
BACKGROUND OF THE INVENTION
Many electronic devices are designed with auxiliary power supplies that turn on when a main power supply fails. For instance, it is desirable and, in fact, standard practice to provide an on-board battery back-up power source in computers for keeping the time and date clock circuitry running when the main power source for the computer is off so that the computer always has the current date and time available. For example, in a laptop computer, a power supply detector circuit must be provided to detect when the portable laptop computer is plugged into an AC outlet. When it is plugged into an AC outlet, power to run the computer is supplied from the AC outlet. However, when the computer is unplugged, that absence of the power from the AC input terminal of the computer must be detected so that the computer can be switched over to operate from the on-board battery power supply.
In general, the power loss detector circuits of the prior art comprise a comparator for comparing the voltage supplied by the main power supply to the voltage supplied by the auxiliary (e.g., battery) power supply. The voltage provided by the main power supply and the voltage provided by the battery are provided to the two inputs of a comparator through respective voltage dividers. The voltage dividers are ratioed so that the comparator output switches states when the main power supply drops below a predetermined threshold. For instance, in a notebook computer, the 120V AC power available from an outlet is converted to 3.3V DC which is used to power all the circuits in the computer. The battery, on the other hand, may be regulated to provide 3V of power. The resistor dividers corresponding to the main power and the auxiliary power may be ratioed such that the comparator output will switch states when the main power supply drops below 2.8V. The output of the comparator is then used as a main power supply loss indicator signal. When that signal switches state, indicating that the main power supply has dropped below 2.8V, a power supply switching circuit switches to auxiliary power.
U.S. Pat. No. 5,457,414 entitled Power Supply Loss Sensor discloses another power loss detector circuit. In the circuit disclosed in that patent, ring oscillators and other digital circuitry are used in the scheme for detecting power loss.
In both of the above-described schemes, the auxiliary or battery power supply is compared to the main power supply. Accordingly, a constant DC drain on the auxiliary power supply is needed for the operation of the power loss detector circuit. Over very long periods of time between rechargings, the battery can be completely drained.
Accordingly, it is an object of the present invention to provide an improved power loss detection method and apparatus.
It is another object of the present invention to provide a power loss detection method and apparatus which does not consume DC power from the auxiliary power supply.
SUMMARY OF THE INVENTION
The invention is a power loss detector circuit for an integrated circuit that asserts a signal when the main power drops below a predetermined threshold. The detector circuit is very simple and comprises two transistors, a resistor divider network and, optionally, an inverter.
The main power source is supplied to one end of the voltage divider. The voltage divider produces a voltage signal that is a fraction of the main power supply voltage. The main power supply voltage also is provided to the source terminal of a transistor. The fractional voltage is provided to the gate terminal of the same transistor. The voltage divider is configured such that the difference between the power supply voltage and the fractional voltage when the power supply voltage is at the minimum acceptable level is equal to the threshold voltage of the transistor. The minimum acceptable level is the point at which it is desired to switch from the main power supply to an auxiliary power supply. Thus, when the main voltage supply is greater than the minimum acceptable level, the transistor is turned on. Otherwise, it is off. The drain of the transistor is coupled to the output node of the detector circuit. This node is also coupled to the drain of a second transistor having its source coupled to ground. The gate of the second transistor is coupled to the auxiliary power supply such that the second transistor is on as long as the auxiliary power supply is sufficient, i.e, greater than the threshold voltage of the second transistor. Alternately, the gate of the second transistor is coupled to a second voltage level through an inverter that is powered by the auxiliary power supply.
Thus, when the main voltage supply is greater than the minimum acceptable level, the transistor is turned on. Otherwise, it is off. The drain of the transistor is coupled to the output node of the detector circuit. This node is also coupled to the drain of a second transistor having its source coupled to ground. The gate of the second transistor is coupled to the auxiliary power supply such that the second transistor is on as long as the auxiliary power supply is sufficient, i.e, greater than the threshold voltage of the second transistor.
The second transistor has a much longer channel than the first transistor so that it has a much higher impedance. Accordingly, regardless of whether the second transistor is on or off, if the main power supply is above the minimum acceptable level, the output node is driven to the voltage provided by the main power supply, i.e., logic high. Thus, a logic high at the output node indicates that the main power supply is operational and should be used to power the integrated circuit. Only when the main power supply drops below the minimum level, thus turning the first transistor off, can the second transistor drive the output terminal to ground (i.e., logic low). Accordingly, a logic low level at the output node indicates that the main power supply is off or has dropped below the predetermined minimum voltage and power should be switched to auxiliary power.
The circuit draws no DC current from the auxiliary power supply. The auxiliary power supply used to power the inverter draws only transient current when the comparator output switches.
REFERENCES:
patent: 4902907 (1990-02-01), Haga et al.
patent: 4984211 (1991-01-01), Tran
patent: 5166546 (1992-11-01), Savignac et al.
patent: 5457414 (1995-10-01), Inglis et al.
patent: 5619156 (1997-04-01), Jandu
patent: 5721502 (1998-02-01), Thomson et al.
Agere Systems Guardian Corp.
Kim Jung Ho
Synnestvedt & Lechner LLP
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