The present application claims priority of Chinese Application No. 200610153719.4 filed Sep. 8, 2006, which is incorporated herein in its entirety by this reference.
The present invention is related to over-voltage comparators and more particularly to an over-voltage comparator circuit that includes a particular power-savings feature during normal voltage operating conditions.
The following symbol conventions are used to describe the prior art as well as the present invention, and are listed below in Table 1.
In general, there are two traditional circuit structures that are used for over-voltage comparators. The first traditional comparator structure 100 is shown in
The comparator circuit 100 takes the threshold voltage of NMOS transistor M1 as a reference voltage. The threshold voltage is given by the following equation:
The bias current is given by the following equation:
The transition threshold of circuit 100 can be derived from equations [1] and [2] as follows:
It can be seen that the threshold of comparator circuit 100 varies with the threshold voltage Vth of NMOS transistor M1, which changes with semiconductor process and temperature. It can also be that the threshold of comparator circuit 100 varies with the accuracy of bias current Ib.
The second traditional over-voltage comparator circuit 200 is shown in
The threshold voltage of circuit 200 is given by the following equation:
The threshold of comparator circuit 200 has better precision than the first solution because it only concerns with VBG, the matching of R1 and R2, and Vdrop. The drawback is that its quiescent current is not zero even if VS is in normal working, as demonstrated in the equation below:
Iquiescent=Iin+Ib [5]
In low quiescent current chip designs, this non-zero current has to be taken into consideration.
What is desired, therefore, is an over-voltage comparator circuit that has a precision threshold, yet does not draw any appreciable quiescent current during normal operation conditions.
An over-voltage comparator circuit includes a power supply voltage to be monitored, a voltage-dropping circuit coupled to the supply voltage, a resistor divider coupled between the voltage-dropping circuit and ground, a transistor coupled to a first output of the resistor divider, a comparator coupled to a second output of the resistor divider having an output for providing an over-voltage indication, and a current mirror having an input coupled to the transistor and an output coupled to the comparator. Using this design, the transistor and comparator consume no quiescent current during normal voltage operating conditions.
The above and other features and advantages of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings, in which:
The over-voltage comparator circuit 300 of the present invention is presented in
When VS is within a normal voltage range, transistor M1 is not conducting. Therefore, no current is drawn from internal five volt supply. When:
transistor M1 conducts and acts as a switch, which provides the current necessary so that comparator 310 is active and prepared to switch. The threshold for providing an over-voltage indication is given by:
Because the VSthreshold is designed to be greater than VSon, comparator 310 is active and well prepared to switch before the VS=VSthreshold.
The over-voltage comparator circuit 310 of the present invention includes the merits of the two traditional over-voltage comparators described above. Comparator circuit 310 saves quiescent current over a normal working range and detects an over-voltage condition using an accurate over-voltage threshold. In part, the design of the comparator circuit of the present invention includes an NMOS transistor in conjunction with other circuit elements to regulate a comparator. Under normal power supply voltages, the comparator stays in standby status and does not consume any quiescent current.
Simulation results 400 are shown in
The simulation results for the three over-voltage comparator circuits are as follows:
Although the present invention has been described with reference to a preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and other will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.
Number | Date | Country | Kind |
---|---|---|---|
2006 1 0153719 | Sep 2006 | CN | national |
Number | Name | Date | Kind |
---|---|---|---|
6486727 | Kwong | Nov 2002 | B1 |
6924533 | Takemura et al. | Aug 2005 | B2 |
7224192 | Fukushi et al. | May 2007 | B2 |
Number | Date | Country | |
---|---|---|---|
20080061844 A1 | Mar 2008 | US |