Abstract:
A voltage detector including a voltage following circuit connected to a power supply and operable to follow a voltage value of the power supply, a selectable threshold point circuit connected to the voltage following circuit and operable to select one of a plurality of values for a threshold point of the power supply, and a switch circuit coupled to the selectable threshold point circuit and the voltage following circuit, the switch circuit cooperating with the selectable threshold point circuit to generate an output indicating whether the value of the power supply has increased above or decreased below the selected value for the threshold point in response to the followed value of the power supply.

Description:
CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY 
   The present application is a continuation of U.S. patent application Ser. No. 09/780,208, filed on Feb. 9, 2001 now abandoned, entitled “Voltage Detector Circuit With A Programmable Threshold Point,” which is assigned to the present assignee and hereby incorporated by reference in its entirety. 

   BACKGROUND 
   1. Field of the Invention 
   The present invention relates generally to voltage detectors and, more particularly, to a voltage detector circuit with a programmable threshold point. 
   2. Related Art 
   Battery-powered portable electronic devices, such as laptop computers and cell phones, require circuits to detect power-on and low power conditions of their battery power supplies. Without such circuits, the portable electronic devices may operate improperly or fail. 
   Circuits that detect power-on conditions are commercially available in the form of power supply monitoring chips. These chips typically have power supply voltage following circuits that track the power supply voltage and output a signal during turn-on when the power supply voltage surpasses a pre-determined threshold point on the rising-edge of the power supply voltage. The threshold point signifies a voltage sufficient for device operation and serves to provide safe startup of the devices by indicating when the power supplies have stabilized at acceptable voltage levels. 
   Circuits that detect low power conditions are also commercially available in the form of microprocessor reset chips. These chips track the power supply voltage and output a signal when the power supply voltage drops below a threshold point. In this case, the threshold point signifies that the power supply voltage has fallen to a level which is insufficient for device operation or that the power supply voltage is decreasing toward a critically low level. 
   One problem with previously developed technologies is that there are no chips having circuits that detect both the power-on and low power conditions. While it is possible to incorporate both a power monitoring circuit and a microprocessor reset circuit on a single chip, doing so would require a substantial amount of surface area for the chip. Additionally, connecting the two circuits unduly increases the complexity of the chip. 
   Another problem with previously developed technologies is that the individual power monitoring circuit chips and microprocessor reset circuit chips themselves are too large to fit inside small portable electronic devices, which are becoming smaller and smaller. One reason for this is that both types of chips require a circuit that generates a reference voltage from which a threshold point value is derived. This reference voltage generation circuit is relatively large and thus increases the size of the individual chips. 
   Another problem with previously developed technologies is that commercial reset chips only provide a small number of programmable threshold point values (e.g., 2 or 3). Chips offering a large number of threshold point values have preset levels for the threshold point values and are only available as discrete components. 
   It is also important that the amount of power or current consumed by circuits or chips in battery-powered portable electronic devices is low. This ensures that the battery-powered portable electronic devices can operate for commercially acceptable periods of time. 
   Accordingly, what is needed is a circuit that can be implemented on a single chip, that is capable of detecting both power-on and low power conditions, that can be programmed to detect a large number of threshold points values, and that consumes a low amount of current. 
   SUMMARY OF THE INVENTION 
   The present invention provides a detector that can be implemented on a single chip, that is capable of detecting both power-on and low power conditions, that can be programmed to detect a large number of threshold point values, and that consumes a low amount of current. 
   In one embodiment of the present invention, a voltage detector is disclosed. The voltage detector includes a voltage following circuit connected to a power supply and operable to follow a voltage value of the power supply, a selectable threshold point circuit connected to the voltage following circuit and operable to select one of a plurality of values for a threshold point of the power supply, and a switch circuit coupled to the selectable threshold point circuit and the voltage following circuit, the switch circuit cooperating with the selectable threshold point circuit to generate an output indicating whether the value of the power supply has increased above or decreased below the selected value for the threshold point in response to the followed value of the power supply. 
   In another embodiment of the present invention, a method for detecting a voltage level performed in a circuit is disclosed. The method includes selecting one of a plurality of values for a threshold point for a power supply, tracking a voltage value of the power supply, and generating an output that indicates whether the voltage value of the power supply has increased above or decreased below the selected value for the threshold point in response to the tracked value of the power supply. 
   In another embodiment of the present invention, a system including a memory, a microprocessor, and a voltage detector coupled to the memory and the microprocessor is disclosed. The voltage detector includes a voltage following circuit connected to a power supply and operable to follow a voltage value of the power supply, a selectable threshold point circuit connected to the voltage following circuit and operable to select one of a plurality of values for a threshold point of the power supply, and a switch circuit coupled to the selectable threshold point circuit and the voltage following circuit, the switch circuit cooperating with the selectable threshold point circuit to generate an output indicating whether the value of the power supply has increased above or decreased below the selected value for the threshold point in response to the followed value of the power supply. 

   
     Other aspects and advantages of the present invention will become apparent from the following descriptions and accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram of a system including a voltage detector, a memory, and a microprocessor, according to an embodiment of the present invention. 
       FIG. 2  is a schematic diagram of a circuit implementation for the voltage detector of  FIG. 1 , according to an embodiment of the present invention. 
       FIGS. 3A-3C  are diagrams illustrating the response of the voltage detector of  FIG. 2  to a varying supply voltage input. 
       FIG. 4  is a schematic diagram of a current source generator block, according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The preferred embodiments of the present invention and their advantages are best understood by referring to  FIGS. 1 through 4  of the drawings. Like numerals are used for like and corresponding parts of the various drawings. 
     FIG. 1  is a block diagram of a system  10  including a voltage detector  100 , a memory  106 , and a microprocessor  118 , according to an embodiment of the present invention. Voltage detector  100  can be implemented on a single chip, is capable of detecting both power-on and low power conditions, can be programmed to detect a large number of threshold point values, and consumes a low amount of current. 
   Voltage detector  100  is connected to a power supply (or supply voltage) Vdd and a ground GND. Voltage detector  100  is also connected to memory  106 , such as an electrically programmable read-only memory (EEPROM), and microprocessor  118 . Memory  106  stores a plurality of bits which correspond to and define a number of values for a threshold voltage that can be programmed in detector  100 . As used herein, the term “threshold voltage” refers to a value of supply voltage Vdd which is less than its maximum value and at which supply voltage Vdd may be considered to be at either a low power or power-off condition. For each threshold voltage value, a respective set of data bits may be provided in memory  106 . The plurality of bits are conveyed from memory  106  to a selectable threshold point circuit block  108  within voltage detector  100  via a plurality of control signals N. In one embodiment, a separate control signal may be provided to selectable threshold point circuit  108  for each bit of a bit set. The number of bits (or control signals N) determines the resolution of the programmable threshold voltage. The use of more data bits in each bit set will allow more values to be defined, and thus provide greater resolution. 
   Selectable threshold point block  108  is connected to a switch circuit at a node CC, a voltage following circuit board  107  at node AA, and to a memory  106 . Selectable threshold point circuit block  108  generally functions to select one of a number of values for the threshold voltage at which supply voltage Vdd is deemed to be at a low power or power-off condition. When the supply voltage Vdd has a value lower than the selected threshold voltage value, then selectable threshold point circuit block  108  may pull the voltage at node CC up to the level of Vdd. 
   Voltage following circuit block  107  is connected to supply voltage Vdd, selectable threshold point circuit block  108  at a node AA, and a current source generator at a node BB. Voltage following circuit block  107  follows or tracks supply voltage Vdd. Due to the operation of voltage following circuit board  107 , the voltage at node BB follows supply voltage Vdd. 
   Current source generator block  110  is connected to node BB and supply voltage Vdd. Current source generator block  110  generally functions to generate a current which is provided to node BB. 
   A switch circuit block  109  is connected to voltage following circuit block  107  and current source generator block  110  at node BB, and to selectable threshold voltage circuit block  108  at a node CC. Switch circuit block  109  generally functions as a switch. When supply voltage Vdd has a value greater then the selected threshold voltage, switch circuit block  109  pulls the voltage of node CC to ground. 
   A voltage level detection circuit block  116  is connected to node CC and external microprocessor  118 . Voltage level detection circuit block  116  generally functions to output a signal which indicates to microprocessor  118  whether the supply voltage Vdd is at a low power condition or a power-on condition. This is further described herein. 
   In operation, one of the bit sets stored in memory  106  is conveyed to selectable threshold point circuit block  108  via control signals N. The bit set essentially programs the selectable threshold point circuit  108 , thus selecting the threshold voltage value associated with that data bit set. From another perspective, the control signals define the magnitude of a pull-up current that flows from node CC through selectable threshold point circuit block  108 . Voltage following circuit block  107  tracks the power supply voltage Vdd and outputs a tracked voltage which appears at node BB, the input to switch circuit block  109 . The tracked voltage controls the magnitude of a pull-down current that flows from node CC through switch circuit block  109 . Node CC functions as a detection node for the threshold point. 
   When the magnitude of the voltage supply Vdd is lower than the selected threshold point, the pull-down current flowing through switch circuit block  109  is less than the pull-up current flowing through selectable threshold point circuit block  108 . This pulls the voltage level of node CC to the voltage level of supply voltage Vdd through selectable threshold point circuitry block  108 . The voltage level detection circuit block  116  detects this voltage value at node CC and, in response, outputs a signal which tracks the supply voltage Vdd. This signifies that the value of supply voltage Vdd is below the threshold point. 
   When the magnitude of the voltage supply Vdd is greater than the selected threshold point, the pull-down current flowing through switch circuit block  109  is greater than the pull-up current flowing through selectable threshold point circuit board  108 . This pulls the voltage level of node CC to ground through switch circuit block  109 . The voltage level detection circuit block  116  then detects this low voltage at node CC and, in response, outputs a driven and clearly defined low output to microprocessor  118 . This signifies that the value of the voltage supply Vdd is above the threshold point. 
     FIG. 2  is a schematic diagram of a circuit implementation for voltage detector  100  of  FIG. 1 , according to an embodiment of the present invention. In particular,  FIG. 2  depicts a number of circuits which correspond to voltage following circuit block  107 , selectable threshold point circuit block  108 , switch circuit block  109 , current source generator block  110 , and voltage level detection circuit block  116 . 
   Voltage following circuit block  107  includes a weak NMOS transistor  230  configured as a source follower. Voltage following circuit block  107  functions to track supply voltage Vdd. The gate terminal of transistor  230  is connected to supply voltage Vdd. Supply voltage Vdd is provided by power source  204 , which can be a battery. The source terminal of transistor  230  is connected to node BB. The drain terminal of transistor  230  is connected to selectable threshold voltage point block  108  at node AA. 
   Selectable threshold point circuit block  108  includes a number of PMOS current mirror transistors  232 ,  234 ,  236 ,  238 , and  240 , which function as current mirrors, coupled to a number of NMOS switch transistors  242 ,  244 ,  246 , and  248 , which function as switches. Selectable threshold point circuit block  108  functions to precisely set the value of the threshold voltage. A PMOS transistor  228  provides a reference for the current mirrors. The source and body terminals of current-reference transistor  228  are connected to supply voltage Vdd. The gate and drain terminals of current-reference transistor  228  are connected to current-mirror-reference node AA. The gate terminals of current mirror transistors  232 ,  234 ,  236 ,  238 , and  240  are connected to current mirror reference node AA. The source and body terminals of current mirror transistors  232 ,  234 ,  236 ,  238 , and  240  are connected to supply voltage Vdd. The drain terminal of current mirror transistor  232  is connected to node CC. The drain terminals of current mirror transistors  234 ,  236 ,  238 , and  240  are connected to the drain terminals of switch transistors  242 ,  244 ,  246 , and  248 , respectively. The source terminals of switch transistors  242 ,  244 ,  246 , and  248  are connected to node CC. The gate terminals of switch transistors  242 ,  244 ,  246 , and  248  are connected to memory  106  to receive control signals  208 ,  210 ,  212  and  214 , respectively, which are generated from bit sets in memory  106 . 
   The selected value of the threshold voltage is determined by the amount of current which is allowed to flow from selectable threshold voltage circuit block  108  to node CC. If more current is allowed to flow, then the value selected for the threshold voltage will be higher. Conversely, if less current is allowed to flow, then the value selected for the threshold voltage will be lower. 
   In operation, when any of the control signals  208 ,  210 ,  212  and  214  have a logic high value, the respective switch transistor  242 ,  244 ,  246 , and/or  248  is turned on. This allows current to flow through the respective current mirror transistor  234 ,  236 ,  238 , and/or  240  to node CC. When any of control signals  208 ,  210 ,  212  and/or  214  have a logic low value, the respective switch transistor  242 ,  244 ,  246 , and/or  248  is turned off. This prevents current from flowing through the respective current mirror transistor  234 ,  236 ,  238 , and/or  240  to node CC. Note that the current always flows through current-mirror transistor  232  to node CC since it is not controlled by a switch transistor. 
   The amount of current conducted by each individual current mirror transistor  232 ,  234 ,  236 ,  238 , and  240  is determined by its width to length (W/L) ratio. In one embodiment, current mirror transistors  232 ,  234 ,  236 ,  238 , and  240  each have a different W/L ratio. Thus, the total amount of current that flows to node CC from selectable threshold point circuit block  108  can be precisely set and controlled depending on which switch transistors  242 ,  244 ,  246 , and  248  have been turned on by respective control signals  208 ,  210 ,  212  and  214  (according to a particular data bit set). 
   In one embodiment, switch transistors  242 ,  244 ,  246 , and  248  have W/L ratios of 5μ/2μ. Current-mirror reference transistor  228  and current-mirror transistor  232  have W/L ratios of 8μ/4μ, current-mirror transistor  234  has a W/L ratio of 3μ/4μ, current-mirror transistor  236  has a W/L ratio of 6μ/4μ, current-mirror transistor  238  has a W/L ratio of 8μ/4μ, and current-mirror transistor  240  has a W/L ratio of 10μ/4μ. 
   Switch circuit block  109  includes a weak NMOS transistor  112 . Switch circuit block  109  allows node CC to be pulled up to the voltage level of supply voltage Vdd or down to ground GND depending on the amount of current being provided to node CC by selectable threshold point circuit block  108  and the value of the tracked supply voltage appearing at node BB. The gate terminal of transistor  112  is connected to node BB. The drain terminal of transistor  112  is connected to node CC. The source terminal of transistor  112  is connected to ground GND. When the value of supply voltage Vdd is above the selected value for the threshold voltage, transistor  112  is turned on, and the voltage at node CC is pulled low. 
   Current source generator block  110  includes an NMOS transistor  250  (which can be a depletion type transistor), NMOS transistors  252  and  254 , and a capacitor  256 . Transistor  252  is configured as a diode-connected current reference transistor and transistor  254  is configured as a current mirror. The drain terminal of transistor  250  is connected to supply voltage Vdd. The gate and source terminals transistor  250 , the gate and drain terminals of transistor  252 , and the gate terminal of transistor  254  are connected together at a node DD. The source terminals of transistors  252  and  254  as well as one terminal of capacitor  256  are connected to ground GND. The other terminal of capacitor  256  and the drain terminal of transistor  254  are connected to node BB. The gate-source connected transistor  250  sets the current level passed to current reference transistor  252 . This current level is relatively constant and independent of voltage source  204 . 
   In operation, transistor  250  provides current to current reference transistor  252 . This current is proportionately mirrored in current mirror transistor  254 . The mirrored current, acting against the pull-up current provided by PMOS transistor  228  (in the selectable threshold voltage circuit block  108 ) and weak NMOS transistor  230  (in the voltage-following circuitry block  107 ), sets the voltage at the node BB, that is, the gate voltage of weak transistor  112  (in switch block  109 ). The voltage at node BB increases with an increasing power supply voltage Vdd. Capacitor  256  stabilizes the voltage at node BB. 
   In one embodiment, NMOS transistors  252  and  254  both have W/L ratios of 4μ/4μ and capacitor  256  has a capacitance of 500fF. 
   Voltage level detection circuit block  116  includes PMOS transistors  260 ,  262 , and  264 , NMOS transistors  266 ,  268 , and  270 , and an inverter  272 . The source and body terminals of transistors  260  and  262  are connected to supply voltage Vdd. The gate terminal of transistor  262  is connected to node CC. The drain terminal of transistor  262  is connected to the source terminal of transistor  264  and to the drain terminal of transistor  260 . The body terminal of transistor  264  is connected to supply voltage Vdd. The gate terminal of transistor  264  is connected to node CC. The drain terminal of transistor  264  is connected to the drain terminal of transistor  266  and to the input terminal of inverter  272 . The gate terminal of transistor  266  is connected to node CC. The source terminal of transistor  266  is connected to the drain terminals of transistors  268  and  270 . The gate terminal of transistor  268  is connected to node CC. The source terminals of transistors  268  and  270  are connected to ground GND. The gate terminals of transistor  270  and transistor  260  are connected to the output terminal of inverter  272 . The inverter  272  outputs an output signal  274 . Transistors  262 ,  264 ,  266 , and  268  are connected as an inverter. This inverter, in combination with inverter  272 , forms a hysteresis circuit. Transistors  260  and  270  provide a feedback loop. 
   In one embodiment, transistor  268  has a W/L ratio of 10μ/2μ, transistor  266  has a W/L ratio of 7μ/1μ, and NMOS transistor  270  has a W/L ratio of 12μ/1μ. Transistor  262  has a W/L ratio of 20μ/2μ, transistor  264  has a W/L ratio of 15μ/1μ, and transistor  260  has a W/L ratio of 24μ/1μ. 
   In operation, voltage level detection circuit block  116  monitors the voltage at node CC, which indicates when supply voltage Vdd is less than or greater than the programmed threshold voltage. The threshold point for supply voltage Vdd is reached when the pull-up current from the selectable threshold point circuit block  108  is equals the pull-down current in transistor  112 . If the magnitude of the supply voltage Vdd is lower than the threshold point, the current flowing through selectable threshold point circuit block  108  pulls the voltage at node CC up to the level of Vdd. This indicates a low power condition. In contrast, if the magnitude of the supply voltage Vdd is greater than the threshold point, the current flowing through transistor  112  pulls the voltage at node CC to ground GND. This indicates a sufficient/adequate power condition. The voltage at node CC is put through the hysteresis circuit (consisting of two inverters) and a feedback loop to sharpen the output signal and to prevent it from responding to small signal perturbations on the node CC at the threshold point. 
     FIGS. 3A-3C  are diagrams illustrating the response of the voltage detector of  FIG. 2  to a varying supply voltage input.  FIG. 3A  shows the magnitude of the supply voltage Vdd and the magnitude of the voltage at node BB on the y-axis versus time on the x-axis.  FIG. 3B  shows the magnitude of the output signal  274  appearing at the output terminal of voltage detector  100  on the y-axis versus time on the x-axis when an exemplary threshold point B has been programmed.  FIG. 3C  shows the magnitude of the output signal  274  appearing at the output terminal of voltage detector  100  on the y-axis versus time on the x-axis when an exemplary threshold point A has been programmed. Although  FIGS. 3B and 3C  only shows the response for two programmed threshold points, threshold point A and threshold point B, skilled artisans will recognize that a large number of programmable threshold voltages can be precisely defined using selectable threshold point circuit block  108 . 
   Initially, as the level of supply voltage Vdd increases from 0.0 volts, both the voltage at node BB and the voltage at the output terminal of voltage detector (output signal  274 ) increase proportionally. When Vdd exceeds the programmed threshold point (approximately 2.5 volts for threshold point B and 4.3 volts for threshold point A), the voltage at node BB increases proportionally, but the voltage at the output terminal of voltage detector  100  (output signal  274 ) falls to approximately 0.0 volts. This signifies a power-on condition. 
   As long as the magnitude of the supply voltage Vdd is above the threshold point, the voltage at the output terminal of voltage detector  100  remains at approximately 0.0 volts. But, as the magnitude of supply voltage Vdd decreases from 5.0 volts, the voltage at node BB decreases proportionally. When the magnitude of the supply voltage Vdd decreases below the programmed threshold point (approximately 2.5 volts for threshold point B and 4.3 volts for threshold point A), the voltage at the output terminal of voltage detector  100  rises from 0.0 volts to the programmed threshold point. This indicates a low voltage condition. Then as the magnitude of the supply voltage Vdd continues to decrease, both the voltage at node BB and the voltage at the output terminal of voltage detector  100  decrease proportionally. 
     FIG. 4  is a schematic diagram of a current source generator block  210 , according to an embodiment of the present invention. Current source generator block  210  can be used as an alternative to current source generator block  110  of FIG.  2 . 
   Current source generator block  210 , like the current source generator block  110 , includes NMOS transistor  250 , NMOS transistors  252  and  254 , node DD, and capacitor  256 . Current source generator block  210  further includes NMOS transistors  300 ,  302 ,  304 , and  306 . NMOS transistors  300  and  302  are connected in series, as are NMOS transistors  304  and  306 . The source terminals of NMOS transistors  302  and  306  are connected to ground GND. The gate terminals of NMOS transistors  302  and  306  are connected to node DD. The drain terminals of NMOS transistors  300  and  304  are connected to node BB. The gate terminals of NMOS transistors  300  and  304  are connected to receive control signals  308  and  310 , respectively. Control signals  308  and  310  may be generated from data stored in memory  106 . 
   Transistor  252  functions as a current reference and each of transistors  254 ,  302 , and  306  function as current mirrors. Transistors  300  and  304  function as switches for current mirror transistors  302  and  306 , respectively. 
   In operation, when any of control signals  308  or  310  have a logic high value, the respective switch transistor  300  and/or  304  is turned on. This allows current to flow through the respective current mirror transistor  302  and/or  306  to node BB. When any of control signals  308  or  310  have a logic low value, the respective switch transistor  300  and/or  304  is turned off. This prevents current from flowing through the respective current mirror  302  and/or  306  to node BB. 
   While particular embodiments of the present invention and their advantages have been shown and described, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.