Abstract:
A system and method for detecting a low battery voltage supplied to a battery operated integrated circuit. A stable reference voltage provided by a bangap reference is compared with the battery voltage. A switched capacitor circuit is used instead of the more conventional resistor combination to supply a scaled representation of the battery voltage. Power requirements are reduced by combining the bandgap reference and the comparator into a single component.

Description:
FIELD OF THE INVENTION 
     This invention relates to battery operated circuits and, more particularly, to a method and system for detecting low battery voltage utilizing a stable reference voltage and a switched-capacitor circuit. 
     BACKGROUND OF THE INVENTION 
     In many battery operated applications, some means for detecting a low battery voltage is needed. In such applications, a warning signal might be generated or, alternatively, vulnerable circuits within a system might be selectively disabled by an appropriate control signal in order to avoid damage to or malfunction of such circuits. A straightforward method of implementing a low voltage detection function is to compare a scaled battery voltage and a stable reference voltage such as a voltage from a bandgap circuit. It is known, as will be discussed in greater detail later, that a bandgap circuit can generate a stable, substantially temperature independent reference voltage. Normal time-continuous bandgap references typically need well-matched resistors and transistors and some sort of trimming to get good accuracy. 
     U.S. Pat. No. 5,196,833, which issued Mar. 23, 1993, to Kemp describes a low voltage detection circuit which includes a bandgap reference and a differential comparator circuit for generating a signal proportional to the level of the supply voltage. Since many battery operated circuits are subject to ambient temperature variations, such as an automobile application as disclosed in the U.S. Pat. No. 5,196,833, it is important that some form of temperature compensation be used in deriving the reference voltage. The bandgap cell, as disclosed in the aforementioned patent, includes bipolar transistors in which the base to emitter voltage is used as a reference source. Typically, two transistors are used having different current densities through each. In the U.S. Pat. No. 5,196,833, the different current densities are achieved by using four parallel transistors in one current path and a single transistor in the second. It has been shown that the base to emitter voltage (V BE ) of a bipolar transistor exhibits a negative temperature coefficient with respect to temperature. On the other hand, it has also been shown that the difference of base to emitter voltages ΔV BE  of the two bipolar transistors operating at different current densities exhibit a positive temperature coefficient with respect to temperature. Thus, the sum of the base to emitter voltage V BE  of a bipolar transistor and a differential voltage ΔV BE  will be relatively independent of temperature when the sum of the voltages equals the energy gap of silicon. Such temperature stable references have been created by generating a V BE  and summing a ΔV BE  of such value that the sum substantially equals the bandgap voltage of 1.205V. 
     U.S. Pat. No. 5,814,995, which issued Sep. 29, 1998, to Tasdighi, discloses a voltage detector for a battery operated device employing two bipolar transistors and an operational amplifier as a comparator. The system of this reference also employs resistors which, as noted hereinbefore, must be well-matched and usually require some sort of trimming to obtain good accuracy. 
     U.S. Pat. No. 4,375,595, which issued Mar. 1, 1983, to Ulmer et al., relates to a temperature independent bandgap reference which employs switched capacitors to input the V BE  and ΔV BE  of the bipolar transistors. A proper selection of the ratio of the switched capacitors is used to get around the need for matched and/or trimmed resistors. The switched capacitor implementation employs clock signals in order to establish a precharge phase and an output reference stage. In the U.S. Pat. No. 4,375,959, three separate clock signals are required. 
     U.S. Pat. No. 5,563,504, which issued Oct. 8, 1996, to Gilbert et al., also relates to a switching bandgap voltage reference in which one bipolar transistor is used with two different current sources providing the current densities needed to obtain the ΔV BE  value. 
     Thus, in an application where a battery voltage detector is required in order to detect a low voltage value, and providing that a suitable clock exists within the application, a switched capacitor architecture can provide an effective solution. 
     SUMMARY OF THE INVENTION 
     The present invention provides lower untrimmed errors by utilizing offset cancellation techniques. This, in combination with a switched capacitor network having well-matched capacitors provide suitable weighting factors. 
     The invention also provides for a reduced component count by combining a voltage reference circuit and a voltage comparator into a single circuit. 
     In a preferred embodiment, the comparator implementation eliminates the need to actually generate a 1.2V bandgap reference voltage. 
     The invention also provides for lower power requirements by eliminating part of the static current by replacing resistors in the bandgap circuit with a switched capacitor circuit. 
     Therefore, in accordance with a first aspect of the present invention there is provided a detector for providing a low battery voltage indication comprising: a stable voltage reference and comparator circuit for comparing the battery voltage and the voltage reference; switch means to switch between a first operational state and a second operational state; a switched capacitor circuit to store charges related to the battery voltage and the reference voltage in each operational state; and a clock to initiate switching between the first and second states, wherein an output from the comparator during the second state indicates whether the battery voltage is below a preset threshold. 
     In accordance with a second aspect of the present invention there is provided a method of detecting a low battery voltage supplied to an integrated circuit comprising: providing a stable voltage reference and comparator circuit for comparing battery voltage against the reference voltage; providing a capacitor circuit for storing charges associated with the battery voltage and the reference voltage; providing switching means to switch between a first phase wherein the capacitors are charged to a first voltage level, and a second phase wherein the capacitors are charged to a second level; and comparing the first and second levels to determine whether the battery voltage is above or below a preset threshold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail with reference to the attached drawings wherein: 
     FIG. 1 illustrates a prior art voltage detector utilizing resistors; 
     FIG. 2 illustrates a switched capacitor, temperature-independent, bandgap reference utilizing three input clocks; 
     FIG. 3 is a simplified schematic of the low battery voltage detector according to the present invention; 
     FIG. 4 illustrates a second embodiment of the bandgap reference circuit; and 
     FIG. 5 is a circuit diagram of the CMOS low voltage battery detector according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a basic, low battery voltage detector  2  according to the prior art. The scaled battery voltage taken across the voltage divider circuit comprising resistors  4  and  5  is supplied to the positive input of an operational amplifier  6  and the voltage reference  7  which in a preferred embodiment is a bandgap reference is supplied to the negative input of the operational amplifier  8 . The output of the operational amplifier is 0 if the battery voltage is low and is a 1 otherwise. 
     A discussed previously, the voltage divider circuit comprising the resistors  4 , 5  increases the power requirement to the detector circuit and can lead to measurement inaccuracies. 
     To overcome the inaccuracies created by the resistor combination, a switched capacitor circuit  10  as shown in FIG. 2 has also been disclosed in the prior art. In this circuit bipolar transistors  12  and  14  are used to obtain the stable bandgap reference voltage. Comparator  42  evaluates the stored voltage across switched capacitors  28  and  34 . Clock  16  provides clock signals A, B and C to switches  30 ,  32 ,  36 ,  38  and  48 . 
     The present invention provides a low voltage monitoring circuit  100  utilizing the switched capacitor arrangement which is illustrated at a high level in FIG.  3 . The circuit  100  includes switches  110 ,  112 ,  114 ,  116 ,  118  and  120 , operational amplifier  122  bipolar transistors  124  and  126 , current source  128 , capacitors  130  (C 1 ),  132  (C 2 ),  134  (C 3 ) and battery voltage input  136 . 
     As mentioned previously, the object of the invention is to compare an appropriately scaled battery voltage to a stable reference voltage. The reference voltage from the bandgap reference is the sum of a V BE  with a negative temperature coefficient and a K 1 ΔV BE  with a positive temperature coefficient, were K 1  is a scaling factor chosen to balance the negative and positive coefficient and ΔV BE  is the difference in V BE  between the two transistors with different current densities. The sum of V BE  and K 1 ΔV BE  is equal to the silicon bandgap voltage or approximately 1.2 volts. 
     In essence, the object of the invention is carried out by determining the sign of the expression V BE +K 1 ΔV BE −K 2 V BAT . In evaluating this expression it is possible to see whether V BAT  is above or below a minimum voltage V min. The value of the Vmin depends on K 2  and is approximately 1.2/K 2  volts 
     In a switched capacitor circuit, the above mentioned three voltages can be sampled on capacitors C 1 , C 2  and C 3  and then summed to get the answer. The size of each capacitor controls the scaling of each voltage and the accuracy of the scaling is normally a magnitude better then if resistors are used. Another inherent advantage of the present invention is that the typical 1.2 volt bandgap reference voltage is actually never created, so in principle the circuit will work for supply voltages below 1.2 volts. Further, there is not a problem in setting Vmin to be less than 1.2 volts. 
     As discussed previously, a clock signal (not shown in FIG. 3) is required in the implementation of the detector circuit. The clock is required to generate two phases or stages which are shown in FIG. 3 as phase 1 (Φ1) and phase 2 (Φ2). During phase 1 or the pre-charge phase switches  110 ,  112 ,  114  and  116  are closed while switches  118  and  120  are open. In phase 2 switches  110 ,  112 ,  114  and  116  are open while switches  118  and  120  are closed. All of the above switches are shown in FIG. 1 as generic switches for simplicity while it is known that these switches in a preferred embodiment are actually MOS devices as shown in greater detail in FIG.  5 . 
     In pre-charge phase 1 switch  114  is closed so that current from current source  128  flows through both transistors  124  and  126 . Hence both transistors are active and the positive input of operational amplifier  122  will be biased at V BE  also known as V BElow . Since switch  116  is also closed the negative input of operational amplifier will also be raised to V BElow . Since switch  110  and  112  are closed and switch  120  open, capacitor  130  and capacitor  132  will be charged to V BElow +V offset  where V offset  is the off set voltage in the operational amplifier  122 . Capacitor  134  will be charged to V offset . 
     During the second stage or phase 2, switches  110 ,  112 ,  114  and  116  are opened while switches  118  and  120  are closed. Now all of the current from source  128  flows through the transistor  126  and this higher current density results in a higher base to emitter voltage across transistor  126 . The positive input of operational amplifier  122  is now raised to V BE2  or V BEhigh . As discussed previously the difference between V BEhigh  and V BElow  is the ΔV BE  value. It has a value kT/qln (N+1) when N is the relative emitter area of transistor  124 . 
     When the battery voltage V BAT  at input  136  is at the threshold or trip point, V min  the negative input of operational amplifier  122  will be at a voltage equal to V BEhigh  plus V offset . Capacitor  130  is charged to V BEhigh +V offset −V BAT  while capacitor  132  and capacitor  134  are charged to V BEhigh +V offset . The transfer of charges between the capacitors when switching between phase 1 and phase 2 can be written has: C 3 V BEhigh +(C 1 +C 2 )ΔV BE −C 1 V BAT . It will be recognized that this is similar to the expression previously given as V BE +K 1 ΔV BE −K 2 V BAT . In this expression the weighting factor K 1  will be determined by (C 1 +C 2 )/C 3  and the factor K 2  is given by C 1 /C 3 . It is significant that the off set voltage in the operational amplifier  122  will not affect the above result as long as it does not change when the reference voltage at the positive input changes from V BElow  to V BEhigh  so a small and simple amplifier can be used to save current and area. 
     At relatively low operating voltages MOS switches can have a sufficient on resistance especially if they are biased at a voltage between the supply voltages. This is not a problem in a switched capacitor circuit as long as the time constants are much shorter then the clock. But the switch in series with the transistor  124  will have a DC current and this will result in a voltage drop and consequently reduced accuracy. A different topography may therefor be used to switch transistor  124  on and off and this alternate topography is shown in FIG.  4 . In this implementation, two current sources are provided and switch  114  is replaced with switches  142  and  144 . In phase 1 switch  144  is closed and the switch  142  is open and in phase 2 switch  142  is closed while switch  144  is open. Therefore in phase 1 the base of transistor  124  is shorted to ground and the transistor is active. In phase 2 the base of transistor  124  is biased at 1V BE  above ground. If the transistors  126  and  140  are matched the value V BE  is the same voltage as the emitter voltage at transistor  124  so that V BE  is 0 volts and the transistor will be turned off. There is still current flowing through switch  144  to ground during phase 1 but only the base current. With the switch connected to ground the on resistance will also be lower. 
     FIG. 5 is a detailed circuit diagram of the CMOS low voltage detector circuit including the circuit topography of FIG.  4 . 
     Although specific embodiments of the invention have been described and illustrated it will be apparent to one skilled in the art that numerous variations can be made without departing from the basic concept. It is to be understood, however that such variations will fall within the true scope of the invention as defined in the appended claims.