Abstract:
A differential voltage monitor is shown using a resistive bridge to produce a sample value representing that differential voltage. A portion of the resistive bridge is located on an IC and a portion is external to the IC. The object is to compare the differential voltage with a reference, producing a logic signal at a threshold level. The threshold level serving as a reference and the sample valve representing the differential voltage are made to be equally dependent on the bridge components located on the IC. In this way, any changes in the IC located bridge resistors compared to the bridge resistors located outside the IC are prevented from affecting the measurement.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of measuring voltages, in particular, differential voltages. 
     BACKGROUND OF THE INVENTION 
     It is well known to measure voltages, such as differential voltages, having a common mode component. Further, it is known to measure a range of differential voltage using an integrated circuit. Where that range of measured voltage is above the capability or input range of the integrated circuit, it has been known to use a resistor bridge using external resistive components to provide a lower voltage representing that differential voltage within the operating range of the integrated circuit. That representation, a fraction of the voltage to be measured, is then applied to the IC and a signal representative of that measured voltage is obtained. 
     A problem raised by this known method of measurement outside the range of an IC is the requirement for a resistor bridge using components exterior to the IC. Reduction of the components by inclusion of at least some of the bridge resistors on the IC compromises the measurement. This follows as the ratio of resistors outside the IC to those resistors inside the IC cannot be maintained with consistent accuracy required for the voltage division. This is due to the variation between a) the resistors built on IC&#39;s, due to processing and during operation, and b) the resistors external to the IC. 
     SUMMARY OF THE INVENTION 
     This invention minimizes the number of components external to an Integrated Circuit (IC), used in measuring a voltage beyond the range or capability of the IC. For example, where a voltage occurs outside an IC and beyond the range of the IC, and where it is desirable to measure that voltage using components on the IC, the inventive principles described here may be applied to make that measurement accurately and with consistency even though the value of the resistors on the IC relative to the value of resistances off the IC, may vary. 
     For example, in the preferred embodiment a voltage located outside an IC is to be measured. Where this voltage is beyond the range of the IC, a voltage divider is used to derive a representation of that voltage at a lower value, within the IC range. According to the inventive principles, some of the voltage divider components are located on the IC thus reducing the number of components external to the IC. Because of variations in voltage divider element values located on the IC relative to element values outside the IC, the lower representative voltage actually measured by the IC may vary. 
     The inventive principles as shown in the preferred embodiment solves this problem by providing a reference voltage that compensates for variations in the lower representative voltage caused by variations in the elements located internally on the IC relative to the elements off the IC. In this way, variations in the IC located elements relative to the associated elements located externally to the IC, are compensated and prevented from affecting the measured value. 
     In the preferred embodiment, a differential voltage is reduced by a voltage divider network to a divided differential voltage level. This divided differential voltage level is translated into a single ended output and compared to a reference voltage. The reference voltage is a portion of the voltage supply VBAT produced through a voltage divider network having elements or components on and off the IC. Comparison of the divided differential voltage level as a single ended output signal with the reference level provides an indication when the differential voltage has changed level and gone above or below the reference voltage level. This indication can be obtained, according to the inventive principles without any change in the measured differential voltage level due to variations between the voltage divider elements on and off the IC by a comparison with a reference level derived from voltage divider components on and off the IC. 
     Accordingly, the differential voltage which will produce an output signal indicative of a change in state when the differential voltage goes above or below the reference signal can be produced independent of elements on and off the IC. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 shows in generalized form the prior art system for measuring a voltage beyond the range of an IC, by the IC. 
     FIG. 2 shows in the preferred embodiment according to the inventive principles, the system for measuring a voltage beyond the range of an IC, using components located on the IC. 
     FIG. 3 shows an alternative embodiment wherein the resistor between the transistor Q1 and ground differs from RB. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows the prior system for measuring a voltage beyond the range of an IC shown within the dashed line box, 1, by components on the IC. As shown in the preferred embodiment, the voltage to be measured is a differential voltage shown as V DIF . Because the common mode component of that voltage, V DIF , is higher than the measurement capability of the IC, a lower representative voltage shown as V IN  is sampled using the bridge network composed of resistances RA and RB. V IN , as is well known, then is applied to a differential to single ended converter 3. The output of that converter 3 is then applied to a comparator 5 which compares a reference voltage to V IN  to produce the measurement voltage at terminal 15. The reference voltage V REF  is derived using divider network R C1  -R C2 . R C1  may be an adjustable or fixed resistor. 
     As is well known, the resistor bridge, composed of the two matched resistors RA and the two matched resistors R B , reduces the common mode voltage at the input of the IC to: ##EQU1## 
     The resistor bridge also reduces the differential voltage V DIF  to be measured by the ratio: ##EQU2## 
     Where the values of R A  and R B  are maintained in a fixed ratio, the ratio V IN  (the representation of V DIF  presented to the IC) to V REF  will remain constant. If the ratio of R A  to R B  changes, then the ratio of V IN  to V REF  will change causing an inaccuracy in the measured value at terminal 15. 
     Where it is desired to minimize components external to the IC1, such as for example, the resistors R B  of the resistance bridge R A  /R B , inaccuracies may occur. In the preferred embodiment, the resistances R B  of the resistance bridge R A  /R B  are placed on the IC, 1, minimizing components off the IC. 
     As it is not possible in most IC technologies to produce IC internal resistors that match either the initial value or temperature coefficients of external resistors, the solution shown in FIG. 1 would lead to unacceptable accuracy in the measured value at terminal 15. 
     The inventive principles with regard to the preferred embodiment are now shown in FIG. 2. 
     This invention solves the problem of locating at least some of the R A  /R B  bridge resistors on the IC1. The preferred embodiment according to the inventive principles solves the problem by providing a reference voltage V REF  that compensates for variations in V IN  caused by variations in the bridge resistors R B  located on the IC relative to resistors R A . In the preferred embodiment, V IN  is a representative voltage of V DIF . In the preferred embodiment the resistance bridge is shown as comprising resistors R A  and R B . Those resistances labeled R B  of the resistance bridge are located on the IC. It is variations in these resistances R B  located on the IC relative to the resistances off the IC, causing variations in the representative voltage V IN  that is compensated by the inventive principles as shown in the preferred embodiment. 
     In FIGS. 1 and 2 the same numerals refer to the same or similar components. A source follower Q 1  is shown between R C  and R B . 
     The differential voltage V IN , at IC1 representative of V DIF  is: ##EQU3## where V DIF  is the differential voltage to be measured and V IN  is the representative differential voltage at the input of the IC. As stated above, V IN  is a representative voltage of V DIF , reduced from V DIF  to accommodate the operating capability of the IC1. 
     V X  is the voltage at node 13 produced by the R A  /R B  voltage divider element connected to VBAT. ##EQU4## 
     Operational amplifier 7, and Q1, shown in the preferred embodiment as a PMOS FET are arranged to force the voltage at the Q1 source at node 14 to be equal to V X  at node 13. The voltage across the resistor R C  then is V BAT  -V X  : ##EQU5## 
     The current flowing through R C  becomes: ##EQU6## 
     This current I RC  is applied to node 21 and to a resistor with the value R B  to develop the voltage reference V REF . The voltage reference VREF has the value: ##EQU7## 
     V REF  appears at the node 21 and is applied to one of the inputs of comparator 5. The IC1 then compares V IN , the representative of differential voltage V DIF  against V REF . 
     Comparator 5 switches its output state at the point where its two inputs are equal, that is V IN  =V REF . 
     If the equations shown above for V IN  and V REF  are set equal to each other the resulting relationship of V DIF  to R A , R C , and V BAT  shown below as: ##EQU8## 
     This reduces to: ##EQU9## 
     Accordingly, the internal resistance, R B , is effectively removed from the process and the relation of V DIF  to V BAT  which causes comparator 5 to switch is a function of external resistors R A  and R C . Accordingly, R B  the internal resistor, does not affect the differential voltage measurement. 
     Resistor RB located between Q1 and ground can be a different value then the value of the other RB resistors. For example as shown in FIG. 3, Resistor RB 1  may be a different value than the two resistors RB 2 . According to the principles of the invention, as disclosed for the preferred embodiment RB 1  can be related to RB 2  by a definite ratio. That ratio can be designated K, that is R B1  =KR B2 . 
     If RB1 and RB2 are substituted for RB and VIN set equal to V REF , then ##EQU10## this reduces to: ##EQU11## substituting ##EQU12## 
     As shown V DIF  is dependent on R A  and R C  and independent of R B . The multiplier K permits flexibility in selecting components and R B1  need not be made equal to R B2 . 
     One application of the inventive principles may be to measure the voltage drop across an electrical component. For example, the electrical component may be an NDMOS. The voltage across that NDMOS may be the V DIF  input. VBAT, representing the voltage supply may have very high positive and negative transients. This circuit may be used to measure excess current flow in the NDMOS by monitoring the voltage drop across the NDMOS. R C  may be used to adjust for a desired threshold level representative of a threshold level for V DIF , for example. 
     According to the inventive principles, a voltage outside the range of an IC may be measured in comparison with a reference using a divider network to produce a representative value of that voltage and where a portion of that divider network is located on the IC. 
     As shown in the preferred embodiment and according to the inventive principles, a signal can be produced at terminal 15 indicative of a change of state where such change of state may represent a signal crossing a trip point or threshold level. In the case of the preferred embodiment, the trip point is a threshold level for the differential input voltage V DIF . The trip point may be set as shown above as a percentage of V BAT  by setting the ratio of R A  and R C  accordingly. 
     V IN  is a divided differential voltage level produced from V DIF  through the resistive voltage divider R A  -R B . The amplifier 3 produces a single ended output to operational amplifier 5 indicative of the level of V IN . Operational amplifier 5 has very high gain and operates as a comparator having as a second input V REF . V REF  is derived through a voltage divider network having components off and on the IC, similar to the voltage divider components used to produce V IN . Accordingly, any variations in the voltage divider network having components on and off the IC and used to produce V REF  and the voltage divider components on and off the IC used to produce V IN  and representative of V DIF , compensate, removing the effect of such variations on the measurement. Accordingly, by proper adjustment of R C , a threshold level or trip level may be established for V DIF  as a percentage of V BAT . A state change signal may be produced at terminal 15 whenever V DIF  changes value and crosses that said threshold level. 
     As would be apparent to those skilled in the art comparative 15 is a high gain amplifier suitable for producing a state change signal. 
     The inventive principles may be applied to measuring any voltage requiring a divider network to produce a representative value. The inventive principles are not limited to resistive bridges or to voltage divider resistive elements partially located on an IC, or to the production of a state signal, but may be applied generally to the use of combined circuit elements located off and on the IC and causing variations on an output signal due to variations between the elements on the IC and off the IC.