Abstract:
A voltage reference circuit ( 40 ) is provided for producing a low temerature-coefficient analogue trim value. A pair of EEPROMs ( 50  and  60 ) are arranged such that the trim value is the difference between two EEPROM transistor threshold voltages. The substantially temperature dependent components of threshold voltage cancel out leaving only the substantially temperature independent trim value. Therefor the temperature coefficient of the voltage reference circuit ( 40 ) is negligible.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to voltage reference circuits and particularly though not exclusively to analogue trim values in integrated circuits.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the field of this invention it is known in integrated circuits to produce an analogue trim value that is used to trim a reference voltage produced by the integrated circuit. It is highly desirable that this trim value is substantially independent of temperature variations, especially in automotive applications where large temperature ranges are specified.  
           [0003]    It is known to provide analogue trim values using fuses, zener zaps and EEPROMs to provide digital trim information to trim a voltage. It is also known to use EEPROMs and resistors together to store analogue trim information. However, a problem with these arrangements is that although a voltage may be trimmed precisely at a given temperature, the nature of analogue signals means that this precision is not well maintained over a range of temperatures. For an integrated circuit used in an automotive application, the operable temperature range may typically be −40° C. to 125° C.  
           [0004]    What is needed is an arrangement in which an analogue trim value is provided which is substantially temperature independent over such a temperature range. In other words, the trim value must have a negligible temperature coefficient.  
           [0005]    It is an object of the present invention to provide an EEPROM circuit, a voltage reference circuit and a method for producing a low temperature-coefficient voltage reference wherein the abovementioned disadvantages may be alleviated.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with a first aspect of the present invention there is provided an EEPROM circuit for providing a low temperature-coefficient voltage, comprising first and second EEPROM cells having first and second transistor threshold voltages and first and second control electrodes respectively, the first and second control electrodes being coupled together wherein the first threshold voltage is programmable independently of the second threshold voltage, such that the low temperature-coefficient voltage is provided as a voltage differential between the first and the second threshold voltages.  
           [0007]    In accordance with a second aspect of the present invention there is provided a voltage reference circuit comprising: the EEPROM circuit in accordance with the first aspect of the invention;  
           [0008]    a bandgap reference circuirt ( 90 ); and  
           [0009]    a current mirror ( 80 ) coupled between the EEPROM circuit ( 45 ) and the bandgap reference circuit ( 90 ), for transferring a scaled voltage value to the bandgap reference circuit ( 9 ),  
           [0010]    Wherein the scaled voltage value is a scaled proportion of the low temerature-coefficient voltage.  
           [0011]    In accordance with a third aspect of the present invention there is provided a method for providing a low temperature-coefficient voltage, the method comprising the steps of:  
           [0012]    programming a threshold voltage of a control electrode of a first EEPROM cell at a first voltage level: and, programming a threshold voltage of a control electrode of a second EEPROM cell at a second voltage level, the control electrode of the second EEPROM cell being coupled to the control electrode of the first EEPROM cell  
           [0013]    Wherein the low temperature-coefficient voltage is provided as a voltage differential between the first and the second threshold voltages.  
           [0014]    The first threshold voltage is preferably programmed independently of the second threshold voltage via a switch arrangement coupled between the first and second control electrodes of the first and second EEPROM cells respectively.  
           [0015]    Preferably the step of programming the threshold voltage of the control electrode of the second EEPROM cell includes the step of switching a switch arrangement coupled between the control electrodes of the first and second EEPROM cells. The low temperature-coefficient voltage is preferably an analogue trim value for trimming a reference voltage.  
           [0016]    In this way low temperature-coefficient analogue trim values are provided, which are particularly beneficial when used to trim voltage values in automotive applications where an integrated circuit may have an operational temperature range of typically −40° C. to 125° C. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    One circuit and method for producing a low temperature-coefficient analogue trim value incorporating the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0018]    [0018]FIG. 1 shows an illustrative circuit diagram of an EEPROM circuit having two EEPROM cells arranged to provide an analogue trim voltage in accordance with the present invention; and  
         [0019]    [0019]FIG. 2 shows a circuit diagram of a voltage reference circuit according to the present invention, using the arrangement of FIG. 1.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    Referring to FIG. 1, there is shown an illustrative circuit diagram showing an arrangement  5 , including first and second EEPROM cells  10  and  20  respectively and a resistor  30 . The first EEPROM cell  10  has a threshold voltage VT 10  and the second EEPROM cell has a threshold voltage VT 20 .  
         [0021]    The resistor  30  develops a trim voltage VR which is the difference between the threshold voltages VT 10  and VT 20  of two EEPROM cells  10  and  20  respectively.  
         [0022]    Referring now also to FIG. 2, there is shown a practical implementation  40  of the arrangement  5 , in which the difference in threshold voltages between two EEPROM cells is used to trim the voltage of a band-gap reference.  
         [0023]    Implementation  40  includes an EEPROM arrangement  45 , a current source  70 , a current mirror  80 , and a bandgap reference circuit  90 .  
         [0024]    The EEPROM arrangement  45  has first and second EEPROM cells  50  and  60  respectively, coupled in a similar fashion to that of the arrangement  5 . Each of the EEPROM cells  50  and  60  respectively has source, gate and drain electrodes. A switch  55  is coupled between the gate electrodes of the EEPROM cells  50  and  60  respectively, and the switch is also coupled to a programming voltage Vp to be further described below. The source electrode of the first EEPROM cell  50  is coupled to ground via a resistor  65  which is arranged to develop a voltage VEE to be further described below. The source electrode of the second EEPROM cell  60  is coupled directly to ground.  
         [0025]    The current source  70  is coupled between a power supply voltage Vcc and the drain electrode of the second EEPROM cell  60 . The current mirror comprises first and second transistors  82  and  87  respectively, which each have source, gate and drain electrodes. The source electrodes of the first and second transistors  82  and  87  respectively are coupled to the power supply voltage Vcc. The drain electrode of the second transistor  87  is coupled to the drain electrode of the first EEPROM cell  50  of the arrangement  45 , and also to the gate electrodes of both the first and second transistors  82  and  87  respectively. The drain electrode of the first transistor  82  is coupled to the bandgap reference circuit in a manner to be further described below.  
         [0026]    The bandgap reference circuit  90  comprises a differential amplifier  95 , a bipolar transistor  96 , first and second resistors  92  and  97  and a bandgap voltage source  93 . The differential amplifier  95  has a non-inverting input coupled to the voltage source  93 , an inverting input coupled to ground via the first resistor  92  and coupled to the drain electrode of the first transistor  82  of the current mirror  80  and an output.  
         [0027]    The bipolar transistor  96  has a base electrode coupled to the output of the differential amplifier  95 , a collector electrode coupled to the power supply voltage Vcc and an emitter electrode coupled to the inverting input of the differential amplifier  95  via the second resistor  97 , and to a reference voltage node  98 , arranged to provide a reference voltage VREF.  
         [0028]    In operation the voltage VEE across resistor  65  is the difference between the thresholds of the first and second EEPROM cells  50  and  60  respectively. The threshold of the second EEPROM cell  60  may be adjusted by programming using the programming voltage VP when the switch  55  is open. In this way the first EEPROM cell  50  remains unprogrammed. During normal operation of the circuit  40  the switch  55  is closed.  
         [0029]    In the circuit  40  of FIG. 2, the output reference voltage VREF is given by:  
               V   REF     =         V   BG     ·         R   1     +     R   2         R   1         -       V   EE     ·       R   2       R   3                   Equation                 1                               
 
         [0030]    The voltage VEE is scaled by resistors  97  and  65  and subtracted from a bandgap voltage VBG of the band-gap voltage source  93  (which in this example is 1.2V). The band-gap voltage VBG has a low temperature coefficient (TC), but the Integrated Circuit fabrication process causes about 3.5% variation in VBG at 1 standard deviation.  
         [0031]    VEE is linearly adjustable and has a low TC. The circuit  40  therefore has the advantages that VEE is scaled down and therefore variation in VEE is also scaled down. Furthermore, in the case of EEPROM failure VREF returns to its untrimmed value.  
         [0032]    Most of the temperature sensitive parameters affecting the threshold voltage of a transistor are due to the silicon substrate characteristics and not the gate oxide or polysilicon gate characteristics. When an EEPROM is programmed, charge is stored on the gate of the EEPROM. Regardless of whether an EEPROM is programmed or not the silicon characteristics remain unchanged (unless many cycles of program and erase are performed). Therefore the EEPROM threshold temperature dependency is controlled by the silicon parameters and these silicon parameters can be subtracted out leaving only the stored charge component.  
         [0033]    The TC of an EEPROM cell threshold voltage is substantially independent of charge stored. In other words the TC of the EEPROM threshold remains almost exactly the same regardless of the amount of charge stored on an EEPROM gate. The mean change in TC between erased and programmed states of a typical EEPROM is only 0.14 mV/C.  
         [0034]    EEPROM cells will normally lose some of their stored charge during their lifetime so the trim value can be expected to vary slightly over time. VEE is scaled down by resistors R 2  and R 3 , so that any variation in VEE will also be scaled down. Temperature accelerates charge loss. Over its life, the under-bonnet area of an automobile will spend typically 500 hours at 150° C. and the rest of the time below 110° C. Since charge loss is highly temperature dependent the time spent at less than 110° C. can be neglected.  
         [0035]    Though the reference circuit itself has a non-negligible TC, the analogue trim circuit adds almost no temperature dependence to VREF. In the above embodiment the analogue trim circuit only changes the temperature dependence by 0.05 mV/C.  
         [0036]    It will be understood that the circuit and method for producing a low temperature-coefficient analogue trim described above provides the following advantages:  
         [0037]    At least one fundamental difference between the present invention and a prior art arrangements in which EEPROM cells are used to store trim values is that the trim value of the present invention is stored as the difference between two EEPROM thresholds rather than as the value of a single programmed EEPROM cell.  
         [0038]    Therefore one advantage of the present invention for trimming voltage references is that the TC of the trim value is negligible. Other known solutions using digital trim to selectively switch resistors have a high TC because the TC of resistors is high. Solutions for analogue trim using single EEPROM cells have much higher TCs because the EEPROM threshold has a large TC.  
         [0039]    An additional advantage is that the circuit  40  is much smaller than other arrangements such as a digital trim arrangement using EEPROM cells. Trimming using an array of EEPROM cells requires a larger semiconductor area and involves high power consumption, high voltage switching circuitry.  
         [0040]    It will be appreciated that embodiments other than those mentioned above are possible. For example, the exact arrangements of the bandgap reference circuit and the current mirror may differ from those described above.