Abstract:
A high shunt regulator provides precise voltage over process, temperature, power supply, and foundries. The HV level is settable by a digital control bits such as fuse bits. A filter network filters out the ripple noise and charge transient. A tracking capacitor divider network speeds up response time. A fractional band gap reference provides fractional bandgap voltage and current, and operates at low power supply and has superior power supply rejection. It is unsusceptible to substrate hot carrier effect. It exposes very little to drain induced barrier lowering effect. The bandgap core has better than conventional transient response and stability. One embodiment has adjustable level control. Complementary TC (temperature coefficient) trimming allows efficient realization of zero temperature coefficients of current and voltage. Higher order curvature correction of voltage and current is integrated. Replica bias for the control loop is presented. A Binary and Approximation Complementary TC search trimming is described. A zero TC fractional voltage less than the theoretical bandgap voltage (&lt;&lt;˜1.2 Volt) is realizable. The bandgap core has a filtering mechanism to reject high frequency noise. A low power startup circuit powers up the bandgap. The bandgap also has variable impedance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to U.S. patent application Ser. No. ______ (Attorney Docket No. 2102397-992410), filed on even date herewith, entitled “Curved Fractional CMOS Bandgap Reference”, inventor Hieu Van Tran, the disclosure of which is incorporated herein by reference. 
     
    
     
       BACKGROUND  
         [0002]    The invention relates to high voltage regulators, and more particularly high voltage regulators including a shunt regulator and/or a bandgap reference generator.  
           [0003]    A conventional mixed mode integrated circuit system frequently uses different voltage supplies. Analog signal processing, such as amplification, comparison, and pulse generation, may be performed at high voltage. A FLASH memory applies an erase signal and a program signal to memory cells. The erase signal and the program signal have voltage levels greater than a supply voltage. Also in multilevel volatile memories, the variation of the voltage level of the program signal falls in a smaller range for the multibit signals stored in the memory cells.  
           [0004]    A high voltage supply is typically used on-chip for non-volatile programming, erasing, and read operations. High voltage is generated typically from a charge pump utilizing capacitors. Regulation of the charge pumped high voltage provides precise voltage level for chip operation. The regulation is typically done using Zener-based techniques.  
         SUMMARY  
         [0005]    In one aspect, the present invention uses a bandgap including a mixed op amp operated in a continuous mode to provide precise voltage over process, temperature, power supply, and foundries. A HV level is provided at different level for different chip operations and is settable by digital control bits, such as fuse bits at power up and/or at initialization of chip operations. A filter network filters out the ripple noise and charge transient. A mixed scheme helps to achieve the regulation, and may have both low voltage and high voltage devices as part of a circuit block to minimize area. The bandgap may also include certain elements to achieve more than one circuit function. A simulated resistor using HV PMOS in a certain configuration to achieve a precision divider ratio. A tracking capacitor divider tracks the simulated resistor ratio to speed up the response time.  
           [0006]    A bandgap architecture is desirable to provide fractional bandgap voltage (&lt;1.2V) and current that is suitable for nano-meter process technology. As technology progresses into the nano-meter regime, transistor performance is susceptible to secondary effect such as channel length modulation (CLM), breakdown (BV), gate or drain induced lowering (GIBL or DIBL), direct tunneling. Hence a circuit architecture that mitigates these effects is desirable. In addition, for nano-meter technology, power supply level is reduced significantly, hence fractional level is desired.  
           [0007]    In another aspect, the present invention provides fractional bandgap voltage and current at the same time. It works at low power supply and has superior power supply rejection. It is not susceptible to substrate hot carrier effect. It has very little exposure to drain induced barrier lowering effect. The bandgap core has better than conventional transient response and stability. One embodiment has adjustable level loop control. Complementary TC (temperature coefficient) trimming allows efficient realization of zero temperature coefficients of current and voltage. Higher order curvature correction of voltage and current is integrated. Replica bias for the control loop is presented. Binary and Approximation Complementary TC search trimming is described. A zero TC fractional voltage less than the theoretical bandgap voltage (&lt;&lt;˜1.2 Volt) is realizable. The bandgap core has a filtering mechanism to reject high frequency noise. The invention includes low power startup circuits to power up the bandgap. The bandgap also has variable impedance. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]    [0008]FIG. 1 is a block diagram illustrating a non-volatile multilevel memory system.  
         [0009]    [0009]FIG. 2 is a block diagram illustrating a high voltage shunt regulator of a high voltage power generator of the non-volatile multilevel memory system of FIG. 1.  
         [0010]    [0010]FIG. 3 is a schematic diagram illustrating a conventional bandgap reference generator.  
         [0011]    [0011]FIG. 4 is a graph illustrating the drain-source current versus drain-source voltage characteristic of a typical sub-micron metal-oxide-silicon field effect transistor (MOSFET).  
         [0012]    [0012]FIG. 5A is a schematic diagram of a bandgap reference generator of the high voltage shunt regulator of FIG. 2.  
         [0013]    [0013]FIG. 5B is a schematic diagram of another bandgap reference generator of the high voltage shunt regulator of FIG. 2.  
         [0014]    [0014]FIG. 6A is a block diagram illustrating a trimmable resistor of the bandgap reference generator of FIG. 5A.  
         [0015]    [0015]FIG. 6B is a block diagram illustrating a trimmable resistor of the bandgap reference generator of FIG. 5B.  
         [0016]    [0016]FIG. 7 is a schematic diagram illustrating a bandgap reference generator having cascoding in an alternate embodiment.  
         [0017]    [0017]FIG. 8 is a schematic diagram illustrating a current summer.  
         [0018]    [0018]FIG. 9 is a schematic diagram illustrating a current to voltage converter.  
         [0019]    [0019]FIG. 10 is a schematic diagram illustrating a bandgap reference generator according to another embodiment.  
         [0020]    [0020]FIG. 11 is a schematic diagram illustrating a bandgap reference generator including a replica biased operational amplifier.  
         [0021]    [0021]FIG. 12 is a schematic diagram illustrating a replica biased operational amplifier of the bandgap reference generator of FIG. 11.  
         [0022]    [0022]FIG. 13 is a schematic diagram illustrating a bandgap reference generator including a startup circuit.  
         [0023]    [0023]FIG. 14 is a schematic diagram illustrating a startup circuit.  
         [0024]    [0024]FIG. 15 is a schematic diagram illustrating a startup circuit.  
         [0025]    [0025]FIG. 16 is a block diagram illustrating a binary complementary trimming circuit.  
         [0026]    [0026]FIG. 17 is a graph illustrating the temperature coefficient current using binary complementary temperature coefficient trimming.  
         [0027]    [0027]FIG. 18 is a graph illustrating the generation of a complementary temperature coefficient current.  
         [0028]    [0028]FIG. 19 is a block diagram illustrating a complementary temperature coefficient current generator.  
         [0029]    [0029]FIG. 20 is a graph illustrating the generation of a complementary temperature coefficient voltage.  
         [0030]    [0030]FIG. 21 is a schematic diagram of a complementary positive temperature coefficient voltage generator.  
         [0031]    [0031]FIG. 22 is a flowchart illustrating an operation of approximation complementary trimming.  
         [0032]    [0032]FIG. 23 is a schematic diagram illustrating a low voltage current mirror bandgap reference.  
         [0033]    [0033]FIG. 24 is a schematic diagram illustrating a current trim circuit.  
     
    
     DETAILED DESCRIPTION  
       [0034]    As used herein, a N-type NMOS enhancement transistor is an enhancement transistor having a gate threshold, for example in the range of approximately 0.3 to 1.0 volts. A P-type transistor is a PMOS enhancement transistor having a gate threshold approximately in the range of −0.3 to −1.0 volts. A NZ NMOS transistor is a native low voltage transistor having a gate threshold approximately in the range of −0.1 to 0.3 volts. An NH NMOS transistor is an enhancement high voltage transistor having a gate threshold approximately in the range of 0.3 to 1.0 volts. A PH PMOS transistor is an enhancement high voltage transistor having a gate threshold of approximately in the range −0.3 to −1.0 volts. An NX NMOS transistor is a native high voltage transistor having a gate threshold voltage approximately in the range −0.1 to 0.3 volts.  
         [0035]    As used herein, the symbol VBE x  is the voltage across the base-emitter of a transistor x, and a resistance R y  is the resistance of a resistor y.  
         [0036]    [0036]FIG. 1 is a block diagram illustrating a non-volatile multilevel memory system  100  according to the present invention.  
         [0037]    The non-volatile multilevel memory system  100  comprises a memory array  102  and a high voltage power generator  104 . The high voltage power generator  104  generates a regulated high voltage supply signal (VSUPHV)  103 . For clarity and simplicity, only one regulated high voltage supply signal  103  is shown and described herein. However, voltage signals having different voltage levels may be generated as appropriate for programming, reading, erasing, and verifying the memory array  102 . The non-volatile multilevel memory system  100  also comprises control logic (not shown).  
         [0038]    The memory array  102  comprises a plurality of memory cells (not shown), a plurality of sense amplifiers (not shown), a plurality of decoders (not shown). The memory cells may include data cells and reference cells. The memory cell may store multilevel digital data. In one embodiment, the memory cells are arranged in 16K rows by 8K columns. In one embodiment, the memory array includes a source side injection flash technology, which uses lower power in hot electron programming and efficient injector based Fowler-Nordheim tunneling erasure. The programming is done by applying a high voltage on the source of the memory tell, a bias voltage on the control gate of the memory cell, and a bias current on the drain of the memory cell. The erase is done by applying a high voltage on the control gate of the memory cell and a low voltage on the source and/or drain of the memory cell. The verify (sensing or reading) is done by placing the memory cell in a voltage mode sensing, e.g., a bias voltage on the source, a bias voltage on the gate, a bias current (or zero current) on the drain, and the voltage on the drain is the readout voltage. In another embodiment, the verify (sensing or reading) is done by placing the memory cell in a current mode sensing, e.g., a low voltage on the source, a bias voltage on the gate, a load (resistive or transistors) coupled to the drain, and the voltage on the load is the readout voltage. In one embodiment, the array architecture is the one disclosed in U.S. Pat. No. 6,282,145, entitled “Array Architecture and Operating Methods for Digital Multilevel Nonvolatile Memory Integrated Circuit System” by Tran et al., the subject matter of which is incorporated herein by reference.  
         [0039]    The high voltage power generator  104  comprises a charge pump  106 , a filter  108 , a fuse circuit  110 , a bandgap generator  112 , and a high voltage shunt regulator  114 .  
         [0040]    In a normal operation mode, the charge pump  106  is enabled to convert a voltage from a power supply (VSUP) to a high voltage suitable for non-volatile memory operation, such as program, erase, and read operation. In one embodiment, the charge pump  106  may be the charge pump disclosed in pending U.S. patent application Ser. No. 10/044,273, entitled “High voltage generation and regulation system for digital multilevel nonvolatile memory”, filed Jan. 10, 2002, the subject matter of which is incorporated herein by reference. The output of the charge pump  106  may be regulated to a precise voltage that functions as a high voltage supply source, and may be wave-shaped and applied to the decoders (not shown) and subsequently to the memory cells (not shown) in the memory array  102 .  
         [0041]    The filter  108  filters out ripple of high frequency noise from the operation of the charge pump  106  to form a high voltage supply signal and also may function as a charge reservoir for transient program, read, or erase operation In one embodiment, the filter  108  is a resistor-capacitor filter. In another embodiment the filter  108  is a diode-capacitor filter, in which a diode substitutes for the resistor in series with a capacitor. In another embodiment, the filter  108  is a diode-resistor-capacitor filter, in which a diode is in series with the resistor in series with the capacitor. The diode may be a PN junction diode or a metal-oxide-silicon (MOS) transistor with gate and drain tied together. Another embodiment of the bandgap does not include the filter  108 .  
         [0042]    The fuse circuit  110  stores digital data that are used to set voltages and control signals. The fuse circuit  110  includes control logic (not shown) that decodes the stored digital data to set the control signals. As described below, the fuse circuit  110  sets an output high voltage level at power up or at the start of an operation, such as program, erase or read. The output high voltage level may be different for program, erase, or read.  
         [0043]    The bandgap generator  112  provides precise voltage level signals over process, temperature, and supply as desired for multilevel programming, erasing, and sensing. The bandgap generator  112  provides a zero temperature coefficient voltage (V0TC)  116  and a zero temperature coefficient current (I0TC)  118 . The zero temperature coefficient voltage (V0TC)  116  and the zero temperature coefficient current (I0TC)  118  may be trimmable based on the control signals from the fuse circuit  110 . The bandgap generator  112  may be, for example, a bandgap reference generator  500  (see FIG. 5), or a bandgap reference generator  700  (see FIG. 7).  
         [0044]    The high voltage shunt regulator  114  regulates the high voltage supply signal from the filter  108  in response to a trimmable zero temperature coefficient voltage V0TC or a trimmable zero temperature coefficient current I0TC from the bandgap generator  112 .  
         [0045]    [0045]FIG. 2 is a schematic diagram illustrating the high voltage shunt regulator  114 .  
         [0046]    The high voltage shunt regulator  114  comprises a trimmable MOS voltage divider  202 , a capacitor divider  204 , an operational amplifier  206 , a selection circuit  208 , and an inverter  210 .  
         [0047]    The trimmable MOS voltage divider  202  comprises a plurality of PMOS  212  through  222  arranged with the drain-source terminals connected in series between the regulated high voltage supply signal (VSUPHV)  103  and an NH NMOS transistor  223  to form a divider chain. In one embodiment, the PMOS transistors  212  through  222  provide a divider chain that simulates a resistor chain.  
         [0048]    The PMOS transistors  212  through  218  are diode connected to eliminate body effect. The PMOS  219  through  222  are selectively diode connected.  
         [0049]    The drain-source terminals of the NH NMOS transistor  223  are coupled between the drain of the PMOS transistor  222  and ground for power down in response to an inverted power down (PDB1) signal  299  applied to the gate of the NH NMOS transistor  223 . The NH NMOS transistor  223  is coupled on the drain-side to eliminate additional error.  
         [0050]    The voltage divider  202  further comprises a selection circuit that includes a PMOS transistor  225  and  226 , a plurality of NH NMOS transistors  227  through  234 , and a plurality of inverters  236  through  238 .  
         [0051]    The selection circuit of the voltage divider  202  selectively shorts out one, two, or three of the PH PMOS transistors  220 ,  221 , and  222 , respectively, to modify the ratio. The selection circuit is arranged so that any voltage drop is at the drain side only, not at the gate so as to not introduce any errors. The selection circuit of the voltage divider  202  selectively diode connects or shorts out the PH PMOS transistors  220 ,  221 , and  222  in response to selection signals (SHORTP1)  253 , (SHORTP2)  254 , and (SHORTP3)  255 . The divider chain formed of the PMOS transistors  212  through  222  generate tap voltages VP3, VP2, VP1, and VP0 on the drain terminals of the PMOS transistors  218 ,  219 ,  220 , and  221 , respectively.  
         [0052]    The selection circuit  208  comprises a plurality of NH NMOS transistors  283  through  286  and a NOR gate  287 . The selection circuit  208  selectively couples the selected divided voltage from the voltage divider  202  to apply it to a voltage node  252 . The NH NMOS transistors  283  through  286  selectively couple the tap voltage, VP3, VP2, VP1, and VP0, respectively, to the voltage node  252  in response to the selection signals (SHORTP3)  255 , (SHORTP2)  254 , (SHORTP1)  253 , and the NOR of the selection signals  253  through  255 , respectively.  
         [0053]    The inverter  210  generates an inverted power down signal  299  in response to a power down signal  298 .  
         [0054]    The capacitor divider  204  comprises a plurality of capacitors  240  through  244 , and a plurality of NH NMOS transistors  245  through  250 . The capacitors  240  and  241  are coupled in series between regulated high voltage supply signal (VSUPHV)  103  and ground, and form a node  252  on which a voltage VF is connected. The capacitors  242 ,  243 , and  244  are coupled between the node  252  and the NH NMOS transistor  245 , the NH NMOS transistors  246  and  247 , and the NH NMOS transistors  248  through  250 , respectively, to form a selectable capacitor divider in response to inverted selection signals  253 ,  254 , and  255 , respectively. The capacitors  240  through  244  form a tracking capacitor divider to speed up the response time of the divider. The NH NMOS transistors  245  through  250  form switches to modify the capacitor ratio appropriately to track the PH PMOS transistor ratio of the voltage divider  202 . In one embodiment, the capacitor  240  may be two or more capacitors coupled in series to buffer the high voltage drop across the capacitor  240 .  
         [0055]    The operational amplifier  206  comprises an amplifier stage  257  and a control stage  258 .  
         [0056]    The amplifier stage  257  comprises a plurality of PMOS transistors  259  through  265  and a plurality of NMOS transistors  266  through  269 . The control stage  258  comprises a PMOS transistor  270 , a plurality of NX transistors  271  through  273 , a plurality of NH NMOS transistors  274  through  276 , a plurality of NMOS transistors  277  and  278 , an inverter  279  and a capacitor  280 .  
         [0057]    The amplifier stage  257  controls the shunt operation of the control stage  258  in response to comparing the divided voltage on the node  252  that is divided from the high voltage supply signal (VSUPHV)  103  and compared to a reference voltage, such as the zero temperature coefficient voltage (V0TC)  116 . A bias current (IBIASN)  281  adjusts the biasing of the amplifier stage  257 . The amplifier stage  257  includes a transconductance operational amplifier. The PMOS transistors  261  and  262  are an input pair for receiving a reference voltage, such as the zero temperature coefficient voltage (V0TC)  116 , and a divided voltage on the node  252 , respectively. The PMOS transistors  260 ,  261  and  262  and the NMOS transistors  266  and  267  are arranged as a differential amplifier. The PMOS transistors  259 ,  263 ,  264 , and  265  and the NMOS transistors  268  and  269  form a bias circuit for providing a voltage VBP to bias the PMOS transistor  260  in response to a bias current (IBIASN)  281 . The PMOS transistor  259  includes a drain terminal coupled to the common node of the gates of the PMOS transistors  260  and  263  to power down the amplifier stage  257  in response to the inverted power down signal (PDBI)  299 .  
         [0058]    The control stage  258  includes a shunt circuit to shunt current from the high voltage supply signal (VSUPHV)  103  as part of a control loop with the amplifier stage  257 . The control stage  258  further includes the HV buffered capacitor  280  for loop stability and to control the ramp rate of the high voltage supply signal (VSUPHV)  103 .  
         [0059]    The NMOS transistor  278  is a low voltage device that functions as a shunt element to shunt away the current from the high voltage supply signal (VSUPHV)  103  to regulate the signal  103 . The NX NMOS transistor  271  buffers the high voltage for the NMOS transistor  278 .  
         [0060]    The PMOS transistor  270  and the NH NMOS transistor  274  bias one terminal of the capacitor  280  at an intermediate voltage so the capacitor  280  can avoid breakdown. In another embodiment, the capacitor  280  may be two capacitors in series which quadruples the circuit area for the same capacitance.  
         [0061]    The NX NMOS transistor  273  serves as a HV buffering for the NMOS transistors  266 ,  277 , and  278  and also serves as a resistor in series with the capacitor  280  for loop stability.  
         [0062]    The capacitor  280  provides loop stability and also together with the current bias from the NMOS transistor  266  control the ramp rate of the high voltage supply signal  103 . This is also to avoid the overshoot if the high voltage supply signal (VSUPHV)  103  rises too fast.  
         [0063]    The NH NMOS transistor  276 , the NX NMOS transistor  272 , the NH NMOS transistor  275 , and the inverter  279  are used to short out the PMOS transistor  270  and the NX NMOS transistor  273  when regulating the high voltage supply signal (VSUPHV)  103  at low voltage levels or improving the loop stability. In one embodiment, the low voltage levels are in the range of 4-6 volts. The inverter  279  is enabled by an enable shunt regulator signal  297 . The NX NMOS transistor  272  buffers the high voltage for the NH NMOS transistor  276 . The NH NMOS transistor  275  disconnects the NH NMOS transistor  274  from shorting the supply voltage VSUP to the node CAPN by effectively acting as a reversed bias diode (with gate and drain tied together). This enabling mode may also be used to assist in stability of the loop when the high voltage supply signal (VSUPHV)  103  reaches a plateau or flat level.  
         [0064]    In another embodiment, the amplifier stage  257  may include the HV transistors instead of low voltage transistors. In another embodiment, the amplifier stage  257  may be powered from a HV supply such as the high voltage supply signal (VSUPHV)  103  instead of the supply voltage VSUP. In this case, appropriate usage of HV devices are used to avoid breakdown. In another embodiment, the amplifier  257  receives power from a filter network such as a RC or a DRC (a diode in series with RC) network. In another embodiment, the filter is coupled from a HV supply such as the high voltage supply signal (VSUPHV)  103 . In this case, the filter network serves to smooth out the ripple and noise from the HV supply signal (VSUPHV)  103  before being supplied to the amplifier  257 .  
         [0065]    Bandgap reference generators are next described. The bandgap generator  112  generates a zero temperature coefficient current (I0TC)  118  that may be formed from a plurality of currents that are summed together by a current summer, such as a current summer  800  (FIG. 8). The zero temperature coefficient current (I0TC)  118  may be converted into a zero temperature coefficient voltage (V0TC)  116  by a current to voltage converter, such as a current to voltage converter  900  (FIG. 9). Each of the currents that are summed to form the zero temperature coefficient current (I0TC)  118  may be generated by bandgap reference generators described below in conjunction with FIGS. 5A, 5B,  7 ,  10 ,  11 ,  13 , and  14 . First, a conventional bandgap reference is described.  
         [0066]    [0066]FIG. 3 is a schematic diagram illustrating a conventional band gap reference generator  300 .  
         [0067]    The conventional band gap reference generator  300  comprises an operational amplifier  302 , a plurality of PMOS transistors  303  through  305 , a plurality of pnp bipolar junction transistors  306  through  308 , and a plurality of resistors  310  and  311 .  
         [0068]    The drain-source terminals of the PMOS transistor  303  and the emitter-collector junction of the PNP bipolar junction transistor  306  are coupled in series between a supply voltage and ground. The drain-source terminals of the PMOS transistor  304 , the resistor  310  and the emitter-collector terminals of the transistor  307  are coupled in series between the supply voltage and ground. The operational amplifier  302  biases the gates of the PMOS transistors  303  and  304  in response to the voltages on the drains of the PMOS transistors  303  and  304  applied to the negative and positive inputs, respectively. The PMOS transistor  305 , the resistor  311  and the transistor  308  are arranged in a similar manner as the respective PMOS transistor  304 , the resistor  310  and the bipolar junction transistor  307  with the exception that the drain of the PMOS transistor  305  forms an output terminal that provides an output bandgap voltage VBG.  
         [0069]    The current I into the emitter of the transistor  306  is:  
           I=dVBE   306-307 /R 310   =dVBE/R   310   (1)  
         [0070]    The current I 310  in the resistor  310  is:  
           I   310 =( VBE   306   −VBE   307 )/ R   310   =dVBE/R   310   (2)  
         [0071]    The output band gap voltage is  
           VBG=VBE+ ( R   311   /R   310 )  dVBE   ( 3 )  
         [0072]    The conventional band gap reference generator  300  provides no zero temperature coefficient (TC) current, has no fractional band gap voltage, and requires a supply voltage VDD greater than 1.2 volts (VBG). Further, the conventional band gap reference generator  300  is susceptible to channel length modulation (CLM), drain induced lowering (DIBL), and near break down condition.  
         [0073]    [0073]FIG. 4 is a graph illustrating the drain-source current versus drain-source voltage characteristic of a typical sub-micron metal-oxide-silicon field effect transistor (MOSFET).  
         [0074]    The current-voltage (I-V) characteristic is poor at medium voltage, and is especially worse at 65 nanometer and 90 nanometer process nodes. Thus, if the band gap core is maintained at low voltage, the channel length modulation (CLM), the drain induced lowering (DIBL) and the near breakdown condition do not affect the precision level.  
         [0075]    Bandgap reference generators in accordance with the present invention are next described.  
         [0076]    [0076]FIG. 5A is a schematic diagram of a band gap reference generator  500 .  
         [0077]    The band gap reference generator  500  comprises an operational amplifier  502 , a plurality of PMOS transistors  503  through  505 , a plurality of pnp bipolar junction transistors  506  and  507 , a resistor  510 , a filter  512 , and a switch  514 .  
         [0078]    In alternative embodiments, the bandgap reference generator  500  comprises one of signal lines  520 ,  521 , and  522 .  
         [0079]    The filter  512  is coupled between an output of the operational amplifier  502  and a voltage node  516 . Another embodiment of the bandgap does not include the filter  512 . The drain-source terminals of the PMOS transistor  503  and emitter-collector generator of the pnp bipolar junction transistor  506  is coupled in series between the voltage node  516  and ground. The drain-source terminals of the PMOS transistor  504 , resistor  510 , and the emitter-collector terminals of the pnp bipolar junction transistor  507  are coupled in series between the voltage node  516  and ground. The gates of the PMOS transistors  503 ,  504  and  505  are coupled together, and coupled to one of the signal lines  520 ,  521 , or  522 . In alternative embodiments, the gates of the PMOS transistors  503  and  504  may be coupled by the signal lines  520 ,  521 , or  522  (shown as dashed lines) to ground, the positive input of the operational amplifier  502 , and the emitter of the transistor  507 , respectively. The drain-source terminals of the PMOS transistor  505  are coupled between the voltage node  516  and an output node  524 , which provides an output current IOUT. The negative input of the operational amplifier  502  is coupled to the drain of the PMOS transistor  503  and the positive input of the operational amplifier is coupled to the no-error resistor divider output node of the resistor  510  (described in FIG. 6A). The switch  514  is coupled in parallel with the collector-emitter terminals of the pnp bipolar junction transistor  507 .  
         [0080]    The output node  524  provides an output current IOUT equal to a current IC that flows through the PMOS transistor  504 , the resistor  510 , and the bipolar junction transistor  507 .  
         [0081]    The current IC flowing in the right portion (through the resistor  510 ) of the band gap reference generator  500  equals either a positive temperature coefficient current IPTC or a negative temperature coefficient current INTC depending on the switch  514  being opened or closed, and a sense current ISENSE. A positive curve temperature coefficient current IPCTC or a negative curve temperature coefficient current INCTC is generated from a positive temperature coefficient current IPTC and a negative temperature coefficient current INTC as described below in conjunction with FIG. 19. A current summer (such as in FIG. 8) provides a final summation current  
           ISUM=IPTC+INTC+ ( IPCTC and/or INCTC )  (4)  
         [0082]    The operation of the bandgap reference generator  500  is next described for the switch  514  being in open and closed states.  
         [0083]    In a configuration in which the switch  514  is open, the positive temperature coefficient current IPTC is:  
           IPTC=dVBE/R   510    =kT/q In a   (5);  
         [0084]    where a=emitter ratio of VBE 507  to VBE 506 ; k=Boltzman constant, q=electron charge, and T=temperature in Kelvin.  
         [0085]    In a configuration in which the switch  514  is closed, the negative temperature coefficient current INTC is  
           INTC=VBE   500   /R   510   (6).  
         [0086]    A typical variation of VBE over temperature is −2 mV/° C. (Celsius).  
         [0087]    The negative curve temperature coefficient current INCTC is an incremental current that is generated to adjust for a temperature coefficient and is defined as:  
           INCTC═IAPX 0− INTC   (7)  
         [0088]    where the negative temperature coefficient current INTC is defined by Equation (6) and the approximate zero temperature coefficient current IAPX0 is the summed output current (equation 9).  
         [0089]    A positive curve temperature coefficient current IPCTC is generated to adjust the current and is defined as follows:  
         [0090]    [0090] IPCTC=IPTC−IAPX 0  (8)  
         [0091]    where the positive temperature coefficient current IPTC is defined by Equation (5).  
         [0092]    The approximate zero temperature coefficient current IAPX0 is defined as the sum of the positive and negative temperature coefficient currents, IPTC and INTC, or may be expressed as:  
           IAPX 0= IPTC+INTC   (9)  
         [0093]    In alternate embodiments, the temperature coefficient currents IPTC and INTC are generated from other than PNP devices, such as MOS devices in sub-threshold operating regime or VT of MOS devices.  
         [0094]    In another embodiment, the output of the filter  512  may be coupled to the gates of the PMOS transistors  503  and  504 .  
         [0095]    The zero temperature compensated voltage V0TC is generated from the summation of different current elements that have ratios that are trimmable, and that are applied across an output resistance. The zero temperature coefficient voltage V0TC is generated from the positive temperature coefficient current IPTC, the negative temperature coefficient current INTC, the positive curve temperature coefficient current IPCTC, and the negative curve temperature coefficient current INCTC. In another embodiment, this trimmable ratio of different current elements may be different at different V0TC levels.  
         [0096]    The zero temperature coefficient current I0TC is generated from the summation of several currents that have an appropriate trimmable ratio. The currents are the positive temperature coefficient current IPTC, the negative temperature coefficient current INTC, the positive curve temperature coefficient current IPCTC, and the negative curve temperature coefficient current INCTC. In one embodiment, the trimmable ratio is generally different from the trimmable ratio of the zero temperature coefficient voltage.  
         [0097]    The resistor  510  may be trimmable without creating additional error. In one embodiment, the resistor  510  is a trimmable resistor  600  described below in conjunction with FIG. 6A.  
         [0098]    The resistor  510  may be controlled to have a variable impedance, for example, a low impedance, e.g., R 510  value is small, to help speed up settling time and/or reject power supply and coupling noise and a high impedance to have low power consumption such as during standby. The low impedance may be done at power up or during certain chip operations that generate a lot of on-chip noises such as memory programming or burst mode reading. This variable impedance provides a bandgap with variable impedance with precision voltage and current because the resistor trimming introduces insignificant error as described below in conjunction with FIG. 6A.  
         [0099]    In an alternate embodiment, the resistor  510  is a fixed resistor and the positive input of the operational amplifier  502  may be coupled to one of the terminals of the resistor  510 . It should be noted that alternate embodiments of FIGS. 5B, 7,  10 , and  13  may similarly include a fixed resistor instead of a variable resistor, and a corresponding coupled of the operational amplifier to the resistor.  
         [0100]    In an alternative embodiment, another filter, such as the filter  512  may be applied to the supply voltage VDD before being applied to the operational amplifier  502  and other circuit blocks (such as the current summer, and startup circuit described below).  
         [0101]    In an alternative embodiment, the bandgap reference generator  500  is operated in a dynamic operation in which the switch  514  is opened and closed to sample the positive temperature coefficient current IPTC and the negative temperature coefficient current INTC, and the corresponding voltages and currents are stored in storage nodes (such as by capacitors (not shown).  
         [0102]    [0102]FIG. 6A is a block diagram illustrating a trimmable resistor  600 .  
         [0103]    The trimmable resistor  600  comprises a plurality of resistors  602 -A through  602 -N, a resistor  603 , a plurality of switches  604 -A through  604 -N, and a plurality of switches  606 -A through  606 -N.  
         [0104]    The plurality of resistors  602 -A through  602 -N and the resistor  603  are coupled in series. The plurality of switches  604 -A through  604 -N are coupled from a node  608  to a respective resistor  602 -A through  602 -N, to selectively short the terminals of the respective resistor to the node  608 . The plurality of switches  606 -A through  606 -N are coupled to a respective resistor  602 -A through  602 -N, to selectively short the terminals of the respective resistors. The resistor  602 -A couples from a node  610  to the resistor  602 -B. The resistor  603  is coupled between a node  612  to the resistor  602 -N-1 (shown as  602 -B in FIG. 6A). As shown in FIG. 5A, the node  608  is coupled to the positive input of the operational amplifier  502 , the node  610  is coupled to the drain of the PMOS transistor  504  and the node  612  is coupled to the emitter of the bipolar transistor  507 .  
         [0105]    In this embodiment, the shorted resistor  606 -A to  606 -N may have a small voltage drop because of the V DS  of the CMOS transistor, but this voltage drop only affects the VDS of the PMOS  504 . However, the shorted resistor  604 -A through  604 -N does not introduce any voltage drop because no current flows through the shorted resistors (which connects to a gate of a MOS input device of the operational amplifier  502 ). The voltage at the positive terminal of the operational amplifier  502  then stays the same after trimming. Accordingly, the resistor trimming does not cause an error. In one embodiment, the switches  604  are CMOS transistors.  
         [0106]    [0106]FIG. 5B is a schematic diagram illustrating a bandgap reference generator  550 .  
         [0107]    The bandgap reference generator  550  comprises an operational amplifier  552 , a plurality of PMOS transistors  553  and  554 , a plurality of pnp bipolar junction transistors  556  and  557 , a plurality of resistors  560 ,  574 , and  575 , a filter  562 , and a switch  564 .  
         [0108]    In alternate embodiments, the bandgap reference generator  550  comprises one of signal lines  570 ,  571 , and  572 . The bandgap reference generator  550  is similar to the bandgap reference generator  500  of FIG. 5A, with the addition of the variable resistors  574  and  575  coupled between the drains of the respective PMOS transistors  553  and  554  and the emitters of the pnp bipolar junction transistors  556  and  557 . The variable resistors  574  and  575  may be the transistor  650  shown in FIG. 6B. The resistors  574  and  575  adjust the voltage levels coupled into the positive and negative terminals of the operational amplifier  552 . The adjusted resistance of the variable resistor  574  is similar to that of the variable resistor  575  to provide similar voltage levels.  
         [0109]    In another embodiment, the resistors  560  and  575  may be combined into a single resistor.  
         [0110]    The use of variable resistors  574  and  575  may be included in the bandgap generators  700  (FIG. 7),  1000  (FIG. 10), and  1100  (FIG. 11).  
         [0111]    [0111]FIG. 6B is a schematic diagram illustrating a trimmable resistor  650 .  
         [0112]    The trimmable resistor  650  comprises a plurality of resistors  652 -A through  652 -N, a resistor  653 , and a plurality of switches  656 -A through  656 -N. By selectively closing the switches  656 -A through  656 -N, corresponding resistors  652  are shorted out to alter the resistance between the nodes  660  and  662 .  
         [0113]    [0113]FIG. 7 is a schematic diagram illustrating a band gap reference generator  700  having cascoding.  
         [0114]    The cascoding described for FIG. 7 also is applicable to the bandgap generators described in conjunction with FIGS. 5B, 10 and  11 .  
         [0115]    The band gap reference generator  700  comprises an operational amplifier  702 , a plurality of PMOS transistors  703 ,  704 ,  716 , and  718 , a plurality of pnp bipolar junction transistors  706  and  707 , a resistor  710  and a switch  714 .  
         [0116]    The bandgap reference generator  700  is arranged in a manner similar to the bandgap reference generator  500  (see FIG. 5) except a cascode PMOS transistor  716  is coupled between the PMOS transistor  703  and the transistor  706 , and a cascode PMOS transistor  718  is coupled between the PMOS transistor  704  and the resistor  710 . The gates of the cascode PMOS transistor  716  and  718  are coupled to a cascode bias voltage (VBPCAS)  730 .  
         [0117]    [0117]FIG. 8 is a schematic diagram illustrating a current summer  800 .  
         [0118]    The current summer  800  may be coupled to the output of a plurality of band gap reference generators to add the currents from the band gap reference generators. The current summer  800  comprises a plurality of PMOS transistors  802  through  805 , a plurality of NZ NMOS transistors  806  and  807 , a plurality of NN NMOS transistors  808  and  809 , and a power down circuit  810 . The power down circuit  810  comprises a PMOS transistor  812  and a plurality of NMOS transistors  813  and  814 . The transistors  802  and  803  represents one input current and the transistors  804  and  805  represent another input current. Multiple input currents are represented by duplicating the transistors  802  and  803  and connecting them in parallel with the transistors  802  and  803  with different input signals INN.  
         [0119]    The PMOS transistors  803  and  805  are biased by a cascode voltage VBPCAS.  
         [0120]    The NZ transistor  807  and the NN transistor  809  are self-cascoding. The NZ transistor  806  and the NN transistor  808  are self-cascoding through the power down circuit  810  in response to the power down circuit  810  being enabled, and are coupled to ground when the power down signal is enabled. The power down circuit  810  disables or enables the self-cascoding of the NZ transistor  806  and the NN transistor  808 , and grounds the gates of the NZ transistor  806  and the NN transistor  808  during power down. The source of the PMOS transistor  812  is coupled to its own well.  
         [0121]    The current I in the NZ NMOS transistor  806  and the NN NMOS transistor  808  is the summation of the currents in the circuit of PMOS transistors  802  and  803  and the circuit of PMOS transistors  804  and  805 . The output current IOUTN in the NMOS transistors  807  and  809  mirrors the summed current I in the NMOS transistors  806  and  808  by any desirable mirror ratio by adjusting the size ratio of the transistors  807  and  809  to that of the transistors  806  and  808 .  
         [0122]    [0122]FIG. 9 is a schematic diagram illustrating a current to voltage converter  900 .  
         [0123]    The current to voltage converter  900  comprises a plurality of PMOS transistors  902  and  903 , and a resistor  904 . The transistor  902  and  903  represents a current sink into the resistor  904 .  
         [0124]    The current to voltage converter  900  may be coupled to the output of the current summer  800  to convert the summed currents from the band gap reference generators into a voltage. The coupling is done for example by two PMOS transistors  902 A and  903 A (not shown) connected in series from power supply VDD (used interchangeably as VSUP) to a node coupled to a node IOUTN of FIG. 8 and to a node IN of FIG. 9. The gate of the transistor  902 A is coupled to the drain of the transistor  903 A. The gate of the transistor  903 A is connected to the bias voltage VBPCAS.  
         [0125]    The resistor  904  may be trimmable in a similar manner as the trimmable resistor  600  described above and thus does not introduce voltage errors. In one embodiment, the resistor  904  is the trimmable resistor  600 .  
         [0126]    The current to voltage converter  900  may generate the zero temperature coefficient voltage (V0TC)  116  by applying the appropriate trimmable summed current from current summer  800  into the resistor  904 .  
         [0127]    [0127]FIG. 10 is a schematic diagram illustrating a band gap reference generator  1000 .  
         [0128]    The band gap reference generator  1000  is similar to the band gap reference generator  500 , and also comprises voltage level shift for the control loop.  
         [0129]    The band gap reference generator  1000  comprises an operational amplifier  1002 , a plurality of PMOS transistors  1003  and  1004 , a plurality of pnp bipolar junction transistors  1006  and  1007 , a plurality of resistors  1010 ,  1015 , and  1016 , a filter  1032 , a switch  1014 , and a plurality of NZ NMOS transistors  1012  and  1013 . In another embodiment, the bandgap does not include the filter  1032 . In another embodiment, the filter  1032  is coupled to the gates of the PMOS transistors  1003  and  1004 . The switch  1014  functions similarly to the switch  514  (FIG. 5A).  
         [0130]    The NMOS transistor  1012  and  1013  and the resistors  1016  and  1015  provide an appropriate low voltage level shift for the control loop. The resistors  1016  and  1015  may be coupled from drains of the transistors  1012  and  1013 , respectively, to a high voltage supply instead of coupled from the sources of the transistors  1012  and  1013 , respectively, to ground and the sources of the transistors  1012  and  1013  are coupled to ground. In this case, the transistors  1012  and  1013  and the resistors  1016  and  1015  constitute common source gain stages, and the loop stability is designed appropriately.  
         [0131]    In another embodiment, the NMOS transistors  1012  and  1013  each are replaced by a PMOS transistor including drain-source terminals coupled to a high voltage supply and the respective resistor  1016  and  1015  to provide an appropriate high voltage level for control loop. Common source gain stages mix alternately as described above for the transistors  1012  and  1013  and resistors  1016  and  1015 .  
         [0132]    In another embodiment, an NMOS transistor is coupled in series to each of the resistors  1016  and  1015  to ground and includes its gate biased by a current bias to provide a current bias to the transistor  1012  and  1013  and resistor  1016  and  1015  control loop. In one embodiment, the current bias can be derived from the temperature coefficient currents (IPTC, INTC) generated from the bandgap.  
         [0133]    [0133]FIG. 11 is a schematic diagram illustrating a band gap reference generator  1100  including a replica biased operational amplifier.  
         [0134]    The band gap reference generator  1100  comprises an operational amplifier  1102 , a plurality of PMOS transistors  1103  and  1104 , a plurality of pnp bipolar junction transistors  1106  and  1107 , a resistor  1110 , a filter  1132 , a switch  1114 , and a plurality of NZ NMOS native transistors  1112  and  1113 .  
         [0135]    A PMOS transistor  1103 , the NMOS transistor  1112  and the bipolar junction transistor  1106  are coupled together in series to form a first leg of the bandgap reference general  1100 . The PMOS transistor  1104 , the NMOS transistor  1113 , the resistor  1110 , and the bipolar junction transistor  1107  are coupled in series to form a second leg. The negative and positive inputs of the operational amplifier  1102  are connected to the drain of the diode connected NMOS transistors  1112  and  1113 , respectively. The filter  1132  is coupled between the output of the operational amplifier  1102  and a common node formed by the sources of the PMOS transistors  1103  and  1104 . The filter  1132  is optional. Alternatively, the output of the operational amplifier  1102  is coupled to a common node formed by the gates of the PMOS transistors  1103  and  1104  with the sources of the PMOS transistors  1103  and  1104  coupled to a high voltage supply, such as VDD. The switch  1114  functions similarly to the switch  514  (FIG. 5A).  
         [0136]    The operational amplifier  1102  has a similar bias configuration as the bandgap core so that the bias is a replica of the bandgap core.  
         [0137]    [0137]FIG. 12 is a schematic diagram illustrating the replica biased operational amplifier  1102 .  
         [0138]    The replica biased operational amplifier  1102  comprises a plurality of PMOS transistors  1202  through  1204 , a plurality of NMOS transistors  1205  through  1207  and a plurality of pnp bipolar junction transistors  1208  through  1210 . The transistors  1202 ,  1203 ,  1205 ,  1206 ,  1208 , and  1209  are arranged as a differential amplifier with the NMOS transistors  1205  and  1206  as the input pair. The transistors  1204 ,  1207 ,  1210  are arranged as an output stage to mirror the current from the differential amplifier portion of the operational amplifier  1102 . The circuit leg formed of the transistors  1202 ,  1205  and  1208  form a replica of the transistors  1103 ,  1112 ,  1106  of the bandgap reference generator  1100  as shown in FIG. 11. The circuit leg formed of the transistors  1203 ,  1206  and  1209  forms a replica of the transistors  1104 ,  1113 ,  1107 , and the resistor  1110  of the bandgap reference generator  1100  as shown in FIG. 11. Alternatively, the NMOS native transistors  1112 ,  1113 ,  1205 ,  1206 , and  1207  may be enhancement NMOS transistors.  
         [0139]    [0139]FIG. 13 is a schematic diagram illustrating a bandgap reference generator  1300  including a startup circuit.  
         [0140]    The bandgap reference generator  1300  comprises a bandgap reference generator  1301  and a resistor  1302 . The bandgap reference generator  1301  is similar to the bandgap reference generator  500 , but without the output PMOS transistor  505 . The resistor  1302  is coupled between the voltage node on the output of the operational amplifier and may supply current at startup until the operational amplifier is sufficiently operational to take over operation of the bandgap reference generator  1301 .  
         [0141]    [0141]FIG. 14 is a schematic diagram illustrating a startup circuit.  
         [0142]    The startup circuit  1400  comprises a sense current generator  1401 , a bias current generator  1402 , and a start current  1403 . The sense current generator  1401  and the start current generator  1403  are coupled to each other in parallel and coupled to the bias current generator  1402 . In one embodiment, a sense current from the sense current generator  1401  is mirrored out from a positive temperature coefficient current IPTC or a negative temperature coefficient current INTC to a bandgap reference generator such as described above. As the supply voltage VCC increases, the bias current from the bias current generator  1402  is reduced. In one embodiment, a bias current generator is a plurality of PMOS transistors coupled in series from VDD to the sense current  1401  with its gate coupled to ground. The start current  1403  is mirrored to be applied to an NMOS device and the bandgap reference generator.  
         [0143]    The starting up of the bandgap operates as follows. If the bandgap is not started up by itself, its bias current (IPTC or INTC) is zero, the start current  1403  is then the same as bias current  1402 , which is then injected into the bandgap to make its bias currents non-zero. Once the bandgap is started up, the sense current  1401 , which is mirrored from the bandgap, then begins to conduct. Once the sense current reaches its designed value, its value is greater than the bias current  1402 , the start current  1403  is then approximately zero. At this point the start current  1403  does not affect the bandgap bias current. In another embodiment, as the supply voltage VDD increases, the bias current from the bias current generator  1402  is reduced. This may be implemented as follows: as the supply voltage VDD increases, a comparator detects if VDD is more than a reference voltage (for example 2V derived from the bandgap) and the output of the comparator is then used to reduce the bias current  1402 , for example, by turning on some additional PMOS transistors in series to realize the bias current  1401  as described above.  
         [0144]    In an alternate embodiment, the start current generator  1403  may be replaced by a start current generator that is coupled between the supply voltage in parallel with the bias current generator, to provide a start current that is applied to a PMOS transistor and to the bandgap reference generator. An example is the transistor  1506  and  1507  portion of a startup circuit  1500  (see FIG. 15).  
         [0145]    [0145]FIG. 15 is a schematic diagram illustrating a startup circuit  1500 . The startup circuit  1500  comprises a bias current generator  1502 , sense current generator  1503 , a plurality of PMOS transistors  1504  through  1507 , a plurality of NZ NMOS transistors  1508  and  1509 , and a plurality of NMOS transistors  1510  and  1511 . The PMOS transistors  1506  and  1507  are arranged as a cascode to provide a startup current IPSTART on a node  1513 . The NMOS transistors  1509  and  1511  are arranged as a cascode to provide a startup current INSTART on a node  1514 . The series connected bias current generator  1502  and sense current generator  1503  provide a bias start voltage to the bias and stage formed of the transistors  1504 ,  1505 ,  1508 , and  1510 . The bias current  1502  and the sense current  1503  are similar to the bias current  1402  and the sense current  1403 , respectively. The start current  1514  is similar to the start current  1403 .  
         [0146]    [0146]FIG. 16 is a block diagram illustrating a binary complementary trimming circuit  1600 .  
         [0147]    The binary complementary trimming circuit  1600  comprises a bit signal generator  1602 , a positive temperature coefficient current generator  1603 , a negative temperature coefficient generator  1604 , a trimmable curve temperature coefficient curve current generator  1605 , and a current summer  1606 .  
         [0148]    The current summer  1606  sums the currents from the positive temperature coefficient current generator  1603 , the negative temperature coefficient generator  1604 , and the trimmable curve temperature coefficient current generator  1605  to generate a zero temperature compensated current ZTC  1608 . The bit signal generator  1602  generates the control bits in response to control signals from the fuse circuit  110 . The bit signal generator  1602  provides the control bits to the generator  1602 ,  1603 ,  1604 , and  1605 . The binary complementary trimming circuit  1600  further comprises an inverting circuit  1610  that provides inverted control signals to the positive temperature coefficient current generator  1603  and negative temperature coefficient generator  1604  to provide complementary trimming. In one embodiment, each incremental trim of the positive temperature coefficient current generator  1603  corresponds to a complementary (or decremental) trimming of the negative temperature coefficient current from the negative temperature coefficient generator  1604 . In an illustrative example, if the positive temperature coefficient current is trimmed upward by one or a plurality of increments, the negative temperature coefficient current is automatically trimmed down by one or a plurality of decrements. Vice versa, if the positive temperature coefficient current is trimmed downward by one or a plurality of decrement; the negative temperature coefficient current is automatically trimmed up by one or a plurality of increments. The trimmable current temperature compensated current generator  1605  generates a trimmable positive temperature coefficient current PCTC 1 and a negative temperature coefficient current NCTC1. The currents PCTC1 and NCTC1 are zero at a temperature less than a desired temperature. The currents PCTC2 and NCTC2 are similar to PCTC1 and NCTC1 except they are zero at a different temperature.  
         [0149]    [0149]FIG. 17 is a graph illustrating the complementary temperature coefficient current using binary complementary temperature coefficient trimming.  
         [0150]    A line  1702  corresponds to the temperature coefficient current generated by the binary complementary trimming circuit  1600  as the sum of the various temperature compensated currents. By altering the individual current characteristics and the adjustable trimming, the temperature compensated current shown in the line  1702  may be varied to have a desired characteristic, such as a flatter curve over a desired temperature range, for example from 0° C. to 70° C. or from −40° C. to 125° C.  
         [0151]    [0151]FIG. 18 is a graph illustrating the generation of a complementary temperature coefficient current.  
         [0152]    The approximate zero temperature coefficient current IAPX0 is derived from equations (8) and (9), described above.  
         [0153]    [0153]FIG. 19 is a block diagram illustrating a curved temperature coefficient current generator  1900 .  
         [0154]    The curve temperature coefficient current generator  1900  comprises a positive temperature coefficient current generator  1902 , a IAPX0 current generator  1903 , and a curve temperature coefficient current generator  1904 .  
         [0155]    The curve temperature coefficient current generator  1900  generates the positive curved temperature coefficient current IPCTC defined above in Equation (8). Similarly, the negative curved temperature coefficient current INCTC is generated.  
         [0156]    [0156]FIG. 20 is a graph illustrating the generation of a complementary positive curve temperature coefficient voltage VPCTC.  
         [0157]    In an alternate embodiment to the generation of a complementary temperature coefficient current of FIG. 18, a curved voltage element may be used instead of a curved current element. In one embodiment, the positive temperature coefficient voltage is generated by applying the positive temperature coefficient current to a resistor. In this embodiment, the approximate zero temperature coefficient voltage VAPX0 equals the positive temperature coefficient voltage VPTC plus the negative temperature coefficient voltage VNTC.  
         [0158]    [0158]FIG. 21 is a schematic diagram of a complementary positive temperature coefficient voltage generator  2100 .  
         [0159]    The complementary positive temperature coefficient voltage generator  2100  comprises a comparator  2102  and a plurality of switches  2104  and  2106 . The comparator  2102  compares the positive temperature coefficient voltage VPTC to the approximate zero temperature coefficient voltage VAPX0. The comparison result is used to generate a difference VPTC minus VAPX0 voltage that is sampled by the switch  2104  to generate the complementary positive curve temperature coefficient voltage VPCTC. If the positive temperature coefficient voltage VPTC is greater than the approximate zero temperature coefficient voltage VAPX0 (VPTC&gt;VAPX0), the switch  2104  is closed to provide an output voltage VPTC minus VAPX0 as the complementary positive curve temperature coefficient voltage VPCTC. If the positive temperature coefficient voltage VPTC is smaller than the approximate zero temperature coefficient voltage VAPX0 (VPTC&lt;VAPX0), the switch  2106  is closed to provide a ground GND as the complementary positive curve temperature coefficient voltage VPCTC. Similarly, a complementary negative curve temperature coefficient voltage VNCTC may be generated.  
         [0160]    In an alternative embodiment, if the positive temperature coefficient voltage VPCT is greater than the approximate zero temperature coefficient voltage VAPX0 (VPTC&gt;VAPX0), the positive temperature coefficient voltage VPTC is provided by the switch from the positive input of the comparator  2102  as the complementary positive curve temperature coefficient voltage VPCTC. In this embodiment, the voltage VPCTC has a higher voltage level.  
         [0161]    [0161]FIG. 22 is a flow chart illustrating an operation of approximation complementary trimming.  
         [0162]    The procedure of approximation complementary trimming measures the voltages (or currents) at maximum, middle, and minimum temperature settings and based on the comparisons adjusts the TC trimming until the resulting maximum, middle and minimum voltages are in a desired range. For example, here a trim step (1*IV step) is assumed. The TC trimming is adjusted in the positive TC (PTC) direction by trimming downward the PTC trim setting. In the process, the negative TC (NTC) trim setting is automatically adjusted upward as described previously. Similarly, the TC trimming is adjusted in the negative TC (NTC) direction by trimming downward the NTC trim setting. In the process, the PTC is automatically adjusted upward.  
         [0163]    The voltage is measured at maximum (max), a middle (mid) and minimum (min) temperature (temp) trim setting (block  2202 ). The measured voltages are compared to determine whether the voltage at the maximum temperature setting is greater than the voltage at the middle temperature setting which is greater than the voltage at the minimum temperature setting and whether the absolute value of the difference between the voltages of the maximum and minimum voltage values is greater than one incremental voltage (IV) step (block  2204 ). In the event that these comparisons are met, the TC trimming is adjusted so that the voltage difference is divided by the incremental voltage step equals the number N for the TC trim setting and the trim settings are reduced in the positive TC direction by the number N trim setting divided by two (block  2206 ) and the voltage measurement is repeated (block  2202 ).  
         [0164]    On the other hand, if the comparison is not met (block  2204 ) and another comparison is performed as to whether or not the voltage at the maximum temperature setting is less than the voltage at the middle temperature setting and whether the voltage at the middle temperature setting is less than the voltage at the minimum temperature setting and that the absolute value of the difference between the voltages of the maximum voltage value and the minimum voltage value is greater than one incremental voltage step (block  2208 ). If the comparison is true, the voltage difference is divided by the incremental voltage step to determine the number N trim setting, and the trim setting is reduced in the negative TC direction by half of the number N (block  2210 ), the procedure returns to measuring the voltages (block  2202 ).  
         [0165]    On the other hand, if the comparison is not true (block  2208 ), a new comparison is performed (block  2212 ). If the voltage of the maximum temperature setting is less than the voltage at the middle of the temperature setting and the voltage at the maximum temperature setting is greater than the voltage at the minimum temperature setting, and the absolute value of the difference between the voltages of the maximum voltage value and minimum voltage value is greater than one incremental voltage step, the TC trim setting is adjusted (block  2214 ). The voltage difference is divided by the incremental voltage step to set a number N of trim settings, and the trim setting is reduced in the positive TC direction by half the number N(N/2) (block  2214 ). The procedure then returns to measuring voltages (block  2202 ).  
         [0166]    On the other hand if the comparison is not true (block  2212 ), another comparison is performed (block  2216 ). If the voltage at the maximum temperature setting is less than the voltage at the middle temperature setting and the voltage at the maximum temperature setting is less than the voltage at the minimum temperature setting, and the absolute value of the difference between the voltages of the maximum voltage value and minimum voltage value is greater than an incremental voltage step, another TC trim adjustment is performed (block  2218 ). The voltage difference is divided by the incremental voltage step to set a number N trim settings and the TC trim setting is reduced in the negative TC direction by the number N divided 2 (N/2) (block  2218 ). The voltages are again measured (block  2202 ).  
         [0167]    On the other hand, if the comparison is not true, the procedure ends (block  2720 ). In this case, the difference between the voltage at the maximum and middle and minimum temperature settings is less than an incremental voltage step.  
         [0168]    [0168]FIG. 23 is a schematic diagram illustrating a low voltage current mirror  2300  that is used in the bandgap reference generator for coupling the current.  
         [0169]    The low voltage current mirror  2300  comprises a plurality of PMOS transistors  2303  and  2304 , an amplifier  2302 , a current source  2305 , and a resistor  2306 . The resistor  2306  represents a load for the transistor  2304 . The load can be a resistor, a MOS or a capacitor. The PMOS transistor  2303  and the current source  2305  form a first leg of the circuit  2300 . The PMOS transistor  2304 , and the resistor  2306  form a second leg of the circuit  2300  with the second leg mirroring the current from the first leg. In this embodiment, the minimum VDD is only approximately two times the VDS at saturation of the PMOS transistors  2303  or  2304 . Each VDS SAT  is used to sustain a current across a MOS transistor. The amplifier  2302  forces the VDS of the PMOS transistors  2303  and  2304  to be equal. Another embodiment has the positive terminal of the amplifier  2302  coupled to a bias voltage.  
         [0170]    [0170]FIG. 24 is a schematic diagram illustrating a current trim circuit  2400  that is used to trim the current for the bandgap reference generator and is used to set the level for the high voltage regulator.  
         [0171]    The current trim circuit  2400  comprises a bias circuit  2402 , a first cascode circuit  2404 , a second cascode circuit  2406 , a third cascode circuit  2408 , a fourth cascode circuit  2410 , a fifth cascode circuit  2412 , and a native NMOS transistor  2414 . The cascode circuits  2404 ,  2406 ,  2408 ,  2410 ,  2412  each comprise three NMOS transistors, the middle of the three being a native NMOS transistor, and the other two being enhancement NMOS transistors. The cascode circuits  2404 ,  2406 ,  2408 ,  2410  and  2412  together with  2414  are self-bias triple cascoding including one bias leg for an input bias current IIN.  
         [0172]    In another embodiment, the native NMOS transistor  2414  is omitted.  
         [0173]    The self-cascoding bias circuit  2402  provides biases for the self-bias triple cascoding circuits  2404 ,  2406 ,  2408 ,  2410 ,  2412  and  2414 . The cascode circuits  2408  and  2410  include switches for selectively disabling or enabling the circuits to selectively trim the output current IOUT.  
         [0174]    In this disclosure, there is shown and described only the preferred embodiments of the invention, but, as aforementioned, it is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.