Patent Publication Number: US-10788851-B2

Title: Self-biased temperature-compensated Zener reference

Description:
BACKGROUND 
     Field of the Disclosure 
     The present disclosure relates generally to reference circuits and more particularly to temperature-compensated reference circuits. 
     Background of the Disclosure 
     Precision references provide a precise analog value, such as a voltage or a current, which can be used by circuits to operate in their intended manners. Zener reference circuits are preferred in highly accurate measurements chains where very low long-term drift is of great importance. They are indeed less sensitive to package stresses and more stable over their lifetimes than the classical bandgap circuits. 
     As was previously done for a battery management system integrated circuit (BMS IC), a Zener reference was biased using an existing reference current generated in a bandgap circuit trimmed over temperature range. This trimming sequence was performed on automated test equipment (ATE) on all the packaged parts at several temperatures, for example, −40° C. and 125° C., and optionally 25° C. 
     Since temperature coefficient trimming of Zener reference bias current was finalized after having measured performance at two boundary temperatures, the Zener reference voltage at both boundary temperatures was not known during first ATE temperature test. Because measurement chain accuracy relies on the reference voltage, which needed to be known during the first ATE temperature test, the Zener reference voltage was measured for all trimming codes, so that a corresponding look-up table (LUT) was created, allowing retrieval of final reference voltages at first temperature insertion once the IC was tested at its second test temperature. 
     This LUT creation required a long test time to measure reference voltage for all trim codes. It added cost and also a measurement error that degraded the measurement chain accuracy. Accordingly, an improved temperature compensated Zener reference is needed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
         FIG. 1  is a block diagram illustrating a self-biased temperature-compensated Zener-diode voltage reference circuit in accordance with at least one embodiment. 
         FIG. 2  is a schematic diagram illustrating a self-biased temperature-compensated Zener-diode voltage reference circuit in accordance with at least one embodiment. 
         FIG. 3  is a flow diagram illustrating a method for providing an output reference voltage in accordance with at least one embodiment. 
     
    
    
     The use of the same reference symbols in different drawings indicates similar or identical items. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     An auto-biased solution for a Zener reference is described. The auto-biasing removes dependencies on the performance or trimming of other blocks. The auto-biasing reduces test time and cost and improves the calibration accuracy. The self-bias current is constant over the temperature range to minimize voltage reference temperature nonlinearities. 
       FIG. 1  is a block diagram illustrating a self-biased temperature-compensated Zener-diode voltage reference circuit in accordance with at least one embodiment. Circuit  100  comprises Zener diode  101 , proportional-to-absolute-temperature (PTAT) current source  102 , complementary-to-absolute-temperature (CTAT) current source  103 , PTAT voltage drop  104 , and output scaling circuit  105 . A supply voltage source is coupled to node  106 , which is connected to a supply voltage input of PTAT current source  102  and to a supply voltage input of CTAT current source  103 . A cathode of Zener diode  101  is connected to node  107 , which is connected to a PTAT current output of PTAT current source  102 , to a CTAT current output of CTAT current source  103 , and to an input of PTAT voltage drop  104 . An anode of Zener diode  101  is connected to ground voltage  110  at node  109 . An output of PTAT voltage drop  104  at node  108  is connected to a temperature-stabilized supply voltage input of PTAT current source  102  and to an input of output scaling circuit  105 . Output scaling circuit  105  provides an output reference voltage VOUT at voltage reference output  112 . Output scaling circuit  105  is connected to ground voltage  110  at node  109 . PTAT current source  102  is connected to node  111 , which is connected to a CTAT control input of CTAT current source  103 . 
     Supply voltage VSUPPLY is provided at node  106  to PTAT current source  102  and to CTAT current source  103 . PTAT current source  102  provides a PTAT current to node  107 . CTAT current source  103  provides a CTAT current to node  107 . A portion of the sum of the PTAT current and the CTAT current is obtained from node  107  and provided to PTAT voltage drop  104  as a Zener diode voltage output current. PTAT voltage drop  104  provides substantially all of the Zener diode voltage output current to output scaling circuit  105  and to PTAT current source  102 . Zener diode  101  passes a Zener diode current equal to the sum of the PTAT current and the CTAT current minus the Zener diode voltage output current. Output scaling circuit  105  provides an output reference voltage VOUT at output node  112 . PTAT voltage drop  104  provides a temperature-stabilized supply voltage signal to a temperature-stabilized supply voltage input of PTAT current source  102  via node  108 . PTAT current source  102  provides a CTAT control signal to CTAT current source  103  via node  111 . 
     By utilizing a combined current obtained from PTAT current source  102  and CTAT current source  103 , the proportional thermal response of PTAT current source  102  and the complementary thermal response of CTAT current source  103  balance each other out to provide a temperature-compensated bias current to supply to Zener diode  101 . To compensate for Zener voltage variation of Zener diode  101  with temperature, PTAT voltage drop  104  provides a voltage reduction proportional to current flow through it. Accordingly, a dropped voltage at the output of PTAT voltage drop  104  is temperature compensated, thereby providing a temperature-stabilized voltage signal, whereas the Zener diode voltage at the input of PTAT voltage drop  104  exhibits a temperature dependence as a characteristic of Zener diode  101 . Output scaling circuit  105  allows the temperature-stabilized voltage signal at the output of PTAT voltage drop  104  to be adjusted to provide an output reference voltage of a desired voltage. Thus, the desired voltage can be stably provided as the output reference voltage regardless of temperature. 
       FIG. 2  is a schematic diagram illustrating a self-biased temperature-compensated Zener-diode voltage reference circuit in accordance with at least one embodiment. Circuit  200  comprises Zener diode  101 , PTAT current source  102 , CTAT current source  103 , PTAT voltage drop  104 , and output scaling circuit  105 . PTAT current source  102  comprises p-channel MOSFET  202 , p-channel MOSFET  203 , NPN bipolar transistor  204 , NPN bipolar transistor  205 , resistor  206 , NPN bipolar transistor  207 , p-channel MOSFET  208 , and p-channel MOSFET  209 . A PTAT current source core of the PTAT current source  102  comprises p-channel MOSFET  202 , p-channel MOSFET  203 , NPN bipolar transistor  204 , NPN bipolar transistor  205 , and resistor  206 . CTAT current source  103  comprises n-channel MOSFET  210 , resistor  211 , p-channel MOSFET  212 , and p-channel MOSFET  213 . PTAT voltage drop  104  comprises resistor  201 . Output scaling circuit  105  comprises resistor  214  and resistor  215 . 
     Supply voltage VSUPPLY is connected to node  106 , which is connected to a source terminal of p-channel MOSFET  208 , to a source terminal of p-channel MOSFET  209 , to a source terminal of p-channel MOSFET  212 , and to a source terminal of p-channel MOSFET  213 . A gate terminal of p-channel MOSFET  208  is connected to a gate terminal of p-channel MOSFET  209  and to a drain terminal of p-channel MOSFET  208 , forming a MOSFET current mirror for PTAT current source  102 . A gate terminal of p-channel MOSFET  212  is connected to a gate terminal of p-channel MOSFET  213  and to a drain terminal of p-channel MOSFET  213  at node  220 , forming a MOSFET current mirror for CTAT current source  103 . A drain terminal of p-channel MOSFET  209  is connected to a cathode of Zener diode  101  at node  107 . A drain terminal of p-channel MOSFET  212  is connected to the cathode of Zener diode  101  at node  107 . The anode of Zener diode  101  is connected to ground voltage  110  at node  109 . The cathode of Zener diode  101  is connected at node  107  to a first terminal of resistor  201  of voltage drop  104 . A second terminal of resistor  201  of voltage drop  104  is connected to node  108 . Node  108  is connected to a source terminal of p-channel MOSFET  202 , to a source terminal of p-channel MOSFET  203 , and to a first terminal of resistor  214 . Resistor  214  is the upper resistor of a voltage divider formed of resistor  214  and resistor  215  connected in series. The second terminal of resistor  214  is connected to reference voltage output  112  and to a first terminal of resistor  215 . The second terminal of resistor  215  is connected to ground voltage  110  via node  109 . The voltage divider formed of resistor  214  and resistor  215  scales the voltage present at node  108  down to lower voltage based on the ratio of the resistance of resistor  215  to the sum of the resistances of resistor  214  and resistor  215 . Thus, the voltage divider formed of resistor  214  and resistor  215  forms output scaling circuit  105 . 
     A gate terminal of p-channel MOSFET  202  is connected to the gate terminal of p-channel MOSFET  203  and to a drain terminal of p-channel MOSFET  203 , forming a MOSFET current mirror. The gate terminal and drain terminal of p-channel MOSFET  203  are connected to node  217 , which is connected to a collector terminal of NPN bipolar transistor  205 . A drain terminal of p-channel MOSFET  202  is connected to node  216 , which is connected to a gate terminal of n-channel MOSFET  210  and to a collector terminal of NPN bipolar transistor  204 . An emitter terminal of NPN bipolar transistor  204  is connected to ground voltage  110  at node  109 . An emitter terminal of NPN bipolar transistor  205  is connected to node  222 , which is connected to a first terminal of resistor  206 . A second terminal of resistor  206  is connected to ground voltage  110  via node  109 . A gate terminal and a drain terminal of p-channel MOSFET  213  are connected to node  220 , which is connected to a drain terminal of n-channel MOSFET  210 . A source terminal of n-channel MOSFET  210  is connected to node  218 , which is connected to a first terminal of resistor  211 , to a base terminal of NPN bipolar transistor  205 , to a base terminal of NPN bipolar transistor  204 , and to a base terminal of NPN bipolar transistor  207 . A second terminal of resistor  211  is connected to ground voltage  110  via node  109 . An emitter terminal of NPN bipolar transistor  207  is connected to ground voltage  110  via node  109 . A collector terminal of NPN bipolar transistor  207  is connected to node  219 , which is connected to the drain terminal and gate terminal of p-channel MOSFET  208  and to the gate terminal of p-channel MOSFET  209 . 
       FIG. 3  is a flow diagram illustrating a method for providing an output reference voltage in accordance with at least one embodiment. Method  300  begins at block  301  and continues to block  302 . Block  302  comprises sub-blocks  303 ,  304 ,  305 ,  306 ,  307 , and  308 , which may be performed simultaneously, sequentially in any order, or in a combination of simultaneous and sequential order. Generally, a self-biased temperature-controlled Zener-diode voltage reference circuit is operated in block  302 . In sub-block  303 , a proportional-to-absolute-temperature (PTAT) current is provided to a Zener diode. In sub-block  304 , a complementary-to-absolute-temperature (CTAT) current is provided to the Zener diode. In sub-block  305 , a Zener diode voltage output current is provided to a voltage drop. The voltage drop is an electrical circuit block exhibiting a linear voltage difference in relation to a current passing through it. The voltage drop may, as examples, be a resistor or another electrical device (e.g., a transconductance circuit, which may be an active transconductance circuit or a passive transconductance circuit) exhibiting a linear voltage difference for a current passing through it. In sub-block  306 , a Zener diode current equal to the sum of the PTAT current and the CTAT current minus the Zener diode voltage output current is passed through the Zener diode. In sub-block  307 , a temperature-stabilized voltage signal (e.g., at node  108 ) received from the voltage drop (e.g., resistor  201 ) is provided to the PTAT current source core (e.g., at the source terminals of p-channel MOSFETs  202  and  203  of PTAT current source  102 ). In sub-block  308 , an output reference voltage is provided from the temperature-stabilized voltage signal received from the voltage drop. For example, the output reference voltage can be a scaled output reference voltage provided by output scaling circuit  105 . 
     In accordance with at least one embodiment, a temperature-compensated auto-bias circuit to optimize Zener reference voltage performance is disclosed. While Zener reference circuits have previously been biased using an existing current generated in a bandgap circuit trimmed over temperature range, the dependency upon a trimmed bandgap current increased the calibration procedure complexity and ATE test time, resulting in increased cost, and degraded the accuracy of the calibration. Technological improvement is provided by the auto-bias aspect disclosed herein, which is temperature compensated using a combination of a PTAT current source and a CTAT current source, avoiding the trimming and calibration complexity, time, cost, and reduced accuracy of previous technology. 
     A reference circuit in accordance with at least one embodiment presented herein is auto-biased, and measurement chain accuracy can be evaluated in one shot. This saves test time and removes the former need for analog reference voltage measurements for creating of a look-up table (LUT) of relevant values. 
     In accordance with at least one embodiment, a core voltage reference, Vz, is generated thanks to a buried Zener. The resulting voltage Vz may vary with temperature, for example, having a positive thermal coefficient (TC) of approximately 1-2 mV/° C. To compensate the Zener voltage variation with the temperature, a PTAT voltage (Proportional To Absolute Temperature voltage) is subtracted from Vz via a voltage drop, which may, for example, be a resistor or another device capable of reducing the voltage by an amount proportional to the current through the device (e.g., having a linear current-to-voltage (I/V) curve). At last, a voltage divider (e.g., a resistor bridge) is used as output scaling circuit  105  to reduce Vz′ to obtain a bandgap-compatible output reference voltage (Vout) approximately equal to 1.25V. 
     The resulting output reference voltage depends on the buried Zener voltage Vz and on the various component value ratios but is otherwise independent. High insensitivity to the package mechanical stresses and long-term stability are provided. 
     Moreover, in order to have a Zener current biasing that is constant over the temperature, the PTAT current IPTAT pulled down by the PTAT circuit (e.g., resistor  206 , NPN bipolar transistors  204  and  205 , and p-channel MOSFETs  202  and  203 ) is reinjected into the Zener via NPN bipolar transistor  207  and p-channel MOSFET transistors  208  and  209 , providing better linearity of output reference voltage Vout with temperature. 
     By providing an auto-biasing Zener reference circuit, at least one embodiment is able to eliminate dependencies on characteristics of an external bias current source. A complementary-to-absolute-temperature (CTAT) current is generated according to operation of resistor  211  and n-channel MOSFET  210 , where the CTAT current ICTAT=Vbe/R0, where Vbe is a base-to-emitter voltage (e.g., of NPN bipolar transistor  204  or NPN bipolar transistor  207 ) and R0 is the resistance of resistor  211 . 
     This CTAT current ICTAT, as well as an additional amount of PTAT current IPTAT, is injected into the node to which the cathode of the Zener diode is attached. ICTAT is injected via n-channel MOSFETs  212  and  213 , and IPTAT is injected via n-channel MOSFETs  208  and  209 . 
     The resulting current into the Zener is stable over the temperature range and equal to: 
             Izener   =         (       3   *   IPTAT     +   ICTAT     )     -     (       2   *   IPTAT     +       Vz   ′     R       )       =       (     IPTAT   +   ICTAT     )     -     (       (     Vz     ⋀   ′       )     ⁢     /     ⁢   R     )               
where Izener is the Zener diode current, where IPTAT is the PTAT current, where ICTAT is the CTAT current, where Vz′ is the temperature-stabilized voltage signal at node  108 , and where R is the sum of the resistance values of resistors  214  and  215 .  FIG. 2  shows current mirror ratios using notations such as “(1×)” and “(3×)” to denote differences in current, which can be obtained, for example, by adjusting the physical dimensions of the respective transistors or portions thereof or, for example, by implementing multiple transistors connected in parallel with each other. The “(1×)” and “(1×)” of p-channel MOSFETs  212  and  213  denote equal currents in the CTAT current mirror comprising p-channel MOSFETs  212  and  213 . The “(1×)” and “(1×)” of p-channel MOSFETs  202  and  203  denote equal currents in the PTAT core current mirror comprising p-channel MOSFETs  202  and  203 . The “(1×)” and “(3×),” respectively, of p-channel MOSFETs  208  and  209  denote p-channel MOSFET  209  passing three times as much current as p-channel MOSFET  208  in the PTAT current mirror. The tripled current of p-channel MOSFET  208  is reflected in the 3*IPTAT term of the Izener equation shown above. The “(1×)” of each of NPN bipolar transistors  204  and  207  and the “(xN)” of NPN bipolar transistor  205  denote NPN bipolar transistor  205  passing “N” times less current density than NPN bipolar transistors  204  and  207 . As an example, NPN bipolar transistor  205  may be implemented using a number N of NPN bipolar transistors of a type similar to NPN bipolar transistors  204  and  207 , where N is equal to two or more.
 
     The (IPTAT+ICTAT) portion of the equation is stable with temperature, as temperature changes of IPTAT and ICTAT balance each other out. The (Vz′/(R)) portion of the equation is stable with temperature. The Izener nominal value and temperature coefficient can be adjusted by changing the ratio of the resistance of resistor  206  to the resistance of resistor  211  and the current mirror transistor ratios of p-channel MOSFETs  209  and  208  and p-channel MOSFETs  212  and  213 . Accordingly, a reference circuit as disclosed herein allowed avoidance of external bias current source dependency while minimizing additional components and power consumption, thereby providing technological improvement. 
     In accordance with at least one embodiment, a Zener reference circuit disclosed herein is self-biased and does not require any external current biasing. Such a Zener reference circuit can be applied, for example, to precision measurement circuits where wide operating temperature range and long-term stability is required. 
     In accordance with at least one embodiment, an apparatus comprises a proportional-to-absolute-temperature (PTAT) current source; a complementary-to-absolute-temperature (CTAT) current source; and a Zener diode. The PTAT current source is coupled to a first Zener diode terminal of the Zener diode, and the CTAT current source coupled to the first Zener diode terminal of the Zener diode. The PTAT current source provides a PTAT current and the CTAT current source providing a CTAT current. The PTAT current and the CTAT current are combined for provide a temperature-compensated bias current. 
     In accordance with at least one embodiment, the apparatus further comprises a voltage drop having a first voltage drop terminal coupled to the first Zener diode terminal and a second voltage drop terminal coupled to a temperature-stabilized supply voltage input terminal of the PTAT current source. In accordance with at least one embodiment, the apparatus further comprises an output scaling circuit having a first output scaling circuit terminal coupled to the second voltage drop terminal, the output scaling circuit providing an output reference voltage. In accordance with at least one embodiment, the voltage drop is a resistor. In accordance with at least one embodiment, a voltage output of the PTAT current source is coupled to a CTAT current source control terminal of the CTAT current source. In accordance with at least one embodiment, the CTAT current source comprises a first transistor having a first terminal coupled to the CTAT current source control terminal, a second terminal coupled to a CTAT current source resistor, and a third terminal coupled to a CTAT current source current mirror. In accordance with at least one embodiment, the third terminal is further coupled to a first PTAT transistor base terminal of a first PTAT transistor, to a second PTAT transistor base terminal of a second PTAT transistor, and to a third PTAT transistor base terminal of a third PTAT transistor. 
     In accordance with at least one embodiment, a method comprises providing a proportional-to-absolute-temperature (PTAT) current to a Zener diode; providing a complementary-to-absolute-temperature (CTAT) current to the Zener diode; providing a Zener diode voltage output current to a voltage drop; and passing through the Zener diode a Zener diode current equal to the sum of the PTAT current and the CTAT current minus the Zener diode voltage output current. In accordance with at least one embodiment, the method further comprises providing a temperature-stabilized voltage signal received from the voltage drop to the PTAT current source. In accordance with at least one embodiment, the method further comprises providing an output reference voltage from the temperature-stabilized voltage signal received from the voltage drop. In accordance with at least one embodiment, the output reference voltage is a scaled output reference voltage. In accordance with at least one embodiment, providing the temperature-stabilized voltage signal received from the voltage drop to the PTAT current source further comprises providing the temperature-stabilized voltage signal to a PTAT current source core of the PTAT current source. In accordance with at least one embodiment, the voltage drop is a resistor. In accordance with at least one embodiment, the method further comprises controlling the CTAT current according to a CTAT current control signal obtained based on the temperature-stabilized voltage signal. 
     In accordance with at least one embodiment, an integrated circuit comprises a proportional-to-absolute-temperature (PTAT) current source; a complementary-to-absolute-temperature (CTAT) current source; and a Zener diode. The PTAT current source is coupled to the Zener diode. The CTAT current source is coupled to the Zener diode. The PTAT current source comprises a first PTAT transistor and a PTAT current source resistor coupled, at a first PTAT current source resistor terminal, to a first PTAT transistor emitter terminal of the first PTAT transistor and, at a second PTAT current source resistor terminal, to a ground voltage. The CTAT current source comprises a CTAT current source resistor coupled, at a first CTAT current source resistor terminal, to a first PTAT transistor base terminal of the first PTAT transistor and, at a second CTAT current source resistor terminal, to the ground voltage. The PTAT current source provides a PTAT current based on a PTAT current source resistor resistance of the PTAT current source resistor. The CTAT current source provides a CTAT current based on a CTAT current source resistor resistance of the CTAT current source resistor. At least a portion of a combination of the PTAT current and the CTAT current bias the Zener diode. 
     In accordance with at least one embodiment, the integrated circuit further comprises a voltage drop comprising a resistor having a first voltage drop terminal coupled to the first Zener diode terminal and a second voltage drop terminal coupled to a temperature-stabilized supply voltage input of the PTAT current source. In accordance with at least one embodiment, the integrated circuit further comprises an output scaling circuit having a first output scaling circuit terminal coupled to the second voltage drop terminal, the output scaling circuit providing an output reference voltage. In accordance with at least one embodiment, a voltage output of the PTAT current source is coupled to a CTAT current source terminal of the CTAT current source. In accordance with at least one embodiment, the CTAT current source comprises a first transistor having a first terminal coupled to the CTAT current source control terminal, a second terminal coupled to the CTAT current source resistor, and a third terminal coupled to a CTAT current source current mirror. In accordance with at least one embodiment, the third terminal is further coupled to the first PTAT transistor base terminal of the first PTAT transistor, to a second PTAT transistor base terminal of a second PTAT transistor, and to a third PTAT transistor base terminal of a third PTAT transistor. 
     The concepts of the present disclosure have been described above with reference to specific embodiments. However, one of ordinary skill in the art will appreciate that various modifications and changes can be made without departing from the scope of the present disclosure as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims.