Patent Publication Number: US-11664648-B2

Title: Programmable overcurrent protection for a switch

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/249,377, filed Mar. 1, 2021, which claims priority to U.S. Pat. No. 10,938,199, issued Mar. 2, 2021, which claims priority to U.S. Provisional Patent Application No. 62/656,700, filed Apr. 12, 2018, all of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     A synchronous switch mode power supply, such as a buck converter, is an electronic power supply that efficiently converts power from a first power regime to a second power regime. Such converters typically incorporate a high-side switch (e.g., a “control” switch), a low-side switch (e.g., a “synchronous” switch), and an inductor. The inductor couples a common node (e.g., a “phase node”) of the switches to a load of the converter. In some applications, the switches are field-effect transistors (FETs). The high-side switch delivers power to the load through the inductor, thereby converting an input voltage at a first level to an output voltage at a second level. In synchronous buck DC-DC applications, when an output short condition occurs, current through the inductor increases to maintain the output voltage level at the inductor. However, this approach may cause the inductor to undesirably saturate, which may damage the FET. 
     SUMMARY 
     In some embodiments, an overcurrent protection circuit is coupled to a switch having an on-state resistance that varies based on a temperature coefficient of the switch. The overcurrent protection circuit has an adjustable overcurrent threshold level determined based on an adjustable voltage generated by the overcurrent protection circuit. The adjustable voltage is generated based on the temperature coefficient of the switch. 
     In some embodiments, a method for overcurrent protection involves generating, in an overcurrent protection circuit coupled to a switch, an adjustable voltage based on a temperature coefficient of the switch, the generated adjustable voltage having a positive temperature coefficient. The method further involves determining an overcurrent threshold level in the overcurrent protection circuit based on the generated adjustable voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a simplified schematic of an overcurrent protection circuit in accordance with one or more example embodiments. 
         FIG.  2    illustrates details of the overcurrent protection circuit of  FIG.  1   , in accordance with some embodiments. 
         FIG.  3    illustrates further details of the overcurrent protection circuit of  FIG.  1   , in accordance with some embodiments. 
         FIG.  4    is an example operation of the overcurrent protection circuit of  FIG.  1   , in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Improved methods and circuits are described herein for a programmable overcurrent protection circuit for one or more switches, such as switches used in a switch-mode power supply (SMPS) circuit having an inductor and one or more field-effect transistor (FET) switches, such as a metal-oxide-semiconductor field-effect transistor (MOSFET). When an output short condition occurs at a load of the SMPS circuit, an overcurrent event may occur. An overcurrent event may cause current through the inductor to increase so to maintain the output voltage. Such an overcurrent event may damage one or more of the FETs of the SMPS circuit. 
     Some overcurrent protection circuits sense a source-drain current level through a FET of the SMPS circuit and compare the sensed current level to an overcurrent threshold. If the sensed current level surpasses the current threshold, one or more FETs of the SMPS circuit are turned off to stop current flow through that FET switch to the load of the SMPS circuit. 
     Current flow through a FET switch is in part related to an on-resistance (Rdson) of a conduction channel formed between a drain region and a source region of that FET. However, the FET on-resistance varies according to a temperature coefficient of resistance (TCR) of the FET as a temperature of the FET varies. As such, at lower temperatures, a sensed source-drain current level through the FET of the SMPS circuit is the same or similar to a current level through the load of the SMPS circuit. However, at higher temperature levels, on-resistance Rdson of the FET varies according to the TCR of the FET. As temperatures vary, a sensed source-drain current level through the FET may diverge from the current level through the load of the SMPS circuit. Thus, a particular overcurrent threshold which is used to accurately determine an overcurrent event at a first temperature of the FET may no longer accurately determine the overcurrent event at another temperature. 
     Described herein is a circuit configured for high precision overcurrent protection that advantageously varies an overcurrent threshold level according to an adjustable temperature coefficient of the circuit as temperature of the circuit varies. The adjustable temperature coefficient of the circuit is adjusted to substantially match the temperature coefficient of resistance of a FET of the circuit. As a result, as the on-resistance of the FET varies with temperature, the overcurrent threshold will also vary proportionally. Thus, the circuit advantageously detects an overcurrent condition of the FET across a range of temperatures. Other improvements or advantages will also be described below or become apparent from the following disclosure. 
       FIG.  1    is a simplified schematic of an overcurrent protection circuit  110 , in accordance with one or more example embodiments. As shown in  FIG.  1   , the overcurrent protection circuit  110  is coupled to a switch M 2 , such as a MOSFET, having an on-resistance Rdson. The on-resistance Rdson varies with temperature based on a temperature coefficient of the switch M 2 . In the example embodiment of  FIG.  1   , the switch M 2  is a low-side switch (e.g., a synchronous switch) in a switch-mode power supply (SMPS) circuit  100 , such as a DC-DC buck converter. The switch M 2  is coupled to and controlled by the overcurrent protection circuit  110 . A high-side switch M 1  and the switch M 2  of the SMPS  100  provide a current I L  via inductor L 1  to an output node Vout. A load (not shown) is typically coupled to the output node Vout. A first terminal of an output capacitor Cout, having a resistance symbolically shown by resistor ESR, is coupled at one end to the inductor L 1  and the Vout node at node  102 . A second terminal of the output capacitor Cout is coupled to a voltage ground (PGND). A node  101 , sometimes referred to as a phase node or switch node, is coupled to a port of the overcurrent protection circuit  110  designated as SW. The overcurrent protection circuit  110  is configured to receive, sense, or measure a voltage and/or current from the node  101 . 
     As shown in  FIG.  1   , the overcurrent protection circuit  110  is coupled to a high gate node (HG) of the switch M 1  and to a low gate node (LG) of the switch M 2 . A control logic and driver circuit  105  of the overcurrent protection circuit  110  controls the turning ON or OFF of the switches M 1 , M 2  by applying driving signal(s) to the gate(s) of one or both of the switches M 1 , M 2 . During an overcurrent event, excess amounts of current I L  flows through L 1 , which can saturate L 1  and damage one or both of the switches M 1  and/or M 2 . 
     As described below and in greater detail in conjunction with  FIGS.  2 - 4   , to protect the SMPS  100 , and in particular, the switch M 2 , from an overcurrent condition, a level of a current or voltage which is proportional to the output current I L  is compared to a threshold current level. The threshold current level can be embodied as a voltage or as a current. If the level of the current or voltage which is proportional to the output current I L  exceeds the threshold current level, the overcurrent protection circuit  110  turns one or both of the switches M 1 , M 2  OFF. 
     The overcurrent protection circuit  110  is configured to generate an adjustable overcurrent threshold level. As mentioned previously, the overcurrent protection circuit  110  advantageously adjusts the overcurrent threshold level with temperature according to a temperature coefficient which is the same or similar to a TCR of the switch M 2 . The adjustable overcurrent threshold level is determined based on an adjustable voltage level Vadj which is generated by an adjustable voltage generator circuit  112 . The voltage level Vadj is generated at least in part based on (e.g., matched or proportionally to) the temperature coefficient of the switch M 2 , and a resistive value of a programmable scaling resistor Rcs coupled to the overcurrent protection circuit  110 , as further described below in conjunction with  FIG.  2   . 
     As shown in  FIG.  1   , the adjustable voltage generator circuit  112  includes a voltage-to-current generator circuit  114 , and a current-to-voltage circuit  115 , amongst other components, which contribute to generating the voltage level Vadj. The generated voltage level Vadj is provided to the detection circuit  113  of the overcurrent protection circuit  110 . As described in greater detail below in conjunction with  FIG.  3   , the detection circuit  113 , having a resistor-divider network  117 , uses the received voltage level Vadj to generate the adjustable overcurrent threshold level which is compared to a voltage or current which is proportional or otherwise representative of the current I L . 
       FIG.  2    illustrates details of the adjustable voltage generator circuit  112  of the overcurrent protection circuit  110  shown in  FIG.  1   . The adjustable voltage generator circuit  112  is configured to generate the voltage level Vadj based on (e.g., matched or proportionally to) the temperature coefficient of the switch M 2 . In an example embodiment, the voltage level Vadj has a positive temperature coefficient that is adjusted to substantially match the temperature coefficient of the on-resistance Rdson of the switch M 2 . As shown in  FIG.  2   , the adjustable voltage generator circuit  112  includes the voltage-to-current generator circuit  114 , which includes a switch T 1 , such as a bipolar junction transistor (BJT) having a base, an emitter and a collector. In some embodiments, the voltage-to-current generator circuit  114  has a negative temperature coefficient (e.g., Vbe of T 1  changes at a rate of approximately −2 mV/degC) and is configured to generate an output voltage V 1  at node  201  based on a reference voltage Vref received at the base of T 1 . V 1  is substantially equal to Vref minus the voltage Vbe between the base and emitter of T 1 . In some embodiments, Vref is pre-adjusted to substantially match the temperature coefficient of the on-resistance Rdson of the switch M 2 . In yet other embodiments, for a different temperature coefficient of the on-resistance Rdson of the switch M 2 , Vref is trimmed such that the Vref-Vbe matches the temperature coefficient of the Rdson of the switch M 2 . 
     The adjustable voltage generator circuit  112  further includes the current-to-voltage circuit  115  coupled to the voltage-to-current generator circuit  114  to receive the voltage V 1  at node  201 . In some embodiments, the current-to-voltage circuit  115  has a net positive temperature coefficient and is configured to generate, based on the voltage V 1 , an output voltage V ILIM . In other embodiments, the voltage-to-current generator circuit  114  has a positive temperature coefficient and the current-to-voltage circuit  115  has a negative temperature coefficient. 
     In the example embodiment of  FIG.  2   , the current-to-voltage circuit  115  is implemented using a resistor-divider configuration with a resistor R 0  which receives voltage V 1  from node  201 , and a variable resistor R 1  coupled in series to resistor R 0  at a common node  202 . The voltage V ILIM  is thus generated based on the following Equation 1: 
     
       
         
           
             
               
                 
                   
                     V 
                     ILIM 
                   
                   = 
                   
                     
                       R 
                       ⁢ 
                       1 
                       * 
                       
                         ( 
                         
                           Vref 
                           - 
                           Vbe 
                         
                         ) 
                       
                     
                     
                       ( 
                       
                         
                           R 
                           ⁢ 
                           1 
                         
                         + 
                         
                           R 
                           ⁢ 
                           0 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                         
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     The generated voltage V ILIM  is output from the common node  202  to a non-inverting (+) input of Op-Amp  203 , which in turn drives a gate of a switch M 3 , such as a MOSFET. The switch M 3  is coupled at a source node to both the inverting (−) input of Op-Amp  203  and to a scaling resistor Rcs. The scaling resistor Rcs is coupled to the adjustable voltage generator circuit  112  between ports I ILM  and ground (AGND), as shown in  FIG.  2   . In an example embodiment, the scaling resistor Rcs is a programmable overcurrent resistor having a selected (i.e., programmed) value which is used in determining the overcurrent threshold level of the overcurrent protection circuit  110 . 
     As also shown in  FIG.  2   , the adjustable voltage generator circuit  112  further includes a current mirror  204  circuit configured to output a threshold current I threshold  based on the voltage V ILIM  and a value of the scaling resistor Rcs. The adjustable voltage generator circuit  112  also includes a trim resistor Rztc coupled at a first terminal to the current mirror  204  via node  205 , and to a ground node (AGND) at a second terminal. In an example embodiment, the trim resistor Rztc is a variable resistor having a temperature coefficient substantially equal to zero. The adjustable voltage generator circuit  112  generates the voltage level Vadj based on the threshold current I threshold  and a value of trim resistor Rztc, and outputs the voltage level Vadj from node  205 . 
     The voltage level Vadj varies with temperature in accordance with a positive temperature coefficient which substantially matches the temperature coefficient of the on-resistance Rdson of the switch M 2  based on the following Equation 2: 
     
       
         
           
             
               
                 
                   Vadj 
                   = 
                   
                     
                       
                         V 
                         ILIM 
                       
                       * 
                       R 
                       ⁢ 
                       z 
                       ⁢ 
                       t 
                       ⁢ 
                       c 
                     
                     
                       R 
                       ⁢ 
                       c 
                       ⁢ 
                       s 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                         
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     Substituting V ILIM  from Equation 1 into Equation 2 results in the following Equation 3: 
     
       
         
           
             
               
                 
                   Vadj 
                   = 
                   
                     
                       R 
                       ⁢ 
                       1 
                       * 
                       
                         ( 
                         
                           Vref 
                           - 
                           Vbe 
                         
                         ) 
                       
                       * 
                       Rztc 
                     
                     
                       
                         ( 
                         
                           
                             R 
                             ⁢ 
                             0 
                           
                           + 
                           
                             R 
                             ⁢ 
                             1 
                           
                         
                         ) 
                       
                       * 
                       R 
                       ⁢ 
                       c 
                       ⁢ 
                       s 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                         
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     Thus, as shown by Equations 1-3, the voltage level Vadj and its positive temperature coefficient is generated based on (a) a temperature coefficient and base voltage (e.g., Vref) of the switch T 1  (which generates Vref-Vbe), (b) respective values of the resistor R 0  and the variable resistor R 1  (which generate voltage V ILIM  from Vref-Vbe), (c) a value of the programmable scaling resistor Rcs, and (d) a value of variable trim resistor Rztc. 
       FIG.  3    illustrates details of the detection circuit  113  of the overcurrent protection circuit  110  shown in  FIG.  1   . As shown in  FIG.  3   , the detection circuit  113  is coupled to and configured to receive the voltage level Vadj from the adjustable voltage generator circuit  112 . The detection circuit  113  is also coupled to the SMPS  100  via the high gate node (HG) of the high-side switch M 1  and the low gate node (LG) of the low-side switch M 2 . During an overcurrent event, an overcurrent event detection signal Voc is received at the control logic and driver circuit  105  of the detection circuit  113 . Upon receiving the overcurrent event detection signal Voc, the control logic and driver circuit  105  reacts by turning one or both of the switches M 1 , M 2  off to end the overcurrent event. The overcurrent event detection signal is based on a scaled level of the received voltage level Vadj, a voltage at the phase node  101 , and a reference voltage (e.g., ground). 
     As shown in  FIG.  3   , the voltage level Vadj generated by the adjustable voltage generator circuit  112 , is received in a non-inverting (+) input of operational amplifier Op-Amp  301  of the detection circuit  113 . Based on the received voltage level Vadj, the Op-Amp  301  is configured to generate a voltage V 3  at node  302 . In an example embodiment, the voltage V 3  is substantially equal to the voltage level Vadj. 
     Node  302  is coupled to an input of the resistor-divider network  117 . In an example embodiment, the resistor-divider network  117  includes series-connected resistors kR and R 2  coupled at a common node  303 . The resistor-divider network  117  is configured to receive the generated voltage V 3  at a first terminal and a voltage/current of the phase node  101  at a second terminal. Based on the received voltage V 3  and the voltage/current of the phase node  101 , the resistor-divider network  117  generates a voltage V OCN  which is provided to a non-inverting (+) input of Overcurrent (OC) Comparator  304 . The OC Comparator  304  compares the voltage V OCN  to a reference voltage (e.g., ground, or another bias voltage) coupled to the inverting (−) input of the OC Comparator  304  and outputs an overcurrent event detection signal V OC . The overcurrent event detection signal Voc is then provided to the control logic and driver circuit  105 . Thus, the adjustment to overcurrent threshold level is advantageously made such that the overcurrent threshold level varies in substantially the same way that Rdson varies across a range of temperatures. 
     In some embodiments, a high output state of the OC Comparator  304  indicates an overcurrent event, and a low output state of the OC Comparator  304  indicates the absence of an overcurrent event. In some embodiments, while the output state of the OC Comparator  304  is low, the control logic and driver circuit  105  cycles the switch M 2  between ON and OFF states. At the moment when V OCN =V OCP  the output state of the OC Comparator  304  is high. In some embodiments, upon receiving a high output state from the OC Comparator  304 , the control logic and driver circuit  105  turns one or both of the switches M 1 , M 2  OFF. When V OCN =V OCP : 
                   Vsw   =       V   ⁢   3     k             (     Equation   ⁢         4     )                           Vsw   =     Ioc   *     Rdson   .               (     Equation   ⁢         5     )               
where I OC  is the overcurrent threshold level of the overcurrent protection circuit  110 .
 
     As previously stated, V 3  is equal, or substantially equal, to the voltage level Vadj determined in Equation 3 above. Substituting the voltage level Vadj for V 3  in Equation 5 leads to the following Equation 6: 
     
       
         
           
             
               
                 
                   Ioc 
                   = 
                   
                     
                       Vsw 
                       Rdson 
                     
                     = 
                     
                       
                         
                           V 
                           ⁢ 
                           3 
                         
                         
                           k 
                           * 
                           Rdson 
                         
                       
                       = 
                       
                         
                           
                             ( 
                             
                               Vref 
                               - 
                               Vbe 
                             
                             ) 
                           
                           Rdson 
                         
                         * 
                         
                           
                             R 
                             ⁢ 
                             1 
                             * 
                             Rztc 
                           
                           
                             k 
                             * 
                             R 
                             ⁢ 
                             0 
                             * 
                             R 
                             ⁢ 
                             c 
                             ⁢ 
                             s 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                         
                     6 
                   
                   ) 
                 
               
             
           
         
       
     
     The voltage Vref can be trimmed to make Vref-Vbe match the temperature coefficient of the on-resistance Rdson of the switch M 2 . Thus, the overcurrent threshold I OC  changes proportionally to a current through M 2  as the temperature changes in the SMPS  100 . A trim resistor Rztc is utilized in some embodiments to trim or otherwise compensate for process variations which may occur during manufacturing of the overcurrent protection circuit  110 . In some embodiments, the trim resistor Rztc is a resistor circuit having a TCR substantially equal to zero. An advantage of the above approach is that the I OC  will have little or no temperature dependency and therefore its value can be more accurately determined. 
       FIG.  4    illustrates an example operation of the overcurrent protection circuit  110 . The process begins at Block  410  in which an adjustable voltage level Vadj is generated based on a temperature coefficient of the switch M 2 , with the generated voltage level Vadj having a net positive temperature coefficient. Next, at Block  420 , an overcurrent threshold level is determined by the overcurrent protection circuit  110  based on the generated voltage level Vadj. Operations in Blocks  410  and  420  are performed in a manner consistent with those described above in detail in conjunction with  FIGS.  1 - 3   . 
     Reference has been made in detail to embodiments of the disclosed invention, one or more examples of which have been illustrated in the accompanying figures. Each example has been provided by way of explanation of the present technology, not as a limitation of the present technology. In fact, while the specification has been described in detail with respect to specific embodiments of the invention, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. For instance, features illustrated or described as part of one embodiment may be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present subject matter covers all such modifications and variations within the scope of the appended claims and their equivalents. These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the scope of the present invention, which is more particularly set forth in the appended claims. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only and is not intended to limit the invention.