Abstract:
The present invention relates to a circuit and method of providing a voltage having a temperature independent current compliance to a load. The circuit includes a first resistive element having a temperature dependent resistivity, a second resistive element, an amplifier, a current module generating a temperature dependent current, and a load current controller. Temperature dependent voltages developed across the resistive elements track each other to enable a constant current limit over a wide temperature range.

Description:
FIELD OF THE INVENTION 
     The invention relates to a current limit circuit and more specifically to a temperature compensated current limit circuit implemented in a solid state switch. 
     BACKGROUND OF THE INVENTION 
     Voltage regulation integrated circuits provide a regulated output voltage to a load. These circuits often include a current limitation feature that prevents current in excess of a predefined limit from flowing in the integrated circuit if the load increases to an unacceptable level. Another class of circuits that rely on accurate current limit protection are solid state switch circuits. These circuits provide a low impedance connection between two nodes and limit current to less than a predetermined value. Without compensating for temperature variations, a shift in the current limit value can occur. For example, the current limit value can shift by more than forty percent over a range of −40° C. to +85° C. due to temperature dependent electrical characteristics of the materials used in the circuit components. Thus, the current limiting portion of the circuit may not provide adequate protection in certain applications. What is needed is a circuit that provides a stable current limit over a wide temperature range. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a circuit and a method of stabilizing the current limit over a range of temperatures. The present invention is directed to limiting the amount of current flow through a metal interconnect, thus providing temperature current limit protection. 
     One aspect of the invention relates to a temperature compensated current limit circuit used in a solid state switch. The circuit, under normal operating conditions, fully enhances an integrated MOSFET, resulting in a reduced voltage drop across the switch. When the load current increases to an unacceptable level, the switch limits the current delivered to a load via a servo loop. The current delivered to a load without exhibiting any current limit behavior is often referred to as current compliance. 
     The circuit includes a first resistive element, a second resistive element, a load current controller, a current module, and an amplifier. The first resistive element has a first terminal adapted to receive an input voltage and has a second terminal. The first resistive element has a temperature dependent resistivity. The second resistive element has a first terminal configured to receive the input voltage and has a second terminal. The load current controller has a first terminal in communication with the second terminal of the first resistive element, a second terminal in communication with a load, and a control terminal adapted to receive a control signal. The amplifier has a first input terminal in communication with the second terminal of the first resistive element, a second input terminal in communication with the second terminal of the second resistive element, and an amplifier output terminal in communication with the control terminal of the load current controller. The amplifier provides the control signal at its output terminal in response to a load current, the resistivity of the first resistive element, the resistivity of the second resistive element, and a temperature dependent current generated by the current module. The current module has a first terminal in communication with the second terminal of the second resistive element and the second terminal of the amplifier. The current module provides a temperature dependent current at its first terminal. 
     In one embodiment, the load current controller is a current controlling transistor. In another embodiment, the current module generates a current that is proportional to absolute temperature (PTAT). In still another embodiment, the second resistive element includes a primary resistive element, and a secondary resistive element, each having first and second terminals. The first terminal of the primary resistive element is in communication with the first terminal of the second resistive element. The first terminal of the secondary resistive element is in communication with the second terminal of the primary resistive element. The second terminal of the secondary resistive element is in communication with the second terminal of the second resistive element. In a further embodiment, the resistivity of the primary resistive element is greater than the resistivity of the secondary resistive element. 
     Another aspect of the invention relates to a method of providing a voltage across a load with a temperature independent current compliance. The method includes the steps of generating a first temperature dependent voltage drop in response to the current through the load, generating a second temperature dependent voltage drop in response to a temperature dependent reference current, and amplifying the difference of the first temperature dependent voltage drop and the second temperature dependent voltage drop to generate a control signal. Additionally, the method includes the step of applying the control signal to a load current controller to provide the voltage having a temperature independent current compliance across the load. The method can be applied repeatedly to achieve a continuing current limitation function. 
     In another aspect, the method includes the steps of comparing a first temperature dependent voltage drop to a second temperature dependent voltage drop, wherein the second temperature dependent voltage drop is responsive to a temperature dependent current, generating a control signal in response to the comparison, and generating the voltage having a temperature independent current compliance in response to the control signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is pointed out with particularity in the appended claims. The advantages of the invention may be better understood by referring to the following description taken in conjunction with the accompanying drawing in which: 
     FIG. 1 is a schematic diagram depicting an embodiment of a temperature compensated current limit circuit according to the present invention; 
     FIG. 2 is a schematic diagram depicting another embodiment of a temperature compensated current limit circuit according to the present invention; 
     FIG. 3 is a schematic diagram depicting another embodiment of a temperature compensated current limit circuit according to the present invention; and 
     FIG. 4 is a flow chart representation of an embodiment of a method for providing a voltage across a load according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, in overview, one embodiment of the present of invention includes a first resistive element  10 , a second resistive element  16 , a load current controller  22 , a current module  28 , and an amplifier  34 . First resistive element  10  includes a first terminal  40  and second terminal  46 , and has a temperature dependent resistance (R 1 ). First terminal  40  of the first resistive element  10  is adapted to receive a supply voltage V SUPPLY . Second resistive element  16  includes a first terminal  52  and second terminal  58 , and has a resistance (R 2 ). First terminal  52  of the second resistive element  16  is adapted to receive supply voltage V SUPPLY . In one embodiment, in which the circuit is fabricated as an integrated circuit, the first resistive element  10  is an aluminum interconnect on an integrated circuit providing a nominal resistance (e.g., approximately 25 mΩ) and the second resistive element  16  is a P+ diffusion resistor providing a substantially greater resistance (e.g., approximately 25 kΩ). In such an embodiment, the ratio of resistances of the first resistive element  10  and the second resistive elements  16  is about 1×10 6 . 
     Load current controller  22  includes a first terminal  64  in communication with the second terminal  46  of the first resistive element  10 , a control terminal  70  configured to receive a control signal CONTROL, and an output terminal  76  in communication with a load  82 . In one embodiment, load current controller  22  is a Metal-Oxide Semiconductor Field Effect Transistor (MOSFET), and first resistive element  10  is the interconnect of the drain terminal of the MOSFET. Current module  28  includes a terminal  88  in communication with the second terminal  58  of the second resistive element  16 . Amplifier  34  includes a first terminal  96  in communication with the second terminal  46  of the first resistive element  10  and the first terminal  64  of load current controller  22 , a second input terminal  102  in communication with the second terminal  58  of the second resistive element  16  and terminal  88  of current module  28 , and an output terminal  108  in communication with the control terminal  70  of the load current controller  22 . 
     During operation, supply voltage (V SUPPLY ) is applied to the first terminals  40  and  52  of first and second resistive elements  10  and  16 , respectively. A drain current (I F ) flows through first resistive element  10 . A voltage (V F ) which is the product of the temperature dependent resistance (R 1 ) and drain current (I F ) exists across the first resistive element  10 . Additionally, current module  28  generates a temperature dependent current (I CM ). In one embodiment, temperature dependent current (I CM ) is proportional to absolute temperature. Consequently, a reference voltage (V R ) which is the product of temperature dependent current (I CM ) and resistance (R 2 ) is generated across the second resistive element  16 . Amplifier  34  amplifies the difference between voltage (V P ) (i.e., V SUPPLY −V F ) applied to its first terminal  96  and voltage (V N ) (i.e., V SUPPLY −V R ) applied to its second terminal  102 . In response, amplifier  34  generates a control signal CONTROL at its output terminal  108 . When voltage (V N ) is less than voltage (V P ) control signal CONTROL remains at the maximum supply voltage applied to the amplifier. In response, load current controller  22  provides a load current (I L ) to load  82  that approximately equals the drain current (I F ). As load current (I L ) and drain current (I F ) increase, the difference between voltages (V P ) and (V N ) decreases. When drain current I F  reaches a predetermined maximum value, the difference between voltage (V P ) and voltage (V N ) becomes zero and load current controller  22  provides load current I L  at a predetermined maximum value in response to the modulation of the current controller  22  according to control signal CONTROL. 
     As the operating temperature varies, the temperature dependent resistance (R 1 ) and temperature dependent current (I CM ) also vary in such a way as to provide a proper temperature compensated current limit. Resistance (R 2 ) and temperature dependent current (I CM ) are selected to define the limit voltage (V N ) which is compared with voltage (V P ) as the temperature varies. In one embodiment, current module  28  is designed such that the temperature dependent current (I CM ) is generated by a PTAT circuit. For example, the PTAT circuit can be a (ΔVbe)/R circuit which includes a resistor comprised of a material having a temperature dependent resistance similar to a temperature dependent resistance (R 2 ) of the second resistive element  16 . Consequently, the temperature dependence of the reference current (I CM ) generated by the (ΔVbe)/R circuit is designed such that the product of the reference current (I CM ) and the resistance (R 2 ) of the second resistive element  16  (i.e., voltage V R ) directly tracks changes in voltage (V F ) due to temperature variations for a fixed load current. 
     FIG. 2 illustrates an embodiment of the circuit of FIG. 1 in more detail. In this embodiment, current controller  22  is implemented as an N-Channel MOSFET  23 . The second resistive element  16  includes a primary resistive element  114  and a secondary resistive element  120 . Primary resistive element  114  includes a first terminal  144  which is the first terminal  52  of the second resistive element  16  and a second terminal  150 , and has a resistivity R P . Secondary resistive element  120  includes a first terminal  156  connected to the second terminal  150  of primary resistive element  114  and a second terminal  162  which is the second terminal  58  of second resistive element  16 , and has a resistivity R S . In one embodiment, the primary resistivity R P  is greater than the secondary resistivity R S . In another embodiment, the primary resistive element  114  and secondary resistive element  120  are both P+ diffusion resistors. 
     In this embodiment, the circuit also includes a charge pump  126 , a reset-switch  132 , and a comparator  138 . Charge pump  126  includes a first input terminal  166  configured to receive a charge pump supply voltage (V SUPPLY2 ), a second input terminal  172  configured to receive a reference voltage (V PREF ), and an output terminal  178  connected to a supply terminal  184  of amplifier  34 . Reset-switch  132  includes a first terminal  190  connected to the gate  71  of MOSFET  23  of load current controller  22 , a second terminal  196  configured to receive a reference voltage (e.g., ground), and a control terminal  202  configured to receive a reset-enable signal (RESETEN). Comparator  138  includes a first input terminal  208  connected to the junction of the second terminal  150  of primary resistive element  114  and the first terminal  156  of the secondary resistive element  120 , a second input terminal  214  connected to the second terminal  46  of the first resistive element  10  and to the first input terminal  96  of amplifier  34 , and a comparator output terminal  220 . 
     In operation, supply voltage (V SUPPLY ) is applied to first terminal  144  of primary resistive element  114 . Consequently, a voltage drop (V PRI ) develops across primary resistive element  114  and a voltage V SEC  develops across secondary resistive element  120 . Comparator  138  generates a flag signal FLAG at output terminal  220  in response to a voltage V N′  (equal to V SUPPLY −V PRI ) existing at common terminals  150 ,  156  of the primary and secondary resistive elements  114  and  120  respectively. Voltage V N′  is slightly greater than voltage V N  because of the additional voltage drop across secondary resistive element  120 . As the current I F  through the first resistive element increases towards a maximum allowable limit, voltage V P decreases. When voltage V P  decreases to less than voltage V N′ , flag signal FLAG transitions to logic HIGH thereby indicating that current I F  is near or at the predetermined current limit. 
     Charge pump  126  provides a pump voltage V PUMP  at output terminal  178  to amplifier  34 . Generally, pump voltage V PUMP  is a magnification of the charge pump supply voltage V SUPPLY2 . In one embodiment, charge pump  126  is a doubler, thereby doubling charge pump supply voltage V SUPPLY2 . In another embodiment, charge pump supply voltage V SUPPLY2  is substantially equal to supply voltage V SUPPLY . The higher pump voltage V PUMP  allows the amplifier  34  to generate a control signal CONTROL of sufficient magnitude to fully enhance MOSFET  23  to operate in the triode region under normal operating conditions when the load current (I L ) is less than the maximum allowable current. 
     Reset-switch  132  receives reset signal RESETEN at control terminal  202 . In response, reset-switch  132  connects or disconnects gate  71  of MOSFET  23  to ground. When gate  71  is coupled to ground, the gate capacitance of the MOSFET  23  is discharged. Consequently, when reset signal RESETEN changes state to activate the circuit, load current I L  gradually increases as the gate capacitance of MOSFET  23  is again charged. 
     Referring to FIG. 3, an alternative embodiment to the circuit of FIG. 2 includes a level shifter  50  and reconfigured comparator  138 ′. The charge pump output terminal  178  is connected to input terminal  54  of the level shifter  50 . The output terminal  56  of the level shifter  50  is connected to gate  71  of MOSFET  23  in load current controller  22 . Comparator  138 ′ has a negative input terminal  214 ′ connected to the output terminal  108  of amplifier  34 . 
     In operation, the level shifter  50  provides the control signal CONTROL to modulate the load current controller  22 . The control signal CONTROL is of sufficient magnitude to fully enhance MOSFET  23  to operate in the triode region under operating conditions when the load current (I L ) is less than the maximum allowable current. Comparator  138 ′ compares the voltage generated at amplifier terminal  108  and a reference voltage (V X ) applied to its positive input terminal  208 ′. The reference voltage (V X ) is selected to correspond to the voltage at terminal  108  of amplifier  34  when the load current I L  is substantially at the maximum allowable current. As current (I F ) increases to equal the maximum allowable current, flag signal FLAG transitions from logic LOW to logic HIGH to indicate that the circuit is operating at the current limit. 
     With reference to FIG. 4, one embodiment of the present invention relates to a method  300  for providing a voltage across a load in which the voltage has a temperature independent current compliance. In step  310  a first temperature dependent voltage (V P ) indicative of a load current is generated. For example, the voltage (V P ) can be generated by conducting the load current through a know resistance. In step  320 , a temperature dependent reference voltage (V N ) is generated. A control signal CONTROL is generated (step  330 ) by amplifying the difference of the temperature dependent voltages (V P ) and (V N ). The control signal CONTROL is applied (step  340 ) to a load controller. The load controller provides a current having a temperature independent current compliance to the load. The steps of the method  300  are preferably directed to a feedback loop therefore, after completing step  340 , the method returns to step  310  to again perform steps  310  through  340 . 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For example, all polarities of logic and voltage signals are shown to represent such polarities in a single functional embodiment. One skilled in the art can easily choose different polarities and arrange the specific components and logic accordingly. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the invention described herein. Scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.