Synchronous-rectified DC to DC converter with improved current sensing

Abstract

A DC to DC buck pulse width modulator converter circuit includes an input, a high side output and a low side output. A high side switch is electrically connected between a common output node and a voltage supply, and controls a flow of current therethrough dependent upon the high side output. A low side switch is electrically connected between the common output node and ground, and controls a flow of current therethrough dependent upon the low side output. A virtual ground amplifier includes a second input electrically connected to ground. A current feedback resistor is electrically connected intermediate the common output node and a first input of the virtual ground amplifier. A variable impedance component is electrically connected to an output of the virtual ground amplifier and to the first input of the virtual ground amplifier. The impedance of the variable impedance component is varied dependent upon the output of the virtual ground amplifier. A sample and hold circuit is electrically connected intermediate the input of the pulse width modulator converter circuit and the variable impedance component. The sample and hold circuit sources a virtual ground current through the variable impedance component, and samples the virtual ground current.

1. A power supply, comprising:
 a DC to DC buck pulse width modulator converter circuit having an input, a high side output and a low side output;   a high side switch electrically connected intermediate a common output node and a voltage supply, said high side switch configured for controlling a flow of current therethrough dependent at least in part upon said high side output;   a low side switch electrically connected intermediate said common output node and ground, said low side switch configured for controlling a flow of current therethrough dependent at least in part upon said low side output;   a virtual ground amplifier having a first input, a second input and an output, said second input electrically connected to ground potential;   a current feedback resistor electrically connected intermediate said common output node and said first input of said virtual ground amplifier;   a variable impedance component electrically connected to said output of said virtual ground amplifier and to said first input of said virtual ground amplifier, said variable impedance component configured to vary in impedance dependent at least in part upon said output of said virtual ground amplifier; and   a sample and hold circuit electrically connected intermediate said DC to DC buck pulse width modulator converter circuit and said variable impedance component, said sample and hold circuit configured to source a virtual ground current through said variable impedance component and to sample and hold said virtual ground current.   
     
     
       2. The power supply of  claim 1 , further comprising a system control circuit, said system control circuit electrically coupled to said sample and hold circuit, said system control circuit issuing a first control signal, said sample and hold circuit being configured to sample and hold said virtual ground current in response to said first control signal, said sample and hold circuit issuing a sample signal dependent at least in part upon said virtual ground current, said system control circuit selectively activating and deactivating at least one of said high side switch and said low side switch dependent at least in part upon said sample signal. 
     
     
       3. The power supply of  claim 2 , wherein said system control circuit issues a sync signal, said sync signal resetting at least one of said high side switch and said low side switch. 
     
     
       4. The power supply of  claim 2 , further comprising an overcurrent detector circuit electrically coupled to said sample and hold circuit and to said system control circuit, said overcurrent detector circuit configured for issuing an overcurrent signal when said sample signal exceeds a predetermined threshold. 
     
     
       5. The power supply of  claim 4 , wherein said system control circuit is configured for shutting down said DC to DC buck pulse width modulator converter circuit in response to said overcurrent signal. 
     
     
       6. The power supply of  claim 5 , wherein said system control circuit is configured to restart said DC to DC buck pulse width modulator converter circuit a predetermined period of time after receiving said overcurrent signal. 
     
     
       7. The power supply of  claim 2 , further comprising a power supply output, a voltage feedback resistor electrically connected intermediate said power supply output and said input of said DC to DC buck pulse width modulator converter circuit. 
     
     
       8. The power supply of  claim 7 , wherein said system control circuit includes a current mirror, said current mirror sourcing a droop current, said droop current being dependent at least in part upon said virtual ground current, said droop current being electrically coupled to said input of said DC to DC buck pulse width modulator converter circuit to modify a feedback voltage across said voltage feedback resistor and thereby adjust an output voltage of said power supply dependent at least in part upon said droop current. 
     
     
       9. The power supply of  claim 1 , further comprising a negative current source, said system control circuit issuing a second control signal, said second control signal being electrically coupled to said negative current source, said negative current source sourcing a negative current in response to said second control signal, said negative current flowing into said first input of said virtual ground amplifier to thereby connect said current feedback resistor to ground when current through said current feedback resistor is negative. 
     
     
       10. The power supply of  claim 1 , wherein said variable impedance component comprises one of a field effect transistor and an NPN-type transistor. 
     
     
       11. The power supply of  claim 1  further comprising an inductor having a first end and a second end, said first end electrically connected to said common output node, said second end configured for being electrically connected to a load. 
     
     
       12. A method of sensing an output current in a power supply, said power supply comprising a DC to DC buck pulse width modulator converter circuit having an input, a high side output and a low side output, said method comprising the steps of:
 electrically connecting a high side switch intermediate a common output node and a voltage supply, said high side switch configured for controlling a flow of current therethrough dependent at least in part upon said high side output;   a low side switch electrically connected intermediate said common output node and ground, said low side switch configured for controlling a flow of current therethrough dependent at least in part upon said low side output;   directing a sensed current to a virtual ground node, said sensed current comprising a known portion of the output current when said low side switch is in an on condition, said sensed current flowing into said virtual ground node in a first direction;   sourcing a virtual ground current into said virtual ground node, said virtual ground current flowing into said virtual ground node in a second direction, said second direction being opposite to said first direction, said virtual ground current being substantially equal to said sensed current and thereby canceling said sensed current at said virtual ground node; and   sampling and holding a value of said virtual ground current.   
     
     
       13. The method of  claim 12 , comprising the further step of selectively activating and deactivating at least one of said high side switch and said low side switch dependent at least in part upon said sampling and holding step. 
     
     
       14. The method of  claim 12 , comprising the further step of selectively activating and deactivating at least one of said high side switch and said low side switch on at least one of a periodic and a random basis. 
     
     
       15. The method of  claim 12 , comprising the further steps of:
 comparing said sampled and held value of said virtual ground current to a predetermined maximum limit; and   shutting down said power supply when said virtual ground current exceeds said predetermined maximum limit.   
     
     
       16. The method of  claim 15 , comprising the further step of restarting said power supply a predetermined period of time after said shutting down step. 
     
     
       17. The method of  claim 12 , comprising the further step of adjusting an output voltage of said power supply dependent at least in part upon said sampled and held value of said virtual ground current. 
     
     
       18. A power supply, comprising:
 a DC to DC buck pulse width modulator converter circuit having an input, a high side output and a low side output;   a high side switch electrically connected intermediate a common output node and a voltage supply, said high side switch configured for controlling a flow of current therethrough dependent at least in part upon said high side output;   a low side switch electrically connected to said common output node and to ground through a sense resistor, said low side switch configured for controlling a flow of current therethrough dependent at least in part upon said low side output;   a virtual ground amplifier having a first input, a second input and an output, said second input electrically connected to ground potential;   a current feedback resistor electrically connected intermediate said sense resistor and said first input of said virtual ground amplifier;   a variable impedance component electrically connected to said output of said virtual ground amplifier and to said first input of said virtual ground amplifier, said variable impedance component configured to vary in impedance dependent at least in part upon said output of said virtual ground amplifier; and   a sample and hold circuit electrically connected intermediate said DC to DC buck pulse width modulator converter circuit and said variable impedance component, said sample and hold circuit configured to source a virtual ground current through said variable impedance component and to sample and hold said virtual ground current.   
     
     
       19. A control apparatus for a DC/DC converter, comprising:
 a converter circuit having an input, a high side output adapted to drive a high side circuit and a low side output adapted to drive a low side circuit;   at least one current sense node that is adapted to receive a current signal when the low side circuit is in an on condition;   an amplifier having at least one input coupled to the at least one current sense node and having an output;   a variable impedance component coupled to the output of the amplifier, the variable impedance component configured to vary in impedance dependent at least in part upon the output of the amplifier; and   a transducer circuit coupled to the converter circuit and the variable impedance component, the transducer circuit configured to generate a signal representative of a current sourced through the variable impendence component.    
     
     
       20. The control apparatus of claim 19, wherein the variable impedance component comprises a transistor with an input coupled to the output of the amplifier.  
     
     
       21. The control apparatus of claim 19, wherein the amplifier has two inputs, the first input coupled to the current sense node and the second input coupled to a reference.  
     
     
       22. The control apparatus of claim 19, wherein the variable impedance component comprises a transistor with a first node coupled to the output of the amplifier, a second node coupled to the current sense node and a third node coupled to the sample and hold circuit.  
     
     
       23. A power supply, comprising:
 a converter circuit having an input, a high side output and a low side output;   a high side switch coupled to a common output node, the high side switch configured for controlling a flow of current therethrough dependent at least in part upon the high side output;   a low side switch coupled to the common output node, the low side switch configured for controlling a flow of current therethrough dependent at least in part upon the low side output;   an amplifier having a first input, a second input and an output, the second input coupled to a reference signal;   a current feedback resistor to provide a signal representative of a load current to the first input of the amplifier;   a variable impedance component coupled to the output of the amplifier, the variable impedance component configured to vary in impedance dependent at least in part upon the output of the amplifier; and   a transducer, coupled to the converter circuit and the variable impedance component, the transducer circuit adapted to generate a signal representative of a current sourced through the variable impendence component.    
     
     
       24. The power supply of claim 23, wherein the variable impedance component comprises one of a field effect transistor and a bipolar transistor.  
     
     
       25. The power supply of claim 23, further comprising an inductor having a first end and a second end, the first end coupled to the common output node, the second end configured for being coupled to a load.  
     
     
       26. The power supply of claim 23, wherein the variable impedance component further includes a node that is coupled to the first input of the amplifier.  
     
     
       27. A control apparatus for a DC/DC converter, comprising:
 a converter circuit having an input, a high side output adapted to drive a high side switch and a low side output adapted to drive a low side switch;   at least one current sense node that is adapted to receive a current signal, wherein the current signal is representative of a load current of the DC/DC converter;   an amplifier having a first input coupled to the at least one current sense node, a second input coupled to a reference and having an output;   a variable impedance component having a first node coupled to the output of the amplifier, a second node coupled to the at least one current sense node and a third node, the variable impedance component configured to vary in impedance dependent at least in part upon the output of the amplifier; and   a transducer, coupled to the converter circuit and the variable impedance component, the transducer circuit adapted to generate a signal representative of a current sourced through the variable impendence component.    
     
     
       28. A current sense circuit, comprising:
 a node coupled to a circuit having a current to be sensed;   an amplifier having an input coupled to the node and having an output;   a variable impedance component, responsive to the output of the amplifier, the variable impedance component providing current to the node based on the output of the amplifier; and   a transducer circuit adapted to generate a signal representative of the current provided to the node through the variable impedance component to thereby provide a measure of the current to be sensed.