Method of setting bi-directional offset in a PWM controller using a single programming pin

Abstract

A user-programmable bi-directional, constant current generator circuit allows external programming of either a positive (+) or a negative (−) polarity output current, for injection into one of two locations of the PWM controller circuit of a DC-DC voltage converter. The parameters of the DC-DC converter's offset voltage will depend upon the connection of a single programming pin to one of two programming resistors. The programming resistors are respectively referenced to different supply rail voltages (VCC and VSS). The polarity of the offset additionally depends upon where, within the PWM-controlled DC-DC converter, the programmed constant current is injected.

1. An apparatus for generating a regulated direct current (DC) voltage comprising:
 a DC-DC converter operative to generate a regulated DC output voltage derived from a supply voltage, said DC-DC converter having a pulse width modulation (PWM) generator which generates a PWM-based output voltage at an output node thereof, said output voltage being coupled through an inductor element to an output voltage terminal for application to a load;  
 a PWM controller for controlling the operation of said PWM generator, said PWM controller including 
 an operational amplifier having a first input coupled to receive a reference voltage, a second input, and an output coupled to said PWM generator, and  
 a current feedback resistor coupled between said output node and said second input of said operational amplifier; and  
 
 a user-programmable bi-directional, constant current generator circuit for controlling the operation of said PWM controller, and being configured to generate a user-programmable one of a positive (+) or a negative (−) polarity current, and to supply said a user-programmable current to a user-selected location of said PWM controller for controlling the operation of said PWM generator.  
 
     
     
       2. The apparatus according to  claim 1 , wherein
 said user-programmable bi-directional, constant current generator circuit comprises first and second operational amplifiers that drive associated switching devices,  
 said first operational amplifier is coupled receive a first DC voltage referenced to a first DC power supply rail, and said second operational amplifier is coupled to receive a second DC voltage referenced to a second DC supply rail,  
 current flow paths through said switching devices are coupled in series between an offset input terminal, to which a programming input pin is coupled and an offset current output terminal from which said user-programmable one of a positive (+) or a negative (−) polarity current is derived, and  
 said programming input pin is coupled through a selected one of a first programming resistor to said first DC voltage supply rail, and a second programming resistor to said second DC voltage supply rail.  
 
     
     
       3. The apparatus according to  claim 2 , wherein, for either a current-sourcing or a current-sinking mode, said programming input pin is coupled through said first programming resistor to said first DC voltage supply rail, causing said first operational amplifier to drive its switching device to conduct, so that said first operational amplifier attempts to make the voltage at its input match said first reference voltage, while said second operational amplifier maintains said second switching device in the off state, whereby a current will flow between said first DC supply rail through said first programming resistor, said input programming terminal and the current path of the turned-on first switching device, said offset current output terminal and said user-selected location of said PWM controller. 
     
     
       4. The apparatus according to  claim 3 , wherein for said current-sourcing mode, said user-selected location of said PWM controller to which said offset current output terminal is applied corresponds to said first input of said operational amplifier of said PWM controller. 
     
     
       5. The apparatus according to  claim 3 , wherein for said current-sinking mode, said user-selected location of said PWM controller to which said offset current output terminal is applied corresponds to said second input of said operational amplifier of said PWM controller. 
     
     
       6. The apparatus according to  claim 2 , wherein, for either a current-sourcing or a current-sinking mode, said programming input pin is coupled through said second programming resistor to said second DC voltage supply rail, causing said second operational amplifier to drive its switching device to conduct, so that said second operational amplifier attempts to make the voltage at its input match said second reference voltage, while said first operational amplifier maintains said first switching device in the off state, whereby a current will flow between said second supply rail through said second programming resistor, said input programming terminal and the current path of the turned-on second switching device, said offset current output terminal and said user-selected location of said PWM controller. 
     
     
       7. The apparatus according to  claim 6 , wherein for said current-sourcing mode, said user-selected location of said PWM controller to which said offset current output terminal is applied corresponds to said second input of said operational amplifier of said PWM controller. 
     
     
       8. The apparatus according to  claim 6 , wherein for said current-sinking mode, said user-selected location of said PWM controller to which said offset current output terminal is applied corresponds to said first input of said operational amplifier of said PWM controller. 
     
     
       9. The apparatus according to  claim 2 , further including a bandgap voltage generator circuit that is operative to generate said first and second DC voltages for application to said first and second operational amplifiers, respectively. 
     
     
       10. A method of generating a regulated direct current (DC) voltage, comprising the steps of:
 (a) providing a DC converter controlled by a pulse width modulation (PWM) generator which generates a PWM based output voltage at an output node thereof that is coupled through an inductor element to an output voltage terminal for application to a load; and  
 (b) controlling the operation of said PWM generator by performing the steps of: 
 1- providing an operational amplifier having a first input coupled to receive a voltage reference, a second input and an output coupled to said PWM generator,  
 2- coupling a current feedback resistor between said output node and said second input of said operational amplifier,  
 3- generating a user-programmable one of a positive (+) or a negative (−) polarity constant current, and  
 4- supplying said user-programmable one of a positive (+) or a negative (−) polarity constant current to a user-selected location of said PWM generator.  
 
 
     
     
       11. The method according to  claim 10 , wherein step (b) 3  comprises:
 providing first and second operational amplifiers that drive associated switching devices, said first operational amplifier being coupled to receive a first DC voltage referenced to a first DC power supply rail, and said second operational amplifier being coupled to receive a second DC voltage referenced to a second DC supply rail, current flow paths through said switching devices being coupled in series between an offset input terminal, to which a programming input pin is coupled, and an offset current output terminal from which said user-programmable one of a positive (+) or a negative (−) polarity current is derived, and wherein said programming input pin is coupled through a selected one of a first programming resistor to said first DC voltage supply rail and a second programming resistor to said second DC voltage supply rail.  
 
     
     
       12. The method according to  claim 11 , wherein, in step (b) 4 , for either a current-sourcing or a current-sinking mode, said programming input pin is coupled through said first programming resistor to said first DC voltage supply rail, causing said first operational amplifier to drive its switching device to conduct, so that said first operational amplifier attempts to make the voltage at its input match said first reference voltage, while said second operational amplifier maintains said second switching device in the off state, whereby a current flows between said first DC supply rail through said first programming resistor, said input programming terminal and the current path of the turned-on first switching device, said offset current output terminal and said user-selected location of said PWM controller. 
     
     
       13. The method according to  claim 12 , wherein step (b) 4  corresponds to said current-sourcing mode, and wherein said user-selected location of said PWM controller to which said offset current output terminal is applied corresponds to said first input of said operational amplifier of said PWM controller. 
     
     
       14. The method according to  claim 12 , wherein step (b) 4  corresponds to current-sinking mode, and wherein said user-selected location of said PWM controller to which said offset current output terminal is applied corresponds to said second input of said operational amplifier of said PWM controller. 
     
     
       15. The method according to  claim 11 , wherein, in step (b) 4 , for either a current-sourcing or current-sinking' mode, said programming input pin is coupled through said second programming resistor to said second DC voltage supply rail, causing said second operational amplifier to drive its switching device to conduct, so that said second operational amplifier attempts to make the voltage at its input match said second reference voltage, while said first operational amplifier maintains said first switching device in the off state, whereby a current will flow between said second supply rail through said second programming resistor, said input programming terminal and the current path of the turned-on second switching device, said offset current output terminal and said user-selected location of said PWM controller. 
     
     
       16. The method according to  claim 15 , wherein step (b) 4  corresponds to current-sourcing mode, and wherein said user-selected location of said PWM controller to which said offset current output terminal is applied corresponds to said second input of said operational amplifier of said PWM controller. 
     
     
       17. The method according to  claim 15 , wherein step (b) 4  corresponds to current-sinking mode, and wherein said user-selected location of said PWM controller to which said offset current output terminal is applied corresponds to said first input of said operational amplifier of said PWM controller. 
     
     
       18. The method according to  claim 11 , wherein step (b) 3  includes generating said first and second DC voltages from a bandgap voltage generator circuit. 
     
     
       19. A user-programmable constant current generator circuit comprising:
 an input terminal;  
 an output terminal from which a user-programmable constant current is derived;  
 a first operational amplifier having a first input coupled to receive a first DC voltage referenced to a first power supply rail, a second input, and an output coupled to drive an associated first switching device;  
 a second operational amplifier having a first input coupled to receive a second DC voltage referenced to a first  second power supply rail, a second input, and an output coupled to drive an associated second switching device;  
 said first and second switching devices having current flow paths therethrough coupled between said input terminal and said output terminal; and  
 a first  programming resistor  input pin coupling said input terminal to one of said first and second power supply rails.  
 
     
     
       20. The user-programmable constant current generator circuit according to  claim 19 , further including a bandgap voltage generator circuit that is operative to generate said first and second DC voltages for application to said first and second operational amplifiers, respectively.