PFM-PWM DC-DC converter providing DC offset correction to PWM error amplifier and equalizing regulated voltage conditions when transitioning between PFM and PWM modes

Abstract

To prevent a voltage glitch in the regulated DC output voltage of a PWM/PFM DC-DC converter when switching between PFM and PMW modes, the error amplifier of the converter's PWM regulation path is provided with a DC voltage offset correction mechanism. This mechanism “zeros-out” DC voltage offsets that may be present in the voltage regulation path, thereby enabling the error amplifier to accurately regulate the converter's output voltage. When the converter transitions between PFM and PWM modes, the DC offset correction mechanism establishes initial conditions of the error amplifier that effectively ensure that the converter's regulated output voltage at the beginning of a new “switched-to” PWM mode cycle is DC offset-free.

What is claimed: 
     
       1. A multi-mode DC-DC converter for supplying a regulated DC output voltage to a load comprising:
 an output power switching stage having upper and lower power switches coupled between an input voltage terminal and a reference voltage terminal and having a common node therebetween coupled by way of an output inductor to an output node from which said regulated DC output voltage is supplied; 
 a control circuit for controlling switching of said upper and lower power switches of said output power switching stage for respectively different modes of operation of said multi-mode DC-DC converter in accordance with respectively different switching control signals applied thereto; 
 a plurality of switching control signal generators, which are operative to generate said respectively different switching control signals, in response to which said control circuit controls switching of said upper and lower power switches of said output power switching stage during said respectively different modes of operation of said multi-mode DC-DC converter; and 
 a mode transition control circuit, coupled with said plurality of switching control signal generators, and being operative to prevent an anomaly in said DC output voltage at a transition between respectively different modes of operation of said multi-mode DC-DC converter. 
 
     
     
       2. The multi-mode DC-DC converter according to  claim 1 , wherein said mode transition control circuit is operative to prevent the occurrence of a DC voltage offset in said DC output voltage at said transition between respectively different modes of operation of said multi-mode DC-DC converter. 
     
     
       3. The multi-mode DC-DC converter according to  claim 1 , wherein said respectively different modes of operation of said multi-mode DC-DC converter correspond to continuous conduction mode (CCM) operation and discontinuous conduction mode (DCM) operation, and wherein said mode transition control circuit is operative to prevent the occurrence of a DC voltage offset in said DC output voltage at a transition from DCM mode operation to CCM mode operation of said multi-mode DC-DC converter. 
     
     
       4. The multi-mode DC-DC converter according to  claim 1 , wherein said respectively different modes of operation of said multi-mode DC-DC converter correspond to pulse width modulation (PWM) mode and pulse frequency modulation (PFM) mode, and wherein said mode transition control circuit is operative to prevent the occurrence of a DC voltage offset in said DC output voltage at a transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter. 
     
     
       5. The multi-mode DC-DC converter according to  claim 4 , wherein said plurality of switching control signal generators include
 a PWM waveform generator which is operative to control the pulse width of a PWM waveform supplied thereby to said control circuit in accordance with a prescribed relationship between said regulated DC output voltage and a reference voltage, and 
 a PFM waveform generator which is operative to control the pulse frequency of said PFM waveform supplied thereby to said control circuit in accordance with a prescribed relationship between said regulated DC output voltage and said reference voltage; and wherein 
 said mode transition control circuit is operative to establish an electrical parameter of said PWM waveform generator that is effective to prevent said DC voltage offset in said DC output voltage, at the time of said transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter. 
 
     
     
       6. The multi-mode DC-DC converter according to  claim 5 , wherein said PWM waveform generator includes an error amplifier, and wherein said mode transition control circuit is operative to establish an electrical parameter of said error amplifier, at the time of said transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter, that is effective to compensate for one or more DC offset voltages in a voltage regulation path of said PWM waveform generator, and thereby equalize the value of said DC output voltage at the beginning of a PWM mode of operation of said converter with the value of said DC output voltage at the end of an immediately previous PFM mode of operation of said converter. 
     
     
       7. The multi-mode DC-DC converter according to  claim 6 , wherein said error amplifier comprises an integrating error amplifier, and wherein said mode transition control circuit includes a DC offset voltage correction circuit, that is operative to place a DC offset correction voltage across said integrating error amplifier at the time of said transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter, that is effective to compensate for said one or more DC offset voltages in said voltage regulation path of said PWM waveform generator. 
     
     
       8. The multi-mode DC-DC converter according to  claim 7 , wherein said DC offset voltage correction circuit is operative, during a PFM mode of operation of said converter, to store information representative of said DC offset correction voltage as placed across said integrating error amplifier during an immediately preceding PWM mode of operation of said converter and, in response to a transition from said PFM mode of operation to a new PWM mode of operation of said converter, to place a DC offset correction voltage across said integrating error amplifier in accordance with said information. 
     
     
       9. The multi-mode DC-DC converter according to  claim 7 , wherein said PWM waveform generator includes a transconductance amplifier coupled to the output of said error amplifier, and wherein said DC offset voltage correction circuit is operative to inject, into the output of said transconductance amplifier, a current representative of said DC offset correction voltage across, and which is reflected back across inputs of said transimpedance amplifier as said DC offset correction voltage, so as to be placed thereby across and charge an integrating capacitor of said integrating error amplifier. 
     
     
       10. The multi-mode DC-DC converter according to  claim 9 , wherein said DC offset voltage correction circuit includes an up/down counter that is controllably incremented or decremented, based upon the polarity of the voltage differential across said integrating error amplifier, by transitions from PFM to PWM mode of operation of said converter, and a current generator, which is operative to generate said current in accordance with the count value of said up/down counter. 
     
     
       11. The multi-mode DC-DC converter according to  claim 10 , wherein said DC offset voltage correction circuit further includes an N-bit counter, which is operative, in response to the number of times that said up/down counter switches back and forth between an increment and a decrement condition reaching a prescribed number, to prevent further operation of said up/down counter by PWM mode transitions, so that the value of said current is fixed by the count value of said up/down counter. 
     
     
       12. A method of controlling the operation of a multi-mode DC-DC converter for supplying a regulated DC output voltage to a load, said multi-mode DC-DC converter including
 an output power switching stage having upper and lower power switches coupled between an input voltage terminal and a reference voltage terminal and having a common node therebetween coupled by way of an output inductor to an output node from which said regulated DC output voltage is supplied, 
 a control circuit for controlling switching of said upper and lower power switches of said output power switching stage for respectively different modes of operation of said multi-mode DC-DC converter in accordance with respectively different switching control signals applied thereto, and 
 a plurality of switching control signal generators, which are operative to generate said respectively different switching control signals, in response to which said control circuit controls switching of said upper and lower power switches of said output power switching stage during said respectively different modes of operation of said multi-mode DC-DC converter, said method comprising the steps of: 
 (a) monitoring an electrical parameter of one of said plurality of switching control signal generators; and 
 (b) in the course of a transition between respectively different modes of operation of said converter, controlling the operation of said one of said plurality of switching control signal generators, in accordance with the electrical parameter monitored in step (a) so as to prevent an anomaly in said DC output voltage for said respectively different modes of operation of said converter. 
 
     
     
       13. The method according to  claim 12 , wherein said respectively different modes of operation of said multi-mode DC-DC converter correspond to continuous conduction mode (CCM) operation and discontinuous conduction mode (DCM) operation, and wherein step (b) comprises controlling the operation of said one of said plurality of switching control signal generators, in accordance with the electrical parameter monitored in step (a) so as to prevent the occurrence of a DC voltage offset in said DC output voltage at a transition from DCM mode operation to CCM mode operation of said multi-mode DC-DC converter. 
     
     
       14. The method according to  claim 12 , wherein said respectively different modes of operation of said multi-mode DC-DC converter correspond to pulse width modulation (PWM) mode and pulse frequency modulation (PFM) mode, and wherein step (b) comprises controlling the operation of said one of said plurality of switching control signal generators, in accordance with the electrical parameter monitored in step (a) so as to prevent the occurrence of a DC voltage offset in said DC output voltage at a transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter. 
     
     
       15. The method according to  claim 14 , wherein said plurality of switching control signal generators include
 a PWM waveform generator which is operative to control the pulse width of a PWM waveform supplied thereby to said control circuit in accordance with a prescribed relationship between said regulated DC output voltage and a reference voltage, and 
 a PFM waveform generator which is operative to control the pulse frequency of said PFM waveform supplied thereby to said control circuit in accordance with a prescribed relationship between said regulated DC output voltage and said reference voltage; and wherein 
 step (b) comprises compensating for the value of the electrical parameter of said PWM waveform generator monitored in step (a), so as to prevent said DC voltage offset in said DC output voltage, at the time of said transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter. 
 
     
     
       16. The method according to  claim 15 , wherein said PWM waveform generator includes an error amplifier, and wherein step (b) comprises establishing the value of said electrical parameter of said error amplifier, at the time of said transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter, that is effective to compensate for one or more DC offset voltages in a voltage regulation path of said PWM waveform generator, and thereby equalize the value of said DC output voltage at the beginning of a PWM mode of operation of said converter with the value of said DC output voltage at the end of an immediately previous PFM mode of operation of said converter. 
     
     
       17. The method according to  claim 16 , wherein said error amplifier comprises an integrating error amplifier, and wherein step (b) comprises placing a DC offset correction voltage across said integrating error amplifier at the time of said transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter, so as to compensate for said one or more DC offset voltages in said voltage regulation path of said PWM waveform generator. 
     
     
       18. The method according to  claim 17 , wherein step (b) comprises storing, during a PFM mode of operation of said converter, information representative of said DC offset correction voltage as placed across said integrating error amplifier during an immediately preceding PWM mode of operation of said converter and, in response to a transition from said PFM mode of operation to a new PWM mode of operation of said converter, placing a DC offset correction voltage across said integrating error amplifier in accordance with said information. 
     
     
       19. The method according to  claim 17 , wherein said PWM waveform generator includes a transconductance amplifier coupled to the output of said error amplifier, and wherein step (b) comprises injecting, into the output of said transconductance amplifier, a current representative of said DC offset correction voltage across, so that said current is reflected back across inputs of said transimpedance amplifier as said DC offset correction voltage, so as to be placed thereby across and charge an integrating capacitor of said integrating error amplifier. 
     
     
       20. For use with a pulse width modulation (PWM) mode to pulse frequency modulation (PFM) mode DC-DC converter for supplying a regulated DC output voltage to a load, said PWM mode to PFM mode DC-DC converter including
 an output power switching stage having upper and lower power switches coupled between an input voltage terminal and a reference voltage terminal and having a common node therebetween coupled by way of an output inductor to an output node from which said regulated DC output voltage is supplied; 
 a control circuit for controlling switching of said upper and lower power switches of said output power switching stage for said PWM and PFM modes of operation of said converter in accordance with respective PWM and PFM switching control signals applied thereto; and 
 PWM and PFM switching control signal generators, which are operative to respectively generate PWM and PFM switching control signals, in response to which said control circuit controls switching of said upper and lower power switches of said output power switching stage during said PWM and PFM modes of operation of said converter, said PWM switching control signal generator including an error amplifier; 
 the improvement comprising: 
 a DC offset voltage correction circuit, that is coupled with said error amplifier, and is operative to place a DC offset correction voltage across said error amplifier at the time of said transition from PFM mode operation to PWM mode operation of said converter, that is effective to compensate for one or more DC offset voltages in a voltage regulation path of said PWM waveform generator. 
 
     
     
       21. A multi-mode DC-DC converter for supplying a regulated DC output voltage to a load comprising:
 an output power switching stage having at least one power switch coupled between an input voltage terminal and a reference voltage terminal and having a node coupled by way of an output inductor to an output node from which said regulated DC output voltage is supplied;   a control circuit for controlling switching of said at least one power switch of said output power switching stage for respectively different modes of operation of said multi-mode DC-DC converter in accordance with respectively different switching control signals applied thereto;   a plurality of switching control signal generators, which are operative to generate said respectively different switching control signals, in response to which said control circuit controls switching of said at least one power switch of said output power switching stage during said respectively different modes of operation of said multi-mode DC-DC converter; and   a mode transition control circuit, coupled with said plurality of switching control signal generators, and being operative to prevent an anomaly in said DC output voltage at a transition between respectively different modes of operation of said multi-mode DC-DC converter.   
     
     
       22. The multi-mode DC-DC converter according to claim 21, wherein said respectively different modes of operation of said multi-mode DC-DC converter correspond to pulse width modulation (PWM) mode and pulse frequency modulation (PFM) mode, and wherein said mode transition control circuit is operative to prevent the occurrence of a DC voltage offset in said DC output voltage at a transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter, and wherein said plurality of switching control signal generators include:
 a PWM waveform generator which is operative to control the pulse width of a PWM waveform supplied thereby to said control circuit in accordance with a prescribed relationship between said regulated DC output voltage and a reference voltage, and   a PFM waveform generator which is operative to control the pulse frequency of said PFM waveform supplied thereby to said control circuit in accordance with a prescribed relationship between said regulated DC output voltage and said reference voltage; and wherein   said mode transition control circuit is operative to establish an electrical parameter of said PWM waveform generator that is effective to prevent said DC voltage offset in said DC output voltage, at the time of said transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter.   
     
     
       23. The multi-mode DC-DC converter according to claim 22, wherein said PWM waveform generator includes an error amplifier, and wherein said mode transition control circuit is operative to establish an electrical parameter of said error amplifier, at the time of said transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter, that is effective to compensate for one or more DC offset voltages in a voltage regulation path of said PWM waveform generator, and thereby equalize the value of said DC output voltage at the beginning of a PWM mode of operation of said converter with the value of said DC output voltage at the end of an immediately previous PFM mode of operation of said converter. 
     
     
       24. The multi-mode DC-DC converter according to claim 23, wherein said error amplifier comprises an integrating error amplifier, and wherein said mode transition control circuit includes a DC offset voltage correction circuit, that is operative to place a DC offset correction voltage across said integrating error amplifier at the time of said transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter, that is effective to compensate for said one or more DC offset voltages in said voltage regulation path of said PWM waveform generator. 
     
     
       25. The multi-mode DC-DC converter according to claim 24, wherein said PWM waveform generator includes a transconductance amplifier coupled to the output of said error amplifier, and wherein said DC offset voltage correction circuit is operative to inject, into the output of said transconductance amplifier, a current representative of said DC offset correction voltage across, and which is reflected back across inputs of said transimpedance amplifier as said DC offset correction voltage, so as to be placed thereby across and charge an integrating capacitor of said integrating error amplifier. 
     
     
       26. The multi-mode DC-DC converter according to claim 25, wherein said DC offset voltage correction circuit includes an up/down counter that is controllably incremented or decremented, based upon the polarity of the voltage differential across said integrating error amplifier, by transitions from PFM to PWM mode of operation of said converter, and a current generator, which is operative to generate said current in accordance with the count value of said up/down counter. 
     
     
       27. The multi-mode DC-DC converter according to claim 26, wherein said DC offset voltage correction circuit further includes an N-bit counter, which is operative, in response to the number of times that said up/down counter switches back and forth between an increment and a decrement condition reaching a prescribed number, to prevent further operation of said up/down counter by PWM mode transitions, so that the value of said current is fixed by the count value of said up/down counter. 
     
     
       28. A method of controlling the operation of a multi-mode DC-DC converter for supplying a regulated DC output voltage to a load, said multi-mode DC-DC converter including:
 an output power switching stage having at least one power switch coupled between an input voltage terminal and a reference voltage terminal and having a node therebetween coupled by way of an output inductor to an output node from which said regulated DC output voltage is supplied;   a control circuit for controlling switching of said at least one power switch of said output power switching stage for respectively different modes of operation of said multi-mode DC-DC converter in accordance with respectively different switching control signals applied thereto; and   a plurality of switching control signal generators, which are operative to generate said respectively different switching control signals, in response to which said control circuit controls switching of said at least one power switch of said output power switching stage during said respectively different modes of operation of said multi-mode DC-DC converter, said method comprising the steps of:
 (a) monitoring an electrical parameter of one of said plurality of switching control signal generators; and 
 (b) in the course of a transition between respectively different modes of operation of said converter, controlling the operation of said one of said plurality of switching control signal generators, in accordance with the electrical parameter monitored in step (a) so as to prevent an anomaly in said DC output voltage for said respectively different modes of operation of said converter. 
   
     
     
       29. The method according to claim 28, wherein said respectively different modes of operation of said multi-mode DC-DC converter correspond to continuous conduction mode (CCM) operation and discontinuous conduction mode (DCM) operation, and wherein step (b) comprises controlling the operation of said one of said plurality of switching control signal generators, in accordance with the electrical parameter monitored in step (a) so as to prevent the occurrence of a DC voltage offset in said DC output voltage at a transition from DCM mode operation to CCM mode operation of said multi-mode DC-DC converter. 
     
     
       30. The method according to claim 28, wherein said respectively different modes of operation of said multi-mode DC-DC converter correspond to pulse width modulation (PWM) mode and pulse frequency modulation (PFM) mode, and
 wherein step (b) comprises controlling the operation of said one of said plurality of switching control signal generators, in accordance with the electrical parameter monitored in step (a) so as to prevent the occurrence of a DC voltage offset in said DC output voltage at a transition from PFM mode operation to PWM mode operation of said multi-mode DC-DC converter.   
     
     
       31. A electronic system comprising:
 a functional circuit; and   a multi-mode DC-DC converter for supplying a regulated DC output voltage to the functional circuit, the multi-mode DC-DC converter comprising:
 an output power switching stage having at least one power switch having a node coupled by way of an output inductor to an output node; 
 a control circuit for controlling switching of said at least one power switch between different modes of operation of said multi-mode DC-DC converter in accordance with respectively different switching control signals applied thereto; 
 a plurality of switching control signal generators to generate said respectively different switching control signals, in response to which said control circuit controls switching of said at least one power switch of said output power switching stage during said respectively different modes of operation of said multi-mode DC-DC converter; and 
 a mode transition control circuit, coupled with said plurality of switching control signal generators, and being operative to prevent an anomaly in said DC output voltage at a transition between respectively different modes of operation of said multi-mode DC-DC converter. 
   
     
     
       32. The system of claim 31, wherein said respectively different modes of operation correspond to continuous conduction mode (CCM) operation and discontinuous conduction mode (DCM) operation, and wherein said mode transition control circuit prevents the occurrence of a DC voltage offset in said DC output voltage at a transition from DCM mode operation to CCM mode operation. 
     
     
       33. The system of claim 31, wherein said respectively different modes of operation correspond to pulse width modulation (PWM) mode and pulse frequency modulation (PFM) mode, and wherein said mode transition control circuit prevents the occurrence of a DC voltage offset in said DC output voltage at a transition from a PFM mode operation to a PWM mode operation of said multi-mode DC-DC converter. 
     
     
       34. The system of claim 33, wherein said plurality of switching control signal generators include:
 a PWM waveform generator to control the pulse width of a PWM waveform supplied thereby to said control circuit in accordance with a prescribed relationship between said regulated DC output voltage and a reference voltage, and   a PFM waveform generator to control the pulse frequency of said PFM waveform supplied thereby to said control circuit in accordance with a prescribed relationship between said regulated DC output voltage and said reference voltage; and   wherein said mode transition control circuit establishes an electrical parameter of said PWM waveform generator that is effective to prevent said DC voltage offset in said DC output voltage, at the time of said transition from the PFM mode operation to the PWM mode operation of said multi-mode DC-DC converter.   
     
     
       35. The system of claim 31, wherein the functional circuit includes a processor. 
     
     
       36. A method of controlling the operation of a multi-mode DC-DC converter for supplying a regulated DC output voltage to a load, said method comprising:
 monitoring an electrical parameter of one of a plurality of switching control signal generators; and   controlling the operation of said one of the plurality of switching control signal generators in the course of a transition between different modes of operation of the multi-mode DC-DC converter, the controlling being based on the monitored electrical parameter, to prevent an anomaly in a DC output voltage for the different modes of operation of the multi-mode DC-DC converter.   
     
     
       37. The method according to claim 36, wherein said different modes of operation correspond to continuous conduction mode (CCM) operation and discontinuous conduction mode (DCM) operation, and wherein controlling the operation of said one of the plurality of switching control signal generators comprises controlling the operation of said one of said plurality of switching control signal generators to prevent a DC voltage offset in said DC output voltage at a transition from DCM mode operation to CCM mode operation. 
     
     
       38. The method according to claim 37, wherein said different modes of operation correspond to pulse width modulation (PWM) mode and pulse frequency modulation (PFM) mode, and wherein controlling the operation of said one of the plurality of switching control signal generators comprises controlling the operation of said one of said plurality of switching control signal generators to prevent the occurrence of a DC voltage offset in said DC output voltage at a transition from a PFM mode operation to a PWM mode operation. 
     
     
       39. The method according to claim 38, wherein monitoring the electrical parameter of one of the plurality of the switching control signal generators comprises monitoring a PWM waveform generator. 
     
     
       40. The method according to claim 39, wherein said PWM waveform generator includes a transconductance amplifier coupled to the output of said error amplifier, and wherein controlling the operation of said one of the plurality of switching control signal generators comprises injecting, into the output of said transconductance amplifier, a current representative of a DC offset correction voltage across, so that said current is reflected back across inputs of said transimpedance amplifier as said DC offset correction voltage, so as to be placed thereby across and charge an integrating capacitor of said integrating error amplifier. 
     
     
       41. The method according to claim 36, further comprising:
 establishing a value of the electrical parameter of an error amplifier in the PWM waveform generator at the time of transition from a PFM mode operation to a PWM mode operation; and   equalizing a value of said DC output voltage at a beginning of the PWM mode of operation of said multi-mode DC-DC converter with a value of said DC output voltage at the end of an immediately previous PFM mode of operation of said multi-mode DC-DC converter.   
     
     
       42. The method according to claim 41, wherein said error amplifier comprises an integrating error amplifier, and wherein establishing the value of the electrical parameter of the error amplifier comprises placing a DC offset correction voltage across the integrating error amplifier at the time of said transition from the PFM mode operation to the PWM mode operation. 
     
     
       43. A control chip for use with a multi-mode DC-DC converter, the control chip comprising:
 a control circuit to control switching between different modes of operation of the multi-mode DC-DC converter in accordance with respectively different switching control signals applied to at least one power switch of an output power switching stage; and   a mode transition control circuit, coupled with a plurality of switching control signal generators, the mode transition control circuit operative to prevent an anomaly in a DC output voltage of the multi-mode DC-DC converter during a transition between the respectively different modes of operation.   
     
     
       44. The control chip of claim 43, wherein the transition between the respectively different modes of operation comprises a transition between a PFM mode and a PWM mode, and wherein the mode transition control circuit comprises:
 a DC offset correction circuit having a VNEG input and a VPOS input, the DC offset correction circuit including;
 a NAND gate having an input coupled to a PWM output of a PFM-PWM mode comparator, 
 a differential amplifier having inputs coupled to the VNEG input and the VPOS input of the DC offset correction circuit, and 
 an up/down counter configured to receive an up/down count control input from the differential amplifier, wherein a clocking operation of up/down counter is controlled by an output of the NAND gate, and wherein transitions in the PWM output are used to clock the up/down counter. 
   
     
     
       45. The control chip of claim 44, wherein the NAND gate provides a count signal to a CETbar input of the up/down counter with each successive transition from the PFM mode to the PWM mode. 
     
     
       46. The control chip of claim 43, wherein the plurality of switching control signal generators coupled with the mode transition control circuit include:
 a PWM waveform generator which is operative to control the pulse width of a PWM waveform supplied to the control circuit, and   a PFM waveform generator which is operative to control the pulse frequency of a PFM waveform supplied to the control circuit, wherein the mode transition control circuit establishes an electrical parameter of said PWM waveform generator that is effective to prevent said DC voltage offset in said DC output voltage, at the time of the transition from a PFM mode operation to a PWM mode operation of the multi-mode DC-DC converter.