Discrete narrow-band switching frequency avoidance of a switch mode power converter

Abstract

The switching frequency of a switch mode PFM power converter is compared with a predetermined frequency range that contains the operating frequency of a nearby clocked sub-system. In response to the switching frequency coming into the range, a parameter of the power converter is changed from an original value, so as to cause the switching frequency to go out of the range.

What is claimed is: 
     
         1 . An electronic circuit for controlling a switching frequency of a switch mode PFM controller, comprising:
 a frequency monitor having an output that can take first, second and third states,   the first state qualitatively, not quantitatively, indicating that a frequency of a power transistor switching control signal for a switch mode PFM controller is above a predetermined frequency range,   the second state indicating qualitatively, not quantitatively, that the frequency is below the range,   and the third state indicating qualitatively, not quantitatively, that the frequency is within the range; and   decision logic that is to, in response to the state output from the frequency monitor, indicate a change, or no change, to a parameter of the controller.   
     
     
         2 . The electronic circuit of  claim 9  wherein the frequency monitor comprises:
 a capacitor; 
 a trimmed current source coupled to charge the capacitor; 
 a reset switch coupled to reset the capacitor to a known voltage; and 
 a window comparator having an input coupled to the capacitor. 
 
     
     
         3 . The electronic circuit of  claim 10  wherein the frequency monitor further comprises a one shot coupled to control the reset switch. 
     
     
         4 . The electronic circuit of  claim 11  wherein the one shot has an input to receive a transistor power stage control signal of the controller. 
     
     
         5 . The electronic circuit of  claim 12  wherein the window comparator provides the state output of the frequency monitor, and wherein the state output is updated in response to each output pulse from the one shot. 
     
     
         6 . A process for operating a pulse frequency modulation (PFM) switch mode power converter in a battery powered portable electronic device, comprising:
 monitoring a switching frequency of a switch mode power converter that is operating in PFM mode in a battery powered portable electronic device;   comparing the switching frequency with a predetermined frequency range that contains a clock frequency of a clocked sub-system in the portable electronic device, wherein the predetermined frequency range corresponds to variation of the clock frequency of the clocked sub-system from its nominal value, due to manufacturing process, supply voltage, and temperature variations; and   in response to the switching frequency coming into the predetermined frequency range, changing a parameter of the power converter from an original value, thereby causing the switching frequency to go out of the predetermined frequency range.   
     
     
         7 . The process of  claim 1  wherein the parameter represents one of an inductor current limit, a transistor control pulse on-time, a transistor control pulse off-time, and a transistor control burst mode interval or count. 
     
     
         8 . The process of  claim 2  wherein the change in the parameter of the power converter is less than 10%. 
     
     
         9 . The process of  claim 1  wherein the variation of the clock frequency of the clocked sub-system from its nominal value is no more than 5%. 
     
     
         10 . The process of  claim 5  wherein the change in the parameter of the power converter is less than 10%. 
     
     
         11 . The process of  claim 1  wherein the change in the parameter of the power converter is less than 10%. 
     
     
         12 . The process of  claim 1  further comprising, in response to the switching frequency coming into the predetermined frequency range a second, consecutive time, changing the parameter back to the original value. 
     
     
         13 . An electronic device comprising:
 a housing having installed therein
 a plurality of sub-systems including a clocked sub-system having an operating frequency; and 
 a switch mode PFM power converter whose switching frequency is to be compared with a predetermined frequency range that contains the operating frequency of the clocked sub-system, and in response to the switching frequency coming into the predetermined frequency range, a parameter of the power converter is changed from an original value, so as to cause the switching frequency to go out of the predetermined frequency range, wherein the predetermined frequency range corresponds to variation of the operating frequency of the clocked sub-system from its nominal value, due to manufacturing process, supply voltage, and temperature variations. 
   
     
     
         14 . The electronic device of  claim 14  wherein in response to the switching frequency coming into the predetermined frequency range a second, consecutive time, the parameter is to be changed back to the original value. 
     
     
         15 . The electronic device of  claim 14  wherein the change in the parameter of the power converter is less than 10%. 
     
     
         16 . The electronic device of  claim 14  wherein the variation of the operating frequency of the clocked sub-system from its nominal value is no larger than 5%. 
     
     
         17 . The electronic device of claim  18  wherein the change in the parameter of the power converter is less than 10%. 
     
     
         20 . The electronic device of  claim 14  wherein the parameter of the power converter represents one of an inductor current limit, a transistor control pulse on-time, a transistor control pulse off-time, and a transistor control burst mode interval or count, and wherein the change in the parameter of the power converter is less than 10%.