Synthetic ripple regulator

Abstract

A synthetic ripple regulator including a synthetic ripple voltage generator that generates a synthetic ripple voltage indicative of the ripple current through an output inductor. The regulator uses the synthetically generated ripple voltage to control toggling of a hysteretic comparator for developing the pulse width modulation (PWM) signal that controls switching of the regulator. In a non-limiting implementation, a transconductance amplifier monitors the phase node voltage of the inductor and supplies an inductor voltage-representative current to a ripple capacitor, which produces the synthetic ripple voltage. Using the replicated inductor current for ripple regulation results in low output ripple, input voltage feed forward, and simplified compensation.

1. A synthetic ripple voltage generator for a DC-DC regulator having a hysteretic comparator that develops a pulse width modulation (PWM) signal provided to a switching circuit that switches an input voltage via a phase node through an output inductor to develop an output voltage at an output node, said synthetic ripple voltage generator comprising:
 a transconductance amplifier circuit having an input for coupling to the output inductor and an output for coupling to an input of the hysteretic comparator; and 
 a ripple capacitor coupled to said output of said transconductance amplifier circuit; 
 wherein said transconductance amplifier circuit provides current to said ripple capacitor which develops a voltage indicative of ripple current through the output inductor. 
 
     
     
       2. The synthetic ripple voltage generator of  claim 1 , wherein said transconductance amplifier circuit comprises a transconductance amplifier having a non-inverting input for coupling to the phase node and an inverting input for coupling to the output node. 
     
     
       3. The synthetic ripple voltage generator of  claim 1 , wherein said transconductance amplifier circuit comprises a transconductance amplifier having a non-inverting input for coupling to the phase node and an inverting input for coupling to a reference voltage. 
     
     
       4. The synthetic ripple voltage generator of  claim 1 , wherein said ripple capacitor has a first end coupled to said output of said transconductance amplifier circuit and a second end for coupling to the output node. 
     
     
       5. The synthetic ripple voltage generator of  claim 1 , wherein said ripple capacitor has a first end coupled to said output of said transconductance amplifier circuit and a second end coupled to ground. 
     
     
       6. The synthetic ripple voltage generator of  claim 1 , further comprising a ripple resistor coupled to said ripple capacitor. 
     
     
       7. The synthetic ripple voltage generator of  claim 6 , further comprising:
 a voltage source referenced to ground; and 
 said ripple resistor having a first end coupled to said ripple capacitor and a second end coupled to said voltage source. 
 
     
     
       8. The synthetic ripple voltage generator of  claim 6 , wherein said ripple resistor has a first end coupled to said ripple capacitor and a second end for coupling to the output node. 
     
     
       9. The synthetic ripple voltage generator of  claim 1 , wherein said transconductance amplifier circuit comprises:
 a first transconductance amplifier having an input for coupling to the output node and an output coupled to said ripple capacitor to discharge said ripple capacitor based on the output voltage; 
 a second transconductance amplifier having an input for receiving the input voltage and an output; and 
 a switch circuit, having an input coupled to said output of said second transconductance amplifier, an output coupled to said ripple capacitor, and a control input for receiving the PWM signal, said switch circuit operative to couple said output of said second transconductance amplifier to said ripple capacitor to charge said ripple capacitor based on the input voltage upon initiation of PWM cycles. 
 
     
     
       10. A method of synthetically generating ripple voltage for a DC-DC regulator including a hysteretic comparator that develops a pulse width modulation (PWM) signal provided to a switching circuit that switches an input voltage via a phase node through an output inductor to develop an output voltage at an output node, said method comprising:
 developing a ripple voltage indicative of current through the output inductor; and 
 applying the ripple voltage to an input of the hysteretic comparator. 
 
     
     
       11. The method of  claim 10 , wherein said developing a ripple voltage indicative of current through the output inductor comprises:
 sensing voltage applied to the output inductor; 
 converting the sensed voltage to a sense current; and 
 charging a capacitive device with the sense current. 
 
     
     
       12. The method of  claim 11 , wherein said sensing voltage applied to the output inductor comprises sensing voltage at the phase node. 
     
     
       13. The method of  claim 11 , further comprising referencing the capacitive device to the output node. 
     
     
       14. The method of  claim 11 , further comprising referencing the capacitive device to ground. 
     
     
       15. The method of  claim 11 , further comprising coupling a resistive device to the capacitive device. 
     
     
       16. The method of  claim 15 , further comprising referencing the resistive device to the output node. 
     
     
       17. The method of  claim 15 , further comprising biasing the resistive device with a voltage source. 
     
     
       18. The method of  claim 10 , wherein said developing a ripple voltage indicative of current through the output inductor comprises:
 converting the output voltage into a first current; 
 discharging a capacitive device with the first current; 
 converting the input voltage into a second current; and 
 charging the capacitive device with the second current upon initiation of each PWM cycle using the PWM signal. 
 
     
     
       19. The method of  claim 10 , wherein said developing a ripple voltage indicative of ripple current through the output inductor comprises:
 sensing ripple current through the output inductor, and 
 converting sensed current into the ripple voltage. 
 
     
     
       20. A synthetic ripple regulator, comprising:
 a hysteretic comparator having a first input and having an output that provides a pulse width modulation (PWM) signal; 
 a switching circuit that alternately couples a phase node to opposite polarities of an input voltage source based on said PWM signal; 
 an output inductor coupled between said phase node and an output node that develops a regulated output signal; 
 a transconductance amplifier circuit having an input coupled to said output inductor and an output coupled to said first input of said hysteretic comparator; and 
 a capacitor, coupled to said output of said transconductance amplifier circuit, that develops a ripple voltage indicative of ripple current through said output inductor. 
 
     
     
       21. The synthetic ripple regulator of  claim 20 , further comprising an error amplifier having a first input receiving a reference voltage, a second input coupled to said output node, and an output coupled to a second input of said hysteretic comparator. 
     
     
       22. The synthetic ripple regulator of  claim 20 , wherein said transconductance amplifier circuit comprises a transconductor amplifier having a first input coupled to said phase node and a second input coupled to said output node. 
     
     
       23. The synthetic ripple regulator of  claim 20 , wherein said transconductance amplifier circuit comprises a transconductor amplifier having a first input coupled to said phase node and a second input coupled to a reference voltage. 
     
     
       24. The synthetic ripple regulator of  claim 20 , wherein said capacitor has a first end coupled to said output of said transconductance amplifier circuit and a second end coupled to said output node. 
     
     
       25. The synthetic ripple regulator of  claim 20 , wherein said capacitor has a first end coupled to said output of said transconductance amplifier circuit and a second end coupled to ground. 
     
     
       26. The synthetic ripple regulator of  claim 20 , further comprising:
 a resistive device having a first end coupled to said output of said transconductance amplifier circuit and a second end; and 
 a voltage source coupled between said second end of said resistive device and ground. 
 
     
     
       27. The synthetic ripple regulator of  claim 20 , further comprising a resistive device having a first end coupled to said output of said transconductance amplifier circuit and a second end coupled to said output node. 
     
     
       28. The synthetic ripple regulator of  claim 20 , wherein said transconductance amplifier circuit comprises:
 a first transconductance amplifier having an input coupled to said output node and an output coupled to said capacitor to discharge said capacitor based on said regulated output signal; 
 a second transconductance amplifier having an input coupled to said input voltage source and an output; and 
 a switch that couples said output of said second transconductance amplifier to said capacitor based on said PWM signal to charge said capacitor based on input voltage during portions of PWM cycles. 
 
     
     
       29. A synthetic ripple voltage generator for a DC-DC regulator having a hysteretic comparator that develops a pulse width modulation signal provided to a switching circuit that controls switching of voltage across an output inductor which is coupled to an output node developing an output voltage, said synthetic ripple voltage generator comprising:
 a transconductance amplifier circuit having an input for coupling to the output inductor and an output for coupling to an input of the hysteretic comparator; and   a ripple capacitor coupled to said output of said transconductance amplifier circuit;   wherein said transconductance amplifier circuit provides current to said ripple capacitor which develops a voltage indicative of tipple current through the output inductor.   
     
     
       30. The synthetic ripple voltage generator of claim 29, wherein said transconductance amplifier circuit comprises a transconductance amplifier having a pair of inputs for coupling across the output inductor. 
     
     
       31. The synthetic ripple voltage generator of claim 29, wherein said tipple capacitor has a first end coupled to said output of said transconductance amplifier circuit and a second end for coupling to either one of the output node and ground. 
     
     
       32. The synthetic ripple voltage generator of claim 29, further comprising a ripple resistor having one end coupled to said ripple capacitor and another end coupled to either one of the output node and a voltage source providing a constant voltage level. 
     
     
       33. A method of synthetically generating ripple voltage for a DC-DC regulator including a hysteretic comparator that develops a pulse width modulation signal provided to a switching circuit that controls switching of voltage applied to an output inductor coupled to an output node which develops an output voltage, said method comprising: developing a ripple voltage indicative of current through the output inductor; and applying the ripple voltage to an input of the hysteretic comparator. 
     
     
       34. The method of claim 33, wherein said developing a ripple voltage indicative of current through the output inductor comprises: sensing voltage applied to the output inductor; converting the sensed voltage to a sense current; and charging a capacitive device with the sense current. 
     
     
       35. The method of claim 33, wherein said sensing voltage applied to the output inductor comprises sensing voltage at the input node. 
     
     
       36. The method of claim 33, wherein said sensing voltage applied to the output inductor comprises sensing voltage across the output inductor. 
     
     
       37. The method of claim 33, further comprising coupling a resistive device to the capacitive device. 
     
     
       38. The method of claim 37, wherein said developing a ripple voltage indicative of ripple current through the output inductor comprises: sensing ripple current through the output inductor; and converting sensed current into the ripple voltage.