PFM and current controlled switching regulator
View Patent ↗A circuit and method for controlling a switching regulator utilize a combination of variable off-time control (or frequency control) and variable peak current control to achieve high efficiency at a wide range of load conditions. A non-linear control circuit receives an error voltage and generates a first control signal for controlling a frequency control circuit and a second control signal for controlling a peak current control circuit. The frequency control circuit and the peak current control circuit operate in conjunction over the entire range of load conditions with the frequency control dominates at light load (or low power) conditions and the variable peak current control dominates at moderate to heavy load (or high power) conditions. The switching regulator transitions smoothly between frequency control and peak current control with continous loop gain throughout the transition region.
1. A circuit for controlling a switching regulator, the switching regulator receiving an input voltage and including a switch circuit coupled to generate a switching output voltage and an output circuit coupled to receive the switching output voltage and to supply current at a regulated output voltage to a load, the circuit comprising:
an error amplifier having a first input node coupled to receive a first voltage indicative of the regulated output voltage, a second input node coupled to receive a reference voltage and an output node providing an error voltage responsive to the difference between the first voltage and the reference voltage;
a first control circuit having an input node receiving the error voltage, a first output node providing a first control signal and a second output node providing a second control signal, the first control circuit generating the first control signal having a magnitude proportional to the error voltage for error voltages in a first voltage range, the first control signal having a maximum value for error voltage values greater than the first voltage range, the first control circuit further generating the second control signal having a minimum value for error voltages in a second voltage range, the second control signal having a magnitude proportional to the error voltage for error voltages greater than the second voltage range, the first and second voltage ranges being associated with low voltage values of the error voltage;
a second control circuit coupled to receive the first control signal and providing an output signal responsive to the first control signal for controlling the switching frequency of the switching regulator; and
a third control circuit coupled to receive the second control signal indicative of a peak current value for the current flowing through the switch circuit, the third control circuit providing an output signal responsive to the second control signal for controlling the peak current flowing through the switch circuit.
2. The circuit of claim 1 , wherein the first and second control signals cause the switching regulator to operate with variable switching frequency and substantially fixed peak current when the error voltage is within the first voltage range and to operate with substantially fixed switching frequency and variable peak current when the error voltage is outside of the second voltage range.
3. The circuit of claim 1 , wherein the second control signal has a maximum value for error voltage values greater than a third voltage range, the third voltage range being associated with high voltage values of the error voltage.
4. The circuit of claim 3 , wherein the first and second control signals operate to cause the switching regulator to operate with substantially fixed switching frequency and substantially fixed maximum peak current when the error voltage is within the third voltage range.
5. The circuit of claim 1 , wherein the maximum error voltage value of the first voltage range is different from the maximum error voltage value of the second voltage range.
6. The circuit of claim 5 , wherein the maximum error voltage value for the first voltage range is greater than the maximum error voltage value of the second voltage range.
7. The circuit of claim 6 , wherein the first and second control signals cause the switching regulator to operate with variable switching frequency and variable peak current when the error voltage is greater than the second voltage range but less than the first voltage range.
8. The circuit of claim 1 , wherein the second control circuit provides the output signal to control an off-time of the switch circuit in response to the first control signal so as to vary the switching frequency of the switching regulator.
9. The circuit of claim 1 , wherein the third control circuit provides the output signal to control the peak current value of the current flowing through the switch circuit, the output signal of the third control circuit turning off the switch circuit when the current flowing through the switch circuit reaches the peak current value indicative by the second control signal.
10. The circuit of claim 1 , wherein the first voltage is a divided-down voltage of the regulated output voltage.
11. The circuit of claim 1 , wherein the first control signal is a current signal and the second control signal is a voltage signal.
12. The circuit of claim 11 , wherein the first control circuit comprises:
a first NPN bipolar transistor having a base terminal coupled to receive the error voltage, a collector terminal providing a first current as the first control signal and an emitter terminal;
a first resistor coupled between the emitter terminal of the first bipolar transistor and a first node;
a second NPN bipolar transistor having a base terminal biased to a second voltage, a collector terminal coupled to a positive power supply voltage and an emitter terminal coupled to the first node, the first node being biased to a third voltage as the second control signal; and
a current source coupled between the first node and a negative power supply voltage and providing a second current,
wherein when the error voltage is less than the second voltage, the first current is zero and the third voltage is clamped to a base-to-emitter voltage less than the second voltage; when the error voltage is greater than the second voltage, the first current is proportional to the error voltage while the third voltage remains clamped to a base-to-emitter voltage less than the second voltage; and when the first current has the same current value as the second current, the first current is clamped at the second current and the third voltage is proportional to the error voltage.
13. The circuit of claim 12 , wherein the first control circuit further comprises:
a diode having an anode terminal coupled to the base terminal of the second NPN bipolar transistor and a cathode terminal;
a voltage source coupled between the cathode of the diode and the negative power supply voltage, wherein the voltage across the diode and the voltage source is the second voltage; and
a second current source providing a third current to the base terminal of the second NPN bipolar transistor and the diode.
14. The circuit of claim 11 , wherein the first control circuit comprises:
a first NMOS transistor having a gate terminal coupled to receive the error voltage, a drain terminal providing a first current as the first control signal and a source terminal coupled to a first node;
a second NMOS transistor having a gate terminal biased to a second voltage, a drain terminal coupled to a positive power supply voltage and a source terminal coupled to the first node, the first node being biased to a third voltage as the second control signal; and
a current mirror coupled to mirror a second current into a third current being N times the second current, the current mirror providing the third current at the first node,
wherein when the error voltage is less than the threshold voltage of the first NMOS transistor, the first current is zero and the third voltage is clamped to a threshold voltage less than the second voltage; when the error voltage is greater than the threshold voltage of the first NMOS transistor, the first current is proportional to the error voltage while the third voltage remains clamped to a threshold voltage less than the second voltage; and when the first current has the same current value as the third current, the first current is clamped at the third current and the third voltage is proportional to the error voltage.
15. The circuit of claim 14 , wherein the first control circuit further comprises:
third and fourth NMOS transistors forming the current mirror, the fourth NMOS transistor having a size N times the size of the third NMOS transistor, the third NMOS transistor receiving the second current and the fourth NMOS transistor providing the third current N times the second current,
wherein the third and fourth NMOS transistors have width and length selected to provide the second voltage at their gate terminals when the second current flows in the third NMOS transistor, the gate terminals of the third and fourth NMOS transistors being coupled to the gate terminal of the second NMOS transistor.
16. The circuit of claim 11 , wherein the second control circuit comprises:
a capacitor and a switch connected in parallel between a second node and a negative power supply voltage, the capacitor having a first plate coupled to be charged by the first control signal; and
a comparator having a first input terminal coupled to receive a reference voltage and a second input terminal coupled to the second node, the comparator providing an output signal having a first state when the voltage at the second node is equal to or greater than the reference voltage,
wherein the switch is turned on to discharge the capacitor when the switch circuit is being turned on and the switch is turned off to allow the capacitor to be charged by the first control signal, thereby establishing an off-time for the switch circuit.
17. The circuit of claim 11 , wherein the third control circuit comprises:
a comparator having a first input terminal coupled to receive the second control signal and a second input terminal coupled to receive a voltage signal indicative of the current flow through the switch circuit, the comparator providing an output signal having a first state when the second control signal has a voltage value that is equal to or greater than the voltage signal at the second input terminal.
18. A method for controlling a switching regulator, the switching regulator receiving an input voltage and including a switch circuit coupled to generate a switching output voltage and an output circuit coupled to receive the switching output voltage and to supply current at a regulated output voltage to a load, the method comprising:
generating an error voltage responsive to the difference between a first voltage and a reference voltage, the first voltage being indicative of the regulated output voltage;
generating a first control signal having a magnitude proportional to the error voltage for error voltages in a first voltage range being associated with low voltage values of the error voltage, the first control signal having a maximum value for error voltage values greater than the first voltage range;
generating a second control signal having a minimum value for error voltages in a second voltage range being associated with low voltage values of the error voltage, the second control signal having a magnitude proportional to the error voltage for error voltages greater than the second voltage range;
controlling the switching frequency of the switching regulator in response to the first control signal; and
controlling the peak current flowing through the switch circuit in response to the second control signal, the second control signal being indicative of a peak current value for the current flowing through the switch circuit.
19. The method of claim 18 , wherein generating the second control signal has a maximum value for error voltage values greater than a third voltage range, the third voltage range being associated with high voltage values of the error voltage.
20. The method of claim 18 , wherein the first and second control signals cause the switching regulator to operate with variable switching frequency and substantially fixed peak current when the error voltage is within the first voltage range and to operate with substantially fixed switching frequency and variable peak current when the error voltage is outside of the second voltage range.
21. The method of claim 19 , wherein the first and second control signals operate to cause the switching regulator to operate with substantially fixed switching frequency and substantially fixed maximum peak current when the error voltage is within the third voltage range.
22. The method of claim 18 , wherein the maximum error voltage value for the first voltage range is greater than the maximum error voltage value of the second voltage range.