Closed loop dummy load to limit minimum operating frequency in current mode control DCDC
Described herein is a method of operating a DC-DC converter to avoid operating in PSM or PFM at light load, due to the possibility that uncontrolled low frequency content could be introduced. The method includes including driving a load demanding a load current with an output current, and when the output current is greater than a minimum output current, regulating the output current to be equal to the load current. When the output current is less than the minimum output current, the method includes regulating the output current to be equal to the minimum output current, and generating a dummy load current and then subtracting the dummy load current from the regulated output current that is equal to the minimum output current. The dummy load current is generated to be equal to a difference between the minimum output current and the load current.
1 . A method of operating a DC-DC converter, comprising:
driving a load demanding a load current with an output current;
when the output current is greater than a minimum output current, regulating the output current to be equal to the load current; and
when the output current is less than the minimum output current:
regulating the output current to be equal to the minimum output current; and
generating a dummy load current and subtracting the dummy load current from the output current that is regulated to equal the minimum output current;
wherein the dummy load current is generated to be equal to a difference between the minimum output current and the load current.
2 . The method of claim 1 , wherein the minimum output current provides for selected on and off times of the DC-DC converter that result in a minimum operating frequency of the DC-DC converter being maintained at a given level.
3 . The method of claim 1 , further comprising determining whether the output current is greater than the minimum output current or less than the minimum output current by:
comparing a feedback voltage indicative of the output current to a first reference voltage to produce a control voltage.
4 . The method of claim 3 , wherein the output current is regulated to be the minimum output current by:
generating a sum voltage as a sum of a voltage ramp and a voltage indicative of current through an inductor of the DC-DC converter;
comparing the control voltage to the sum voltage to produce a PWM signal; and
controlling switching of the DC-DC converter based upon the PWM signal.
5 . The method of claim 4 , wherein the dummy load current is generated as a function of a difference between a reference dummy voltage representing a level of the control voltage at which the load current is equal to the minimum output current and the control voltage.
6 . The method of claim 5 , wherein the reference dummy voltage is determined based upon an input voltage to the DC-DC converter, an output voltage from the DC-DC converter, the load current, and a switching period of the DC-DC converter.
7 . A DC-DC converter system, comprising:
a DC-DC converter driving a load demanding a load current with an output current at an output node;
a control voltage generation circuit configured to generate a control voltage based upon a comparison between a reference voltage and a feedback voltage indicative of the output current;
wherein the DC-DC converter is configured to regulate the output current to be equal to the load current when the control voltage is indicative of the output current being greater than a minimum output current;
wherein the DC-DC converter is configured to regulate the output current to be equal to the minimum output current when the control voltage is indicative of the output current being less than the minimum output current; and
a dummy load configured to generate a dummy load current that is equal to a difference between the minimum output current and the load current and subtract the dummy load current from the output current equal to the minimum output current when the control voltage is indicative of the output current being less than the minimum output current.
8 . The DC-DC converter system of claim 7 , wherein the dummy load comprises:
a dummy current source transistor configured to generate the dummy load current as a function of a dummy gate control voltage; and
a dummy control loop configured to generate the dummy gate control voltage as a function of a comparison between the control voltage and a reference dummy voltage.
9 . The DC-DC converter system of claim 8 , further comprising processing circuitry configured to generate the reference dummy voltage based upon an input voltage to the DC-DC converter, an output voltage from the DC-DC converter, the load current, and a desired switching period of the DC-DC converter.
10 . The DC-DC converter system of claim 9 , wherein the dummy control loop comprises:
a first amplifier having a non-inverting input coupled to the control voltage through a first resistor, with a second resistor being connected between the non-inverting input and an output of the first amplifier, and with the first amplifier having an inverting input coupled to the reference dummy voltage;
a first n-channel transistor having a drain connected to an input of a current mirror, a source coupled to ground through a third resistor, and a gate coupled to the output of the first amplifier;
a second n-channel transistor having a drain connected to an output of the current mirror, a source coupled to ground through a fourth resistor, and a gate coupled to the reference dummy voltage; and
a fifth resistor connected between the output of the current mirror and the output node;
wherein the dummy gate control voltage is generated at the output of the current mirror;
wherein a gain between the control voltage and the dummy gate control voltage is a function of a product of a first ratio and a second ratio, the first ratio being a ratio between resistances of the second resistor and the first resistor, the second ratio being a ratio between resistances of the fifth resistor and the third resistor.
11 . The DC-DC converter system of claim 10 , wherein the control voltage generation circuit comprises:
a second amplifier having a non-inverting input coupled to a comparison voltage, and an inverting input connected to an output of the second amplifier, wherein the output of the second amplifier is connected to the first resistor.
12 . The DC-DC converter system of claim 11 , further comprising a third amplifier having a non-inverting input coupled to the feedback voltage, an inverting input coupled to the reference voltage, and an output at which the comparison voltage is generated.
13 . The DC-DC converter system of claim 12 , further comprising a feedback divider coupled between a top voltage and the output node, with the feedback voltage being produced at a tap of the feedback divider.
14 . A method of operating a DC-DC converter to as to avoid operation in pulse skipping mode (PSM) or pulse frequency modulation (PFM) in light load conditions, the method comprising:
driving a load demanding a load current with an output current;
when the output current is less than a minimum output current:
regulating the output current to be equal to the minimum output current; and
generating a dummy load current and subtracting the dummy load current from the output current that is regulated to equal the minimum output current;
wherein the dummy load current is generated to be equal to a difference between the minimum output current and the load current.
15 . The method of claim 14 , wherein the minimum output current provides for selected on and off times of the DC-DC converter that result in a minimum operating frequency of the DC-DC converter being maintained at a given level.
16 . The method of claim 14 , further comprising determining whether the output current is greater than the minimum output current or less than the minimum output current by:
comparing a feedback voltage indicative of the output current to a first reference voltage to produce a control voltage.
17 . The method of claim 16 , wherein the output current is regulated to be minimum output current by:
generating a sum voltage as a sum of a voltage ramp and a voltage indicative of current through an inductor of the DC-DC converter;
comparing the control voltage to the sum voltage to produce a PWM signal; and
controlling switching of the DC-DC converter based upon the PWM signal.
18 . The method of claim 17 , wherein the dummy load current is generated as a function of a difference between a reference dummy voltage representing a level of the control voltage at which the load current is equal to the minimum output current and the control voltage.
19 . The method of claim 18 , wherein the reference dummy voltage is determined based upon an input voltage to the DC-DC converter, an output voltage from the DC-DC converter, the load current, and a switching period of the DC-DC converter.