Method of forming a PWM controller and structure therefor
View Patent ↗In one embodiment, a PWM controller is configured to form a drive signal that has an operating frequency that varies around a center by a percentage of the center frequency.
1. A PWM controller comprising:
a first input configured to form a first signal;
a second input configured to receive a second signal that varies responsively to a maximum peak value of a bulk voltage;
an oscillator circuit having a first portion that forms an on-time of a clock signal formed by the oscillator circuit and having a second portion that forms an off-time of the clock signal;
a first control circuit of the oscillator circuit configured to control the first portion and the second portion to form an operating frequency of the clock signal, the first control circuit configured to set a center frequency of the operating frequency responsively to the first signal formed on the first input and configured to vary the operating frequency around the center frequency a maximum and minimum amount that is a percent of the center frequency; and
a second control circuit of the oscillator circuit configured to vary a maximum duty cycle of the clock signal responsively to the second signal from the second input.
2. The PWM controller of claim 1 wherein the first control circuit is configured to form a first current through the first input and to use the first current to vary the on-time and the off-time.
3. The PWM controller of claim 2 wherein the first control circuit varies a value of the first current the percent around a center value of the first current.
4. The PWM controller of claim 2 wherein the first control circuit includes a current source that forms a first control current having a first value that is approximately a positive one half of the percent of the first value of the first control current.
5. The PWM controller of claim 4 wherein the first control circuit includes a control oscillator that forms a second control current having a second value that is approximately a negative half of the percent of the first value of the first control current.
6. The PWM controller of claim 5 wherein the first control circuit is configured to sum the first control current and second control current to form the first current wherein the first current varies the percent around the value of the first control current.
7. The PWM controller of claim 5 further including a first current mirror having an input portion that receives the second control current and a mirror portion that receives the first control current wherein the current mirror sums the first and second control currents to form a summed current.
8. The PWM controller of claim 7 further including a voltage-to-current converter that receives a voltage that is representative of the summed current and forms an input current through the first input that is representative of the summed current, and including a second current mirror having an input portion coupled to the first input to receive the summed current and for a charging current that is representative of the summed current.
9. The PWM controller of claim 8 further including a first mirror portion of the second current mirror coupled to supply a first charging current to a first capacitor wherein the first charging current controls a charging time of the first capacitor and also controls the on-time of the clock signal.
10. The PWM controller of claim 9 further including a second mirror portion of the second current mirror coupled to supply a second charging current to a second capacitor wherein the second charging current controls a charging time of the second capacitor and also controls the off-time of the clock signal.
11. The PWM controller of claim 1 wherein the clock signal is used to control a frequency of a switching control signal that is used to control a power switch that is used to regulate an output voltage to a desired value.
12. A method of forming an oscillator of a PWM controller comprising:
configuring an oscillator circuit to form a clock signal having an operating frequency and to form a center frequency of the operating frequency responsively to a first signal on a first input of the oscillator circuit;
configuring the oscillator circuit to vary the operating frequency a percent of the center frequency around the center frequency; and
configuring the oscillator circuit to vary a duty cycle of the operating frequency responsively to a second signal received on a second input of the oscillator circuit.
13. The method of claim 12 wherein configuring the oscillator circuit to form the clock signal having the operating frequency includes coupling a current source to form a first control current having a first value and to form a second control current having a second value that is summed to cause the first value to vary the percent.
14. The method of claim 13 wherein configuring the oscillator circuit to form the clock signal having the operating frequency includes configuring the oscillator circuit to use the first and second control currents to control a charging interval of a first capacitor to set an on-time of the clock signal and to use the first and second control currents to control a charging interval of a second capacitor to set an off-time of the clock signal.
15. The method of claim 14 further including coupling a first comparator to compare a voltage on the first capacitor to a reference value and responsively control the on-time.
16. The method of claim 15 further including coupling a second comparator to compare a voltage on the second capacitor to the reference value and responsively control the off-time.