Circuit and method for switching PFM and PWM
The switching method between pulse frequency modulation and pulse width modulation signals is first based on an output voltage of a power transistor to generate a corresponding pulse frequency modulation signal. Next, it is determined whether the corresponding pulse frequency modulation signal has reached its maximal frequency. If so, the initial pulse width modulation signal is adjusted to have the same width as the pulse frequency modulation signal. Thereafter, the adjusted pulse width modulation signal is outputted.
1. A circuit for switching pulse frequency modulation (PFM) and pulse width modulation (PWM) based on an output voltage of a power transistor, the circuit comprising:
an oscillator for generating triangular waves;
an oscillator-controlling unit connected to the oscillator for generating PFM signals;
a PFM comparison unit connected to the oscillator-controlling unit;
a mode transfer detection unit connected to the PFM comparison unit for detecting if the PFM signals have reached a maximal frequency;
a PWM comparison unit configured to adjust the initial width of the PWM signals to be equal to the pulse width of the PFM signals when the maximal frequency occurs;
a multiplexer for selecting either the PWM or PWM signals; and
a logic-controlling unit configured to control the multiplexer according to the result of the mode transfer detection unit.
2. The circuit of claim 1 , wherein the PWM comparison unit includes a unit gain amplifier and a switch, wherein the switch is turned on when the PFM signals reach the maximal frequency.
3. The circuit of claim 1 , wherein the PFM comparison unit includes a PFM comparator and a latch, wherein the latch determines if it is necessary to enable the oscillator to generate triangular waves when the PFM signals have not reached the maximal frequency.
4. The circuit of claim 1 , wherein the oscillator-controlling unit includes a unit gain amplifier, a switch and a capacitor, wherein the switch is always turned on when the PFM signals have reached the maximal frequency.
5. The circuit of claim 1 , wherein the mode transfer detection unit is for use in detecting if the PFM signals have escaped from the maximal frequency.
6. The circuit of claim 3 , wherein the PWM comparator is for use in comparing the output voltage and a first reference voltage in order to determine a start-up frequency of the oscillator.
7. The circuit of claim 1 , wherein the PWM comparison unit includes a PWM comparator and an error amplifier, one end of the PWM comparator connects to the output of the error amplifier, and the other end of the PWM comparator connects to the output of the oscillator.
8. A method for switching PFM and PWM, comprising the steps of:
generating PFM signals based on an output voltage of a power transistor;
determining if the PFM signals have reached a maximal frequency;
adjusting the initial pulse width of the PWM signals to be equal to the pulse width of the PFM signals if the maximal frequency occurs; and
switching the PWM signals as output signals.
9. The method of claim 8 , further comprising the step of disabling PFM.
10. A method for switching PFM and PWM, comprising the steps of:
determining if the PFM signals have left maximal frequency based on an output voltage of a power transistor;
adjusting the pulse width of the PWM signals to be equal to the pulse width of the PFM signals; and
switching the PFM signals as output signals.
11. The method of claim 10 , further comprising the step of disabling PWM.