Patent ID: 6768657
Filing Date: 2004-07-27
Classification: H02M

Abstract:
A current-driven PWM controller having a power saving means comprising:a current-driven off-time modulator having an input terminal and three output terminals, wherein the input terminal is connected to a feedback current derived from a feedback loop, wherein the first output terminal outputs a feedback voltage for the PWM controller, the second and third output terminals output a discharge current and a minimum discharge current respectively for determining an off-time of as witching period; an oscillator having an input terminal and an output terminal, in which the discharge current is applied to an input terminal of the oscillator and a pulse-signal generated at the output terminal of the oscillator; wherein a maximum on-time of the pulse-signal is kept as a constant and an off-time of the pulse-signal is increased in accordance with the decrease of the discharge current; a control circuit having an input terminal and an output terminal, wherein the input terminal is connected to the supply voltage, the output terminal outputs an OFF-signal to switch on/off the minimum discharge current; wherein the control circuit outputs a logic low OFF-signal to enable the minimum discharge current and restarts the switching as the supply voltage falls below a low threshold, thereby preventing the PWM controller from being turned off due to an insufficient supply voltage; a switch, which is controlled by the OFF-signal to switch on/off the minimum discharge current; a RS register, wherein the set-input is connected to the pulse-signal and the reset-input is controlled by a compared signal; an AND-gate having two input terminals for outputting a PWM signal, in which the first input terminal of the AND-gate is connected to the pulse-signal and the second input terminal of the AND-gate is connected to the output of the RS register; wherein the PWM signal is off in response to the off state of the pulse signal; and a comparator having a negative input terminal, a positive input terminal and an output terminal wherein the positive input terminal is connected to the feedback voltage, the negative input terminal is connected to the sense voltage derived from the switching current, and the output terminal generates the compared signal to reset the RS register.