Audible noise avoiding circuit and DC-DC boost converter having the same

A DC-DC boost converter, operating in a pulse frequency modulation mode, comprising:

• an inductor, coupled between an input voltage node and a phase node;
• a first switch, coupled between the phase node and an output voltage node;
• a capacitor, coupled between the output voltage node and a ground;
• a second switch, coupled between the phase node and the ground;
• a control circuit, transmitting a first switching signal to a first control node of the first switch and a second switching signal to a second control node of the second switch for controlling the conducting status of the first switch and the second switch according to a feedback voltage responding to the voltage at the output voltage node;
• and
• an audible noise avoiding circuit comprising:
• a timing controller having a timing capacitor charged by a current source, the timing controller coupled to the first control node and the second control node, the timing controller discharging the timing capacitor to a low voltage when receiving the first switching signal and the second switching signal;
• and
• a linear regulator, having a sink output stage, a compensation unit and an operational amplifier, the sink output stage coupled to the output voltage node, an inverted input terminal of the operational amplifier receiving a threshold voltage, a non-inverted input terminal of the operational amplifier receiving the voltage of the timing capacitor, an output terminal of the operational amplifier coupled to the compensation unit and a control terminal of the sink output stage, wherein with the voltage of the timing capacitor is higher than the threshold voltage the operational amplifier outputs a control voltage to the compensation unit and the control terminal of the sink output stag, the compensation unit compensates the control voltage for gradually turning on the sink output stage in order to gradually reduce the voltage of the output voltage node;
• wherein a predetermined charging time interval is defined by the time interval of charging the timing capacitor from the low voltage to the threshold voltage, the reciprocal of the predetermined time interval is a frequency in the ultrasonic wave frequency range.