Switching regulator control circuit for a PFM control

#The present invention relates to a switching regulator control circuit for a PFM control having a stable duty and noise immunity when an output voltage extremely approaches a set voltage. 2. Description of the Related Art

#According to a switching regulator control circuit, the basic operation is as follows. An output voltage is fed back and compared with a reference voltage by an output voltage control comparator. When the output voltage becomes smaller than a set voltage, the comparator is operated to send an oscillation start signal to an oscillating circuit. Then, the oscillating circuit outputs a switching pulse, so that an output transistor is turned ON or OFF to thereby adjust the output voltage.

#Here, FIGS. 5A and 5B show an example of a step-up DC-DC converter to which a PFM control switching regulator is applied.

#In the accompanying drawings:

#FIG. 1 is a block diagram showing Embodiment 1 of the present invention;

#FIG. 2 is a block diagram showing a conventional switching regulator control circuit for a PFM control;

#In FIGS. 5A and 5B, although it is similar to a conventional case, as an example to which a switching regulator control circuit for a PFM control of the present invention is applied, a block diagram of a step-up DC-DC converter and its operating characteristic are shown, respectively. In FIG. 5A, in order to obtain an output voltage Vout higher than an input voltage Vin, it is constructed that a power source of Vin and a coil L are added in series.

#Specific operation will be described. First, a transistor Tr is turned ON by a PFM control switching regulator 10 to make a charging current ion flow, so that energy is stored in the coil L. Next, when the transistor Tr is turned OFF by the PFM control switching regulator 10, an electromotive force is generated in the coil L by the stored energy. When a voltage is equal to or larger than Vin, a discharging current ioff flows into an output to rise the voltage Vout. In addition, the energy stored for an ON period of Tr is adjusted by the PFM control switching regulator 10 to stabilize Vout. Note that the PFM control switching regulator to which the switching regulator control circuit for a PFM control according to the present invention is applied can be applied not only to the step-up DC-DC converter but also to a step-down DC-DC converter, an inversion DC-DC converter and the like.

#FIG. 1 shows a basic logic structure of the switching regulator control circuit for a PFM control according to the present invention as Embodiment 1 of the present invention. The output of a reference voltage circuit 1 and that of voltage dividing circuits 2 and 3 are connected with the input of an output voltage control comparator 4. A ring oscillator oscillating circuit 5 outputs a signal for controlling a transistor which controls an output voltage of a DC-DC converter. The output of the ring oscillator oscillating circuit 5 and that of the output voltage control comparator 4 are inputted to a logic OR circuit 6. The output of the logic OR circuit 6 is connected with the input of the ring oscillator oscillating circuit 5.