Abstract:
A switching regulator control circuit for PFM control of a DC-DC converter includes a reference voltage producing circuit, a voltage dividing circuit for dividing an output voltage of the DC-DC converter, a comparator for comparing the reference voltage and the divided voltage and outputting a comparison signal, a ring oscillator for outputting a signal for controlling the output transistor, which controls the output voltage of the DC-DC converter, and a logic OR circuit having a first input connected to an output of the ring oscillator and a second input connected to the comparison signal and an output connected to an input of the ring oscillator. Even when the output of the comparator varies frequently, such as when an output voltage closely approaches a set voltage, the ON period of the output transistor remains stable.

Description:
BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION 
     The present invention relates to a switching regulator control circuit for a PFM control circuit having a stable duty cycle and noise immunity when an output voltage closely approaches a set voltage. 
     The basic operation of a switching regulator control circuit 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. 
     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 PFM control switching regulator, an ON period of an output transistor is always constant. Thus, an OFF period is adjusted so that an output voltage is stably kept to a constant value. 
     FIG. 2 shows a conventional switching regulator control circuit for a PFM control. As shown in FIG. 2, the output of a reference voltage circuit  11  and that of voltage dividing circuits  12  and  14  are connected with the input of an output voltage control comparator  14 . The output of the output voltage control comparator  14  is connected with the input of a ring oscillator circuit  15 . The ring oscillator circuit  15  outputs a signal for controlling a transistor that controls an output voltage of a DC-DC converter. 
     For a PFM switching regulator  10 , the switching regulator control circuits for PFM control as shown in FIG. 2 are widely used. A switching pulse output by the ring oscillator circuit  15  is output by controlling an output signal of the above output voltage control comparator  14 . 
     In FIG. 2, when the output signal, Vosc_EN, of the comparator  14  is “H”, an oscillating circuit of the ring oscillator circuit  15  is being operated, so that a switching pulse is outputted to EXT. On the other hand when Vosc_EN is “L”, the ring oscillator circuit  15  is stopped, so that EXT becomes “L”. 
     When the Vosc_EN signal is changed from “L” to “H”, EXT immediately becomes from “L” to “H”. Hereinafter, oscillation is continued. When the Vosc_EN signal is changed from “H” to “L”, EXT immediately becomes “L”. 
     However, when the output voltage closely approaches a set voltage, chattering of the Vosc_EN output signal of the above comparator  14  is frequently caused due to the influence of noise. Thus, there may arise such an abnormal duty problem that an ON period shorter than a general switching ON period is successively generated in accordance with the chattering. 
     When an abnormal duty is caused, an OFF period becomes shorter as an output ON period shortens. Thus, a consumption current of the switching regulator control circuit is increased, so that efficiency in the case of a light load is greatly influenced thereby. 
     In order to solve the above-mentioned problem, there is also given a method for providing hysteresis in a subsequent stage of the output voltage control comparator  14  to suppress chattering of the output signal of the comparator  14 . However, in such a method, a new problem is caused for the reason such as reduction in yield due to variation in manufacturing an integrated circuit. Therefore, a more simplified and effective solving method is required. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is therefore to provide a switching regulator control circuit for providing a PFM control with a stable duty cycle and noise immunity, having a circuit structure which is simplified and effective and in which the difficulty in manufacturing an integrated circuit is eliminated as compared with a conventional hysteresis circuit system, using a simple logic structure which is devised in order to solve the above-mentioned conventional problems. 
     According to the present invention, a simple logic structure is used, a feedback signal for turning ON an output transistor is fed back, and then a logic signal between the feedback signal and an output signal of an output voltage control comparator is input to a ring oscillator circuit. Thus, even in the case of a frequent variation in the output signal of the comparator, which is liable to occur at a time when an output voltage closely approaches a set voltage, a determined ON period for the output transistor is ensured in the ring oscillator circuit, thereby solving the foregoing problems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 providing PFM control; 
     FIG. 3 is a block diagram showing an example of a conventional switching regulator control circuit for providing PFM control; 
     FIG. 4 is a block diagram showing Embodiment 2 of the present invention; and 
     FIG. 5A is a block diagram showing an example of a step-up DC-DC converter to which the switching regulator control circuit for a PFM control is applied and 
     FIG. 5B is an operating wave diagram of the step-up DC-DC converter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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 of the circuit will be described. First, a transistor Tr is turned ON by a PFM control switching regulator  10  to make a charging current i on  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 the voltage is equal to or larger than Vin, a discharging current i off  flows into an output to raise the voltage Vout. In addition, the energy stored during an ON period of Tr is adjusted by the PFM control switching regulator  10  to stabilize Vout. Note that the PPM control switching regulator  10  to which the switching regulator control circuit for providing PPM 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 providing PFM control according to a first embodiment 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 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 circuit  5  and that of the output voltage control comparator  4  are input to a logic OR circuit  6 . The output of the logic OR circuit  6  is connected with the input of the ring oscillator circuit  5 . 
     When various logic circuits are combined based on such a basic logic structure, a duty guarantee oscillator circuit devised according to the present invention can be realized. Here, an embodiment of the present invention will be described with reference to the drawings. 
     As shown in FIG. 1, an EXT signal for turning ON an output transistor is fed back, the fed back signal and a Vosc_EN signal as an output signal of the output voltage control comparator  4  are ORed by the OR circuit  6 , and a resultant signal New_Vosc_EN is returned as an oscillation start control signal to the ring oscillator circuit  5 . 
     When the output transistor is being turned ON, EXT is “H”. During a period for which EXT is “H”, a change in Vosc_EN signal is neglected. 
     When the output transistor is being turned OFF, EXT is “L”. Thus, New_Vosc_EN=Vosc_EN. At this time, if the Vosc_EN signal is “H”, the oscillation is started and a switching pulse is outputted. An ON period of the pulse is determined to be a time constant of the ring oscillator oscillating circuit  5 . If the Vosc_EN signal is “L”, the oscillation is stopped and the output transistor is continuously turned OFF. In other words, the Vosc_EN signal for controlling the output of the ring oscillator oscillating circuit  5  is neglected during a period for which the output transistor is being turned ON. 
     FIG. 3 shows an example of a conventional switching regulator control circuit for a PFM control widely used. 
     In FIG. 3, the output of a reference voltage circuit  21  and that of voltage dividing circuits  22  and  23  are connected with the input of an output voltage control comparator  24 . The output of the output voltage control comparator  24  is connected with the input of a ring oscillator circuit  25 . The ring oscillator circuit  25  outputs a signal for controlling the transistor which controls an output voltage of the DC-DC converter. A rectangular wave pulse Vosc signal outputted from an oscillating circuit and the oscillation start signal Vosc_EN outputted from the output voltage control comparator  24  are ANDed by an AND circuit. A resultant signal is divided into two routes. With respect to one route, the signal is returned to the oscillating circuit, thereby composing the ring oscillator circuit. With respect to the other route, the signal is outputted as the EXT signal for controlling the output transistor. A logic equation is as follows. 
       EXT=Vosc*Vosc   —   EN   (equation 1). 
     When the Vosc_EN signal is “H”, EXT=Vosc. Thus, the oscillating circuit starts to oscillate, so that a switching pulse is outputted. An ON period of the pulse is determined to be a time constant of the oscillating circuit. 
     On the other hand, when the Vosc_EN signal is “L”, EXT=L. Thus, the oscillating circuit stops to oscillate. In such a circuit, when chattering of the output signal Vosc_EN of the above-mentioned comparator  24  is caused, there is such an inconvenience that a narrow pulse with an ON period shorter than the ON period determined by the oscillating circuit is successively generated as the switching pulse outputted to EXT. 
     FIG. 4 shows Embodiment 2 of the present invention. The output of a reference voltage circuit  31  and that of voltage dividing circuits  32  and  33  are connected with the input of an output voltage control comparator  34 . A ring oscillator circuit  35  outputs a signal for controlling the transistor which controls an output voltage of the DC-DC converter. The output of the ring oscillator circuit  35  and that of the output voltage control comparator  34  are inputted to a logic OR circuit  36 . The output of the logic OR circuit  36  is connected with the input of the ring oscillator oscillating circuit  35 . 
     An EXT signal for turning ON the output transistor is fed back, the fed back signal and the output signal Vosc_EN of the output voltage control comparator are ORed by an OR circuit  36 . A resultant signal New_vosc_EN is used as an oscillation start signal, and the resultant signal is divided into two routes. With respect to one route, the signal is returned to the oscillating circuit, thereby composing the ring oscillator circuit  35 . With respect to the other route, the signal is outputted as the EXT signal for controlling the output transistor. A logic equation is as follows.                      EXT   NEXT     =            Vosc   *   New_Vosc      _EN                 =            Vosc        (     Vosc_EN   +   EXT     )                     (     equation                 2     )                                
     Symbol EXT NEXT  in the equation 2 shows a next state of EXT. In other words, EXT shows a current state of an EXT terminal and EXT NEXT  shows a next state of the EXT terminal. The current state EXT of the EXT terminal is fed back, so that the next state EXT NEXT  of the EXT terminal is influenced thereby. When the output transistor is being turned ON, EXT is “H”, and therefore EXT NEXT =Vosc. In other words, during a period for which EXT is “H”, a change in Vosc_EN signal is neglected. 
     On the other hand, when the output transistor is being turned OFF, EXT is “L”. Thus, EXT NEXT =Vosc*Vosc_EN (this is equivalent to the equation 1). At this time, if the Vosc_EN signal is “H”, EXT NEXT =Vosc, so that the oscillation is started and a switching pulse is outputted. An ON period of the pulse is determined to be a time constant of the oscillating circuit. If the Vosc_EN signal is “L”, EXT NEXT =L, so that the oscillation is stopped and the output transistor is continuously turned OFF. In other words, it is realized that the Vosc_EN signal for controlling the output of the oscillating circuit is neglected during a period for which the output transistor is being turned ON. 
     When the oscillation stop state is transferred to the oscillation start state, EXT is “L” and the Vosc_EN signal is “H”. According to the equation 2, EXT NEXT =Vosc. Thus, when the duty guarantee circuit devised according to the present invention is added, the initialization of an original oscillating circuit is not affected accordingly. 
     As described above, according to the present invention, a logically simplified and effective circuit is used, and a determined ON period for the output transistor is ensured in the oscillating circuit even in the case of a frequent variation in output signal of the output voltage control comparator, which is liable to occur at a time when an output voltage extremely approaches a set voltage, in a light load with respect to the PFM control switching regulator. Thus, a switching regulator in which a stable duty is ensured and which has noise immunity can be realized. The difficulty in manufacturing an integrated circuit is eliminated by a conventional hysteresis circuit system, so that circuit operation is ensured. In addition, a circuit scale is small, with the result that the present invention is effective in cost. As described above, the present circuit system is most suitable for the switching regulator control circuit for a PFM control.