PWM switching regulator and electronic apparatus

Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...

Reexamination Certificate

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Reexamination Certificate

active

06737843

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a PWM (pulse width modulation) switching regulator which is capable of changing an internal oscillation frequency into a known oscillation frequency in accordance with load condition to reduce power consumption and to enhance efficiency.
2. Description of the Related Art
A conventional PWM switching regulator circuit is usually operated at a fixed internal oscillation frequency irrespective of load condition. For example, there is known a control circuit for use in a PWM switching regulator shown in a circuit diagram of
FIG. 4
(for example, refer to JP 11-155281 A (page. 4 and FIG. 1)).
The operation of the conventional PWM switching regulator controlling circuit will hereinbelow be described with reference to FIG.
4
. An error amplifying circuit
102
amplifies a signal exhibiting a difference between a voltage signal Va which has been obtained by dividing a detected voltage signal Vout at an output terminal with bleeder resistors
100
and
101
and a voltage signal Vref outputted from a reference voltage circuit
106
to output the resultant signal in the form of an output voltage signal Verr. Then, a comparison circuit
103
compares a triangular wave Vosc outputted from an oscillator
104
and the level Verr of the output signal with each other to output the resultant signal in the form of a pulse signal EXT used to control an external switch through a buffer
105
to thereby carry out the control in such a way that the terminal voltage Vout becomes a desired constant output voltage. The outputted pulse signal EXT shows an ON/OFF state at the same cycle as that of an internal oscillation frequency Vosc because of the fixed internal oscillation frequency Vosc.
As described above, in an example of the circuit of the PWM switching regulator, since the internal oscillation frequency is usually fixed, the buffer is also usually operated at the same frequency as the internal oscillation frequency, and the external switch is also operated at the same frequency as the internal oscillation frequency to thereby carry out the control in such a way that the output voltage Vout becomes the desired constant voltage. In addition, since the PWM switching regulator is operated so as to hold the output voltage constant by repeatedly turning ON/OFF of the external switch, the noise having the same frequency as the oscillation frequency is contained in the output voltage.
However, since the PWM switching regulator usually oscillates at the same oscillation frequency, a filter for removing the noise having the oscillation frequency is provided in the output voltage line to allow the noise to be readily removed so that the stable power source can be readily obtained.
However, in the conventional PWM switching regulator, the oscillation circuit, the buffer and the external switch are operated at the fixed frequency irrespective of the condition of a load connected to the output terminal. For this reason, there is encountered a problem in that if the load connected to the output terminal supplying the voltage Vout becomes light, then a quantity of power consumed in the PWM switching regulator controlling circuit becomes more than that consumed in the load, which results in the remarkably deteriorated efficiency. The circuits which mainly consume the power in the PWM switching regulator controlling circuit are the internal oscillation circuit and the buffer which is operated at the same frequency as the internal oscillation frequency.
SUMMARY OF THE INVENTION
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problem associated with the prior art, and it is, therefore, an object of the present invention to provide a PWM switching regulator with which an internal oscillation frequency of a PWM switching regulator controlling circuit is halved while the switching regulator is operated under a light load to nearly halve the power consumption of the PWM switching regulator controlling circuit to thereby obtain the high efficiency even while the switching regulator is operated under a light load. Thus, the feature of the present invention is that even the respect, as the advantage of a PWM switching regulator, that the noise contained in the output voltage is readily removed is not injured since an oscillation frequency is held at a known value.
A PWM switching regulator controlling circuit according to the invention of the present application includes: an oscillation circuit for generating an oscillation signal for a PWM switching regulator; a first pulse generator for receiving as its input the signal from the oscillation circuit to output a signal PULSE-MIN; and a second pulse generator for receiving as its input the signal from the oscillation circuit to output a signal PULSE-MAX. In addition, the PWM switching regulator controlling circuit further includes: a comparison circuit for comparing the signal from the oscillation circuit with a reference signal from a reference voltage circuit to output a signal Vcomp used to control an external switch; and a logical circuit for receiving as its input the signal PULSE-MIN, the signal PULSE-MAX, and the signal Vcomp to output a signal used to control an oscillation frequency of the oscillation circuit.
Further, in the PWM switching regulator controlling circuit, the logical circuit includes: an S/R latch circuit; a first Exclusive-OR circuit for receiving as its input the signal Vcomp and the signal PULSE-MIN to output a signal; and a second Exclusive-OR circuit for receiving as its input the signal Vcomp and the signal PULSE-MAX to output a signal. In addition, the logical circuit further includes: a second AND circuit for receiving as its input the output signal from the first Exclusive-OR circuit and the output signal from the second Exclusive-OR circuit to output a signal; a third AND circuit for receiving as its output the output signal from the second AND circuit and the output signal from the S/R latch circuit to output a signal Fup to a reset terminal of the S/R latch circuit; and a buffer circuit for receiving the signal Vcomp to output a signal EXT used to control an external switch. Further, the first AND circuit, in response to the output signal from the first Exclusive-OR circuit and the signal PULSE-MIN, outputs a signal Fdown to a set terminal of the S/R latch circuit, and the output signal from the S/R latch circuit is inputted to the oscillation circuit.
The PWM switching regulator controlling circuit according to the invention of the present application includes: an oscillation circuit for generating an oscillation signal for a PWM switching regulator; a comparator for receiving as its input the oscillation signal from the oscillation circuit and an output signal obtained by comparing a voltage signal obtained through division of a voltage Vout with a reference voltage of a reference voltage circuit in an error amplifying circuit to output a signal Vcomp used to control an external switch signal; and a logical circuit for receiving as its input the signal Vcomp to output a signal used to control an oscillation frequency of the oscillation circuit.
Further, in the PWM switching regulator controlling circuit, the logical circuit includes: an S/R latch circuit; a first time integrating circuit for receiving as its input the signal Vcomp to integrate a period of time during which an external switch signal is in an OFF-state to output a signal Fdown; and a second time integrating circuit for receiving as its input the signal Vcomp to integrate a period of time during which the external switch signal is an ON-state to output a signal Fup, and the output signal from the first time integrating circuit is inputted to a set terminal of the S/R latch circuit, the output signal from the second time integrating circuit is inputted to a reset terminal of the S/R latch circuit, and the output signal from the S/R latch circuit is inputted to the oscillation circuit.
In the PWM switching regulator co

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