Fujitsu Corporation

Akira Haga

Kazuhiro Kobayashi

Atsushi Matsuda

Yuji Segawa

Masaru Otsuka

Yoshinori Yoshikawa

Osamu Kikuchi

A D/A converter in which reference voltage around which voltages outputted vary is not generated by an amplifier in order to reduce electric current consumption. A resistor circuit divides power supply voltage and outputs a plurality of voltages which make up an arithmetic progression. A decoder circuit decodes a digital signal and outputs a control signal. A switching circuit is turned on/off in accordance with the control signal sent from the decoder circuit and selects and outputs one of the plurality of voltages outputted from the resistor circuit. In a conversion circuit, a voltage equal to half of the power supply voltage is obtained by dividing the power supply voltage by resistors and inputted to an inverting input terminal of an operational amplifier which forms a non-inverting amplifier, and the voltage inputted from a switching circuit to a non-inverting input terminal of the operational amplifier is converted into a voltage which varies around a voltage equal to half of the power supply voltage and the converted voltage is outputted. As a result, there is no need to use an amplifier for outputting the voltage equal to half of the power supply voltage and electric current consumption can be reduced.

D/A converter and semiconductor device

The inverter functioning as a comparator, dummy inverter having the same electric characteristics as the inverter, and control circuit are provided. Vth detecting input voltage output from the control circuit is input into the dummy inverter, Vth detecting output voltage output from the dummy inverter is input into the control circuit, and the threshold voltage of the dummy inverter is detected. The threshold voltage of the inverter is controlled by controlling the back gate voltages of the MOS transistors of the dummy inverter and the inverter in such a manner that the threshold voltage of the dummy inverter coincide with an external reference voltage.

Comparator and voltage controlled oscillator circuit

A reset signal generating circuit has a voltage dividing circuit for dividing a power source voltage into a divided voltage, a first switching part controlled responsive to the divided voltage from the voltage dividing circuit and turned ON when the power source voltage rises to a value in a vicinity of a predetermined value with an arbitrary rising speed, a second switching part controlled responsive to an output signal of the first switching part and turned ON during a time in which the first switching part is ON, and a charging and discharging part including a capacitor supplied with an output signal of the second switching part for starting a charging operation from a first time when the second switching part turns ON. The charging anc discharging part outputs a reset signal for a constant time from the first time to a second time and stops to output the reset signal after the second time.

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Kawasaki, Japan

Akira Haga