Voltage control circuit

1. A circuit for controlling voltage, comprising:

a condenser having a first end coupled to an input node of a target circuit, and a second end coupled to a predetermined fixed potential;

a constant current source coupled to the first end of the condenser; and

a control circuit configured to charge or discharge the condenser by the constant current source, to measure a first time period passing from start timing of the charge or discharge to timing at which a voltage at the first end of the condenser reaches a predetermined voltage, and to change an input condition or operation condition of the target circuit upon passage of a second time period from the start timing where the second time period is responsive to the first time period.

2. The circuit for controlling voltage as claimed in claim 1 , further comprising a switch, wherein the target circuit is an amplifier, and the switch is coupled to another input node of the amplifier that is different from said input node of the amplifier, and wherein the control circuit is configured to charge the condenser by the constant current source, and operates the switch upon the passage of the second time period from the start timing to switch nodes to which said another input node of the amplifier is connected.

3. The circuit for controlling voltage as claimed in claim 1 , wherein the target circuit is an amplifier, and the control circuit is configured to discharge the condenser by the constant current source and to deactivate the amplifier upon the passage of the second time period from the start timing.

4. The circuit for controlling voltage as claimed in claim 1 , further comprising a comparator configured to compare the predetermined voltage with the voltage at the first end to produce an output indicative of a result of the comparison, wherein the control circuit is configured to measure the first time period by measuring timing of a change in the output of the comparator based on a clock signal.

5. The circuit for controlling voltage as claimed in claim 4 , wherein the control circuit includes:

a counter configured to start operating at the start timing to count pulses of the clock signal;

a register configured to store a value responsive to a count of the counter in response to the change in the output of the comparator; and

a circuit configured to detect an end timing of the second time period starting from the start timing in response to the value stored in the register and a count value of the counter.

6. The circuit for controlling voltage as claimed in claim 4 , further comprising a clock signal oscillating circuit configured to generate the clock signal, wherein the control circuit is configured to stop the clock signal oscillating circuit upon the passage of the second time period from the start timing.

7. The circuit for controlling voltage claimed in claim 1 wherein the constant current source includes a current mirror circuit.

8. The circuit for controlling voltage as claimed in claim 7 , wherein the current mirror circuit includes a transistor for discharging the condenser and a transistor for charging the condenser.

9. The circuit for controlling voltage as claimed in claim 1 , wherein the constant current source includes:

an amplifier having a first input node coupled to a predetermined power supply voltage;

a first transistor having a first end of a channel thereof coupled to an output of the amplifier, a second end of a channel thereof coupled to a second input node of the amplifier, and a control end coupled to the second end of the channel;

a resistor to couple between the second end of the channel and a ground voltage;

a second transistor sharing an identical gate voltage and an identical source voltage with the first transistor; and

a plurality of transistors each configured to produce a flow of a current having an amount identical to a current amount flowing through the second transistor.