Charge detecting circuit and fingerprint sensor using the same
View Patent ↗A capacitance detecting circuit detects the variations of capacitances at intersections of a plurality of column lines and a plurality of row lines opposite to the column lines and then converts the variations of capacitances into electrical signals. The capacitance detecting circuit includes a column line driving means that drives the column lines, a comparator that is connected to the row lines to convert charges stored in capacitors formed at the intersections of the driven column lines and the row lines into a measured voltage, and that compares the measured voltage with a predetermined set value to output charging or discharging signals, a constant current source that performs charging or discharging on the charges in response to the charging or discharging signals, and a capacitor that stores charges by means of the charging or discharging current. In the capacitance detecting circuit, a voltage between both ends of the capacitor is output as the electrical signal.
1. A capacitance detecting circuit that detects the variations of capacitances at intersections of a plurality of column lines and a plurality of row lines opposite to the column lines and that converts the variations of capacitances into electrical signals, comprising:
a column line driving means that drives the column lines;
a comparator that is connected to the row lines to convert charges stored in capacitors formed at the intersections of the driven column lines and the row lines into a measured voltage, and that compares the measured voltage with a predetermined set value to output charging or discharging signals;
a constant current source that performs charging or discharging of charges in response to the charging or discharging signals; and
a capacitor that stores charges by means of charging or discharging current,
wherein a voltage between both ends of the capacitor is output as the electrical signal,
wherein the comparator outputs, as the charging or discharging signal, a charging signal indicating the charging when the measured voltage is larger than the predetermined set value, and outputs a discharging signal indicating the discharging when the measured voltage is smaller than the predetermined set value,
wherein the constant current source includes a constant current source for charging and a constant current source for discharging, and
wherein the constant current source performs a charging operation on the capacitor when the charging signal is input and performs a discharging operation on the capacitor when the discharging signal is input.
2. The capacitance detecting circuit according to claim 1 ,
wherein the comparator is composed of an inverter, which is a digital circuit.
3. The capacitance detecting circuit according to claim 1 ,
wherein the constant current source is composed of a current mirror circuit using transistors, and an output transistor of the current mirror circuit is divided into a plurality of transistors whose on or off states are controlled by switches, and
a current ratio is set to a plurality of values.
4. A fingerprint sensor that detects the unevenness of a fingerprint by the variations of capacitances at intersections of a plurality of column lines and a plurality of row lines opposite to the column lines, the variation of capacitances being detected by the capacitance detecting circuit according to claim 1 .
5. A fingerprint sensor that detects the unevenness of a fingerprint by the variations of capacitances at intersections of a plurality of column lines and a plurality of row lines opposite to the column lines, the variation of capacitances being detected by the capacitance detecting circuit according to claim 2 .
6. A fingerprint sensor that detects the unevenness of a fingerprint by the variations of capacitances at intersections of a plurality of column lines and a plurality of row lines opposite to the column lines, the variation of capacitances being detected by the capacitance detecting circuit according to claim 3 .
7. A method of detecting variations of capacitance at intersections of a plurality of column lines and a plurality of row lines opposite to the column lines in a capacitance detecting circuit, the capacitance detecting circuit including a first constant current source for charging and a second constant current source for discharging, said method comprising:
driving the column lines;
converting a charge stored in a capacitor formed at an intersection of one of the driven column lines and one of the row line into a measured voltage;
comparing the measured voltage with a predetermined set value and outputting charging or discharging signals, said outputting including:
outputting a charging signal indicating charging if the measured voltage exceeds the predetermined set value; and
outputting a discharging signal indicating discharging if the measured voltage is smaller than the predetermined set value;
performing, by a constant current source, charging or discharging of charges in response to the charging or discharging signals, said performing including:
performing by the first constant current source a charging operation on a storage capacitor if the charging signal is input; and
performing by the second constant current source a discharging operation on the storage capacitor if the discharging signal is input;
storing charges in the storage capacitor by means of charging or discharging current; and
outputting a voltage between both ends of the storage capacitor as an electrical signal.