Method and circuit for controlling the refresh rate of sampled reference voltages
The present invention relates to controlling the refresh rate of the reference voltage on a sampling capacitor (C samp ). A comparator (COMP) compares the voltage on a first capacitor (C 1 ) with the voltage on a second capacitor (C 2 ). These capacitors have the capacitance of the sampling capacitor (C samp ). Upon each refresh the first capacitor (C 1 ) samples a first voltage (V a ) and the second capacitor samples a lower second voltage (V b ). The first capacitor (C 1 ) is discharged at a first current I a via a first leakage current source (D 1 ). The second capacitor (C 2 ) is discharged at a second current I b via a second leakage current source (D 2 ). The comparator (COMP) triggers a refresh when the voltages equal. The first current I a is preferably an integer N times the second current I b .
1. An integrated electronic device comprising circuitry for controlling the refresh rate of a reference voltage on a sampling capacitor (C samp ) circuitry comprising:
a comparator (COMP) having a first input (+), a second input (−)and an output (RF) connected to trigger a refresh of the sampling capacitor (C samp ) when the a voltage at the first input (+) equals a voltage at the second input (−);
a first capacitor (C 1 ) having the capacitance of the sampling capacitor (C samp ) and coupled to a first input (+) of the comparator (COMP);
a second capacitor (C 2 ) having the capacitance of the sampling capacitor (C samp ) and coupled to a second input (−) of the comparator (C samp );
a first leakage current source (D 1 ) coupled to the first capacitor (C 1 ) for discharging the first capacitor (C 1 ) with a first current (I a ); and
a second leakage current source (D 2 ) coupled to the second capacitor (C 2 ) for discharging the second capacitor (C 2 ) with a second current (I b ), wherein the first and second leakage current sources (D 1 , D 2 ) are dimensioned to provide different amounts of leakage currents.
2. The electronic device of claim 1 , further comprising:
a first voltage source (V a );
a first switch (SW a ) connecting the first voltage source (V a ) to the first capacitor (C 1 ) each time the output (RF) of the comparator (COMP) triggers a refresh;
a second voltage source (V b ) and
a second switch (SW b ) connecting the second voltage source (V b ) to the second capacitor (C 2 ) each time the output (RF) of the comparator (COMP) triggers a refresh.
3. The electronic device of claim 1 , wherein:
the first voltage source (V a ) has a voltage of 1.6 V; and
the second voltage source (V b ) has a voltage of 1.2 V.
4. The electronic device of claim 1 , wherein:
the first and second leakage current sources (D 1 , D 2 ) are constructed whereby the first leakage current (I a ) is an integral number N times the second leakage current (I b ).
5. The electronic device of claim 4 , wherein:
the second leakage current source (D 2 ) is constructed with a first structure; and
the first leakage current source (D 1 ) is constructed with N copies of the first structure connected in parallel.
6. The electronic device of claim 4 , wherein:
N is an integer between 5 and 20.
7. The electronic device of claim 1 , wherein:
at least one of the first and second leakage current sources (D 1 , D 2 ) is a reverse biased diode.
8. A method for controlling the refresh of a reference voltage sampled on a sampling capacitor (C samp ), comprising:
sampling the reference voltage on the sampling capacitor (C samp );
sampling a first voltage (V a ) on a first capacitor (C 1 );
sampling a second voltage (V b ) smaller than the first voltage (V a ) on a second capacitor (C 2 );
discharging the first capacitor (C 1 ) by a first leakage current (I a );
discharging the second capacitor (C 2 ) by a second leakage current (I b ) less than the first leakage current (I a );
comparing the voltages on the first capacitor and the second capacitor during the discharging step, and
triggering a refresh of the reference voltage on the sampling capacitor (C samp ) when the voltage levels on the first and second capacitors are equal.