Capacitive fingerprint sensor and the panel thereof
A capacitive fingerprint sensor comprises a fingerprint capacitor, an integrator, a first transistor, a second transistor and a third transistor. The fingerprint capacitor has a capacitance that is either a valley capacitance C FV or a ridge capacitance C FR , wherein C FV is smaller than C FR . The integrator has a reference capacitor C fb . The first transistor is configured to control the fingerprint capacitor during a scan line period. The second transistor is configured to discharge the fingerprint capacitor. The third transistor is configured to precharge the fingerprint capacitor and to redistribute the charges between the fingerprint capacitor and the reference capacitor C fb .
1. A capacitive fingerprint sensor, comprising:
a sensor cell comprising:
a fingerprint capacitor having a capacitance that is either a valley capacitance C FV or a ridge capacitance C FR , and C FV is smaller than C FR ;
a first transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by a scan line, the input terminal is connected to the fingerprint capacitor, and the output terminal is connected to a readout line;
a second transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by an evaluate signal, the input terminal is connected to a second voltage VB, and the output terminal is connected to the fingerprint capacitor;
a control circuit comprising:
a third transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by a first reset line, and the input terminal is connected to the readout line;
an operational amplifier, wherein one input end of the operational amplifier is connected to the output terminal of the third transistor, and the other input end of the operational amplifier is connected to a first voltage VA;
a reference capacitor C fb connected input and output ends of the operational amplifier; and
a fourth transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by
a second reset line, and the input and output terminals are connected to the input and output ends of the operational amplifier.
2. The capacitive fingerprint sensor of claim 1 , which is serially operated in precharge, transition and evaluation phases.
3. The capacitive fingerprint sensor of claim 2 , wherein in the precharge phase, the scan line and the first reset line are asserted such that the operational amplifier transfers the first voltage VA to the fingerprint capacitor.
4. The capacitive fingerprint sensor of claim 3 , wherein in the precharge phase, the second reset line is further asserted.
5. The capacitive fingerprint sensor of claim 3 , wherein in the transition phase, the first reset line is unasserted.
6. The capacitive fingerprint sensor of claim 5 , wherein in the transition phase, the second voltage VB is in connection with the fingerprint capacitor.
7. The capacitive fingerprint sensor of claim 2 , wherein in an evaluation phase, the scan line and the first reset line are asserted, and the second reset line is unasserted.
8. A capacitive fingerprint sensor, comprising:
a sensor cell comprising:
a fingerprint capacitor having a capacitance that is either a valley capacitance C FV or a ridge capacitance C FR , wherein C FV is smaller than C FR ;
a first transistor controlling the fingerprint capacitor during a scan line period;
a second transistor configured to discharge the fingerprint capacitor;
a control circuit comprising:
an integrator having a reference capacitor C fb ;
a third transistor precharging the fingerprint capacitor and redistributing the charges between the fingerprint capacitor and the reference capacitor C fb .
9. The capacitive fingerprint sensor of claim 8 , wherein in a precharge phase, the third transistor is precharging the fingerprint capacitor to a first voltage VA, which is connected to the integrator.
10. The capacitive fingerprint sensor of claim 8 , which is serially operated in precharge, transition and evaluation phases.
11. The capacitive fingerprint sensor of claim 10 , wherein the integrator further comprises a fourth transistor connected to the reference capacitor C fb in parallel, and the fourth transistor is turned on during the precharge and transition phases.
12. The capacitive fingerprint sensor of claim 10 , wherein the third transistor is used in the precharge and evaluation phases.
13. The capacitive fingerprint sensor of claim 10 , wherein the second transistor is controlled by an evaluate signal, which is initiated only during the transition phase.
14. The capacitive fingerprint sensor of claim 10 , wherein the integrator has an output voltage of
VA
+
(
VA
-
(
VB
+
(
VA
-
VB
)
×
ⅇ
-
t
RC
FR
)
)
×
C
FR
C
fb
or
VA
+
(
VA
-
(
VB
+
(
VA
-
VB
)
×
ⅇ
-
t
RC
FV
)
)
×
C
FV
C
fb
during the evaluation phase, R represents a turn-on resistance of the second transistor, and t represents passing time.
15. A panel system, comprising:
an active matrix area having fingerprint sensor cells, each fingerprint sensor cell comprising:
a fingerprint capacitor having a capacitance that is either a valley capacitance C FV or a ridge capacitance C FR , and C FV is smaller than C FR ;
a first transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by a scan line, the input terminal is connected to the fingerprint capacitor, and the output terminal is connected to a readout line;
a second transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by an evaluate signal, the input terminal is connected to a second voltage VB, and the output terminal is connected to the fingerprint capacitor;
a scan driver controlling scan lines to the active matrix area;
a readout circuit, comprising:
a third transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by a first reset line, and the input terminal is connected to the readout line;
an operational amplifier, wherein one input end of the operational amplifier is connected to the output terminal of the third transistor, and the other input end of the operational amplifier is connected to a first voltage VA;
a reference capacitor C fb connected to input and output ends of the operational amplifier; and
a fourth transistor having a gate terminal, an input terminal and an output terminal, wherein the gate terminal is controlled by a second reset line, and the input and output terminals are connected to the input and output ends of the operational amplifier; and
an image processing circuit connected to the readout circuit.
16. The panel system of claim 15 , wherein the readout circuit comprises an output multiplexer and a comparator, the input end of the output multiplexer is connected to the output of the operational amplifier, and the output end of the output multiplexer is connected to the comparator.
17. The panel system of claim 16 , wherein the output multiplexer outputs one bit to the image processing circuit.
18. The panel system of claim 16 , wherein the comparator has a threshold voltage V REF , and
VA
+
(
VA
-
(
VB
+
(
VA
-
VB
)
×
ⅇ
-
t
RC
FR
)
)
×
C
FR
C
fb
>
V
REF
>
VA
+
(
VA
-
(
VB
+
(
VA
-
VB
)
×
ⅇ
-
t
RC
FV
)
)
×
C
FV
C
fb
.
19. The panel system of claim 15 , wherein one scan line is enabled during the operation of the capacitive fingerprint sensors connected to the scan line.
20. The panel system of claim 15 , wherein the active matrix area further includes image pixels.