a compact low-power shunt proximity touch sensor and ... · a compact low-power shunt proximity...

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.3, JUNE, 2016 ISSN(Print) 1598-1657 http://dx.doi.org/10.5573/JSTS.2016.16.3.380 ISSN(Online) 2233-4866

Manuscript received Jul. 4, 2015; accepted Mar. 7, 2016 1 Department of Information Display, Sun Moon University, Korea 2 Department of Electrical and Computer Engineering, The University of Akron, USA 3 Department of Digital Media and App., Seoul Women’s University, Korea E-mail : Corresponding author, HYPERLINK "mailto:[email protected]" [email protected]

A Compact Low-Power Shunt Proximity Touch Sensor and Readout for Haptic Function

Yong-Min Lee1, Kye-Shin Lee2, and Taikyeong Jeong3,*

Abstract—This paper presents a compact and low-power on-chip touch sensor and readout circuit using shunt proximity touch sensor and its design scheme. In the proposed touch sensor readout circuit, the touch panel condition depending on the proximity of the finger is directly converted into the corresponding voltage level without additional signal conditioning procedures. Furthermore, the additional circuitry including the comparator and the flip-flop does not consume any static current, which leads to a low-power design scheme. A new prototype touch sensor readout integrated circuit was fabricated using complementally metal oxide silicon (CMOS) 0.18 μm technology with core area of 0.032 mm2 and total current of 125 μA. Our measurement result shows that an actual 10.4 inches capacitive type touch screen panel (TSP) can detect the finger size from 0 to 1.52 mm, sharply. Index Terms—Shunt proximity touch sensor, touch screen panel (TSP), readout circuit

I. INTRODUCTION

Due to the increasing demand for user-friendly interface and easy manipulation, touch screen panels (TSP) are widely used in tablet computers, personal

digital assistants (PDAs), smart handhelds, and etc [1-3]. To these days, the low-cost resistive type touch screens were generally used [4]; however, capacitive type touch screens are becoming more popular due to responsive characteristic, high light-transmittance and long-lifetime. Furthermore, capacitive type touch screens can realize multi-touch features with improved accuracy along with a function for zoom-in and -out of screen size. Because human body is an electrical conductor, touching the object makes a distortion of the object’s electrostatic field and characteristics. The capacitive type touch screens use this phenomenon, which detects the variation of the touch panel capacitance with touch sensor readout circuits.

For conventional capacitive type touch sensor readout circuits, the finger or stylus has to physically contact the touch screen panel in order to detect the touch location from the panel capacitance variation. However, proximity touch sensing is another touch sensing technique that can detect the touch location when the finger is near the touch panel (without contact) [5].

This feature will be a requirement for next generation touch screens, which will enable easier manipulation of the touch panel in situations such as the user, is driving or walking. To meet this requirement, developing a compact and low-power proximity touch sensor readout blocks are highly demanding.

In this paper, a complete on-chip shunt proximity touch sensor readout circuit is proposed. In Section II, describes a theoretical method to transfer frequencies to the converters. Section III discusses a simulation result that influences on the outputs of a proposed circuit associated with a chip layout scheme. Section IV describes proximity touch sensing operation with low-

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.3, JUNE, 2016 381

power capacitive type touch panel’s measurement result. Section V makes a conclusion of this research work.

With this approach, the panel capacitance variation can be detected even when the finger is near the touch panel. Consequently, the proposed scheme shows lower power consumption and smaller die size, compared to conventional schemes.

II. PROPOSED TOUCH SENSOR READOUT

CIRCUIT

In this section, we are discuss about a proposed touch sensor readout circuit which is currently fabricated using CMOS 0.18 μm technology with core area of 0.032mm2 and total current of 125 μA. Fig. 1(a) shows a block diagram of the proposed proximity touch sensor readout circuit which includes the oscillator, AC to DC convertor; low pass filter (LPF), analog-to-digital converter (ADC) and the external reference resistor R. The AC voltage from the sine wave oscillator is converted to a level DC by the AC to DC converter, and the AC voltage amplitude is controlled by the impedance divider.

In addition, for the actual touch panel, there will be multiple panel capacitances CP which require an

additional selection circuitry. Furthermore, the LPF filters out high frequency noise and the ADC generates a binary output that indicates the condition of the touch panel (proximity level of the finger). The advantage of the proposed readout circuit is a very simple structure and low power consumption compared to the conventional schemes which convert capacitance into a frequency or time domain quantity that require longer detection time and additional circuit components such as the counter. This makes the proposed touch sensor readout circuit suitable for large sized touch panels as well.

Fig. 1(b) describes the operation of the proposed shunt proximity touch sensor. In the readout circuit, the touch panel capacitance is sensed as impedance.

The impedance RP is given as

1/ (2 )p in PR f Cp= (1)

where fin is the input signal frequency. As the finger gets closer to the panel, CP will decrease and RP will increase. In addition, the input voltage of the detection circuit VX is given as

/ ( )X in P PV V R R R= + (2)

(a)

(b)

Fig. 1. (a) Block diagram of proposed touch sensor readout circuit, (b) Shunt proximity touch sensor operation.

382 YONG-MIN LEE et al : A COMPACT LOW-POWER SHUNT PROXIMITY TOUCH SENSOR AND READOUT FOR HAPTIC …

When the finger gets closer, CP will decrease, and the node voltage VX is given as

( ) / ( )X in P P P PV V R R R R R= + D + + D (3)

where VX will be compared with the reference voltages generated from VREFP and VREFN. If R is fixed, VX will have a different level depending on the value of the touch panel capacitance CP.

In general, the variation of CP in the touch screen panel is around 10 pF. In this case, the touch panel capacitance, CP, can increase because of the increased proximity to ground which reduces sensitivity and continue to investigate signal-to-noise ratio (SNR) and other measurement parameters. In order to see a variation of CP, if we assumed that the distance between finger and the sensor is changed (reduced by half), the sensitivity will change double. We can now understand the importance of parasitic capacitance, CP for the touch sensors application, so that CTotal (the total capacitance) consists of CP (the parasitic capacitance) and CF (the finger's capacitance). It is a total sum of two characteristics so that the stronger the effect of CP, the less you can see CF, the change in capacitance due to a finger.

To measure an average level of power consumption, all valid inputs from fingers (Inputs) are set at the inputs of the detection circuit’s one after another. As a small signal analysis, gm, is not easy to set in relation with the different oscillation frequencies from 2/π to 2π. The average power consumption in the aforementioned proposed method at different oscillation frequencies are measured and shown in [6]. The combination of a converter circuit and an ADC converter circuit has the lowest power consumption. In general, flash ADCs, are the fastest way to convert an analog signal to a digital signal - as already known, flash ADCs are ideal for applications requiring very large bandwidth, but they consume more power than other ADC architectures [7], Therefore, we have decide that generally applied to reduce a power consumption for this TSP application.

In addition, Fig. 2(a)-(c) shows that a detail architecture of detection circuit. Fig. 2(a) shows the sine-wave oscillator circuit. The oscillator oscillates at fOSC = gm/2πC, where the gm is given as

1/ 2[2 ( / ) ]m n OX biasg C W L Im= (4) Fig. 2(b) shows the AC to DC converter circuit. The

circuit is composed of a rectifier and LPF. The AC to DC converter generates constant amplitude output from the input that varies in time for the ADC. Fig. 2(c) shows the ADC. For rapid detection time and simple circuitry, the flash ADC architecture is used. For low power operation of the comparator, the pre-amplifier stage is combined with the positive feedback stage.

III. SIMULATION RESULTS

A new approach is proposed in this paper to handle a

(a)

(b)

(c)

Fig. 2. (a) Sine-wave oscillator circuit, (b) AC to DC converter circuit, (c) Analog to Digital Converter (ADC) circuit.


low-power consumption proximity touch sensor. This new proposed circuit is not to change the entire circuitry but to reduce a significant power consumption of the circuit. The ADC circuits of the new method produce one technical result, an increased capacitance output, to determine whether the value on the remaining outputs is reliable or unreliable.

The operation of the proposed touch sensor interface was verified through circuit level simulations. The input sine-wave signal frequency was set to 250 kHz and VREFP

and VREFN were set to 0.7 V and 0.3 V, respectively. The binary output of the whole readout circuit was verified with touch panel capacitance that changes from 10 pF to 20.9 pF, where CP = 20.9 pF is assumed for the un-touched case and CP = 10 pF is assumed for the touched case.

Consequently, Fig. 3(a) is the binary output of the whole readout circuit with CP changing from 20.9 pF (un-touched) to 10 pF (touched). Because 2-bit binary output ADC is a basic fundamental building block of many digital/analog circuits, we will also do an enhanced signal testing (n-th bit) will be expanded. We can estimate a signal-to-noise ratio (SNR) by calculating a µU (the un-pressed average) and µP (the pressed average) in Section IV, Measurement. In this case, the reference resistor R was set to 30 kΩ. The binary output of the whole readout circuit represents the digital output code Dout1~4 shown in Fig. 2(c). When CP = 20.9 pF the output code shows 0011 and the output shows 1111 when CP = 10 pF, respectively. Fig. 3(b) shows the measured binary output waveform with touch panel capacitance that changes from 20.9 pF to 10 pF.

The proposed touch sensor readout circuit was fabricated in CMOS 0.18 μm technology with core size of 0.032 mm2 and total current of 125 μA. Fig. 3(c) shows the layout of the proposed proximity touch sensor readout integrated circuit.

While design of a simple capacitance proximity type sensor, we recognized that a simulation result is significantly related to the original relaxation oscillator design [8]. The capacitance to be sensed forms a portion of the oscillator's RC or LC circuit. It should be noted that the capacitance can be calculated by measuring the charging time required to reach the threshold voltage (of the relaxation oscillator), or equivalently, by measuring the oscillator's frequency. As Fig. 3(c) indicated that the proposed touch sensor interface is given with a previse

simulation result along with a proper fabrication technique.

IV. MEASUREMENT

In order to verify the proximity touch sensing operation, the proposed touch sensor readout circuit is connected to a projective capacitance type 10.4 inches touch panel through the reference resistor R, where the ADC output is monitored by changing the distance of the finger from the touch panel (in Fig. 4(a)).

As the finger gets closer to the touch panel, the capacitance between the top and the bottom Indium Tin Oxide (ITO) electrode reduces, and as a result the equivalent impedance Rp will increase, which will increase the ADC output level. In our experiment measurement results are required to implement on the touch screen panel and our oscillation measurement values are linear with a distance between finger and panel, as we described.

Fig. 4(b) shows the ADC output with different finger distance, d, where the ADC output increases as the d decreases. The proposed proximity touch sensor circuit is able to detect the finger within the distance from 0 to 1.52 mm. Compared to the recent capacitive proximity sensing integrated circuit [7] with die size of 0.21 mm2

and total current of 370 μA, the proposed touch sensor readout circuit shows lower power consumption (total current of 125 μA) [9] and smaller die size (0.032 mm2). This is, certainly, due to the compact and simple circuit.

In this case, the objective of this paper is to define the optimal design result based on the proposed scheme when developing low-power capacitive touch readout circuit. It will address by designing the hardware of touch sensor panel application fabricated by readout ICs. Hardware design for touch sensor panel application will be obtained to help maximize the off peak-to-peak signal-to-noise ratio (SNR) of this real application. It can be measured by a difference between µU (the unpressed average) and µP (the pressed average), after that divided by σ, (the unpressed stdandard deviation). Therefore, in this measurement case, (20.9- 10.0) / 9.2, the SNR will be approximately, 1.2. In order to adjust the sensor size to maximize the sensitivity, the more parasitic capacitance CP (Parasitic capacitance), (See in Section III, Simulation results) will be able to lower the sensitivity.


(a)

Bit1 Bit2

Bit3 Bit4 (b)

(c)

Fig. 3. (a) Simulated output waveforms, (b) Measured output waveforms, (c) Layout of the proposed touch sensor interface.


Moreover, in order to evaluate power consumption and maximum operation speed, the touch sensor readout circuits are designed by a 0.18 mm library with all the parameters of the transistors set at default value. The circuits are fabricated with a commercial design tool, Cadence tool. In addition, we summarized a detailed comparison with other work in Table 1. Table 1 show that a detailed comparison results with other work. The ICs for capacitive touch sensors are from programmable resistive sensing conditioner with digital and analog output, so that conditions its input signals by amplification and digitization [10]. With the user programmed software, this can perform linearization, pressure compensation, and other user’s new algorithms including proximity function.

It should be noted that we presents a chip layout with actual fabrication process in Fig. 3(c) in Section IV, instead of chip photograph because there is no difference between layout and picture.

V. CONCLUSION

A new proximity readout circuit method was proposed to handle power consumption in a touch sensor panel.

Our capacity type touch sensor scheme (oscillator, AC to DC convertor; low pass filter (LPF), analog-to-digital converter (ADC) and the external reference resistor R), has shown the full detection coverage of any signal without extra delay and with low-power consumption. Working together with the existing low-cost resistive type touch methods which are not concern all sensitive and power consumption, the readout integrated circuit help the conventional methods extend the full detection coverage of sensitivity.

In this work, an on-chip shunt proximity touch sensor readout circuit based on impedance divider is proposed. The proposed scheme can directly convert the touch panel capacitance into a voltage level, which does not require additional circuitry for detection. Moreover, the power consumption of the detection circuit is minimized.

Bottom line, the proposed scheme is suitable for complete on-chip capacitive touch sensor interface circuits due to low power consumption and simple circuitry.

ACKNOWLEDGMENT

This work has been supported by the Basic ScienceResearch Program through NRF of Korea2014R1A1A2058886, 2014R1A1A2059115. This work was supported by aresearch fund of Seoul Women’s University in 2016.

REFERENCES

[1] I. S. Yang, and O.K. Kwon, “A Touch Controller using Differential Sensing Method for On-cell Capacitive Touch Screen Panel Systems,” IEEE Transactions on Consumer Electronics, vol. 57, pp.

Fig. 4. (a) Proximity touch sensing operation with capacitive type touch panel, (b) ADC output with different finger distance.

Table 1. Comparison results

Constrains ICs for low-cost resistive touch sensors including

proximity type

ICs for capacitive touch sensors including proximity

type Detection objects

(finger) 0 ~ 1.52 mm 0 ~ 1.75 mm

Touch panel size 10.4 inches 8.0 inches Technology 0.18 μm 0.18 μm Core area 0.032 mm2 0.21 mm2 Current 125 μA 370 μA

Signal frequency 250 kHz 200 kHz


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[5] P. Rodriguez, A.V. Timbus, R. Teodorescu, and M. Liserre, “Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults,” IEEE Transactions on Industrial Electronics, vol. 54 , no. 5, pp. 2583 - 2592, Aug. 2007

[6] L. Bin, T. W. Rondeau, J. H. Reed, and C. W. Bostian, “Analog-to-digital converters,” IEEE Signal Processing Magazine, vol. 22, no. 6, pp. 69-77, Nov. 2005

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[10] X. Zhang, M. Liu, H. Chen, C. Zhang, and Z. Wang, “A wide Dynamic Range and Fast Update Rate Integrated Interface for Capacitive Sensors Array,” in Proceedings of IEEE International Symposium on Circuits and Systems (ISCAS 2012), pp. 2961 – 2964, May 2012

Yong-Min Lee received the B.S. degree of electronic engineering from Inha University, Incheon, Korea, in 1986, and the Ph.D. degree from the University of Ediburgh, U.K., in 2005, in electrical engineering. He was with Samsung , Suwon, Korea,

from 1986 to 2007. He joined Sunmoon University in 2008 and currently an associate Professor with the Department of Information Communication & Display Engineering, Sunmoon University, Asan, Korea. His current research interests include analog integrated circuits for touch sensor applications and LCoS display technology for various applications.

Kye-Shin Lee (S’02–M’06) received the B.S. degree from Korea University, Seoul, Korea, in 1992, the M.S. degree from Texas A&M University, College Station, TX, USA, in 2002, and the Ph.D. degree from the University of Texas at Dallas,

Richardson, TX, USA, in 2005, all in electrical engineering. He was with Texas Instruments Inc., Dallas, TX, USA, from 2005 to 2008. In 2009, he was an Assistant Professor with the Department of Electronics, Sun Moon University, Asan-si, Chungnam, Korea. He is currently an Assistant Professor with the Department of Electrical and Computer Engineering, The University of Akron, Akron, OH, USA. His research interests include analog integrated circuits and data converters for sensor applications.

Taikyeong Ted. Jeong received the Ph.D. degree from the Dept. of Electrical and Computer Eng., the University of Texas at Austin in 2004 and he was a recipient of the research grants of NASA, worked on high performance computing for next-

generation system, and energy harvesting system. Prof. Jeong is currently working as an Assistant Professor at the Dept. of Digital Media at Seoul Women’s University, Korea. His research interests include energy harvesting, IoT and power management design; He is a member of IET, IEICE and IEEE.

a compact low-power shunt proximity touch sensor and ... · a compact low-power shunt proximity...

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