th July 2011 24 www.semtech.com SX8654/SX8655/SX8656 H" />
參數(shù)資料
型號: SX8655IWLTRT
廠商: Semtech
文件頁數(shù): 17/62頁
文件大?。?/td> 0K
描述: IC TOUCH SCREEN 12BIT 20QFN
標準包裝: 1
類型: 電阻式,觸感驅(qū)動器
觸摸面板接口: 4,5 線
輸入數(shù)/鍵: 1 TSC,雨刷,鑰匙
分辨率(位): 12 b
評估套件: *
數(shù)據(jù)接口: I²C,串行
電壓基準: 內(nèi)部
電源電壓: 2.3 V ~ 3.6 V
工作溫度: -40°C ~ 85°C
安裝類型: 表面貼裝
封裝/外殼: 20-WFQFN 裸露焊盤
供應商設備封裝: 20-QFN(4x4)
包裝: 標準包裝
其它名稱: SX8655IWLDKR
ADVANCED COMMUNICATIONS & SENSING
Rev 1 – 25
th July 2011
24
www.semtech.com
SX8654/SX8655/SX8656
Haptics Enabled 4/5-Wire
Resistive Touchscreen Controller with Proximity Sensing
5.2
Analog Front-End (AFE)
5.2.1
Capacitive Sensing Basics
Capacitive sensing is the art of measuring a small variation of capacitance in a noisy environment.
As
mentioned above, the chip’s proximity sensing interface is based on capacitive sensing technology. In order to
illustrate some of the user choices and compromises required when using this technology it is useful to
understand its basic principles.
To illustrate the principle of capacitive sensing we will use the simplest implementation where the sensor is a
copper plate on a PCB but the exact same principles apply if the sensor is the touchscreen’s top plate.
The figure below shows a cross-section and top view of a typical capacitive sensing implementation. The sensor
connected to the chip is a simple copper area on top layer of the PCB. It is usually surrounded (shielded) by
ground for noise immunity (shield function) but also indirectly couples via the grounds areas of the rest of the
system (PCB ground traces/planes, housing, etc). For obvious reasons (design, isolation, robustness …) the
sensor is stacked behind an overlay which is usually integrated in the housing of the complete system. When the
touchscreen is used for sensing the overlay corresponds to the thin and flexible protection film covering the top
panel.
PCB dielectric
Ground
Cut view
Top view
PCB copper
Sensor
Overlay
Figure 28 – Typical Capacitive Sensing Implementation
When the conductive object to be detected (finger/palm/face, etc) is not present, the sensor only sees an
inherent capacitance value CEnv created by its electrical field’s interaction with the environment, in particular
with ground areas.
When the conductive object (finger/palm/face, etc) approaches, the electrical field around the sensor will be
modified and the total capacitance seen by the sensor increased by the user capacitance CUser. This
phenomenon is illustrated in the figure below.
Figure 29 – Proximity Effect on Electrical Field and Sensor Capacitance
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