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AN11157 の電気的特性と機能

AN11157のメーカーはNXP Semiconductorsです、この部品の機能は「Capacitive touch sensing」です。


製品の詳細 ( Datasheet PDF )

部品番号 AN11157
部品説明 Capacitive touch sensing
メーカ NXP Semiconductors
ロゴ NXP Semiconductors ロゴ 




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AN11157 Datasheet, AN11157 PDF,ピン配置, 機能
AN11157
Capacitive touch sensing with high EMC performance
Rev. 1 — 3 February 2012
Application note
Document information
Info Content
Keywords
EMC performance, Touch sensing, Design, Guidelines, PCF8885,
PCA8885
Abstract
NXP’s touch sensor family provides one of the highest immunity sensor
circuits on the market. Nevertheless some considerations in the design
are necessary to get a high EMC performance in the application. A
description and discussion about the EMC immunity and how it can be
improved are made in this document.

1 Page





AN11157 pdf, ピン配列
NXP Semiconductors
AN11157
Capacitive touch sensing with high EMC performance
1. Introduction
NXP’s touch sensor family provides one of the highest immunity sensor circuits on the
market. Nevertheless some considerations in the design are necessary to get a high EMC
performance in the application. A description and discussion about the EMC immunity and
how it can be improved are made in this document.
2. High-immunity cap sensing
There are four important features of NXP’s touch sensor family which provides one of the
highest immunity sensor circuits on the market.
1. Wide input capacitance range: The steady state input capacitance on the sensor
input is allowed to be in the range of 10 pF to 40 pF (60 pF for PCF8883 and
PCF8886) without affecting the sensitivity.
2. External low-pass filter: There may be a resistor (up to 5 k) in series between the
sensor plate and the sensor input with minor impact on the sensitivity.
3. Internal low-pass filter: There is also an internal low-pass filter suppressing
high-frequency noise.
4. Digital signal processing: The decision on capacitive events is based on a simple
and yet very efficient digital signal processing.
Thanks to the auto-calibration feature of the NXP touch sensors, the steady state
capacitance is compensated for by unloading the input through an internal shunting
current source. The distributed capacitance between traces and GND and the parallel
plate capacitance between sensor plates and GND layers is considered as parasitic
capacitance. Many applications with competitor products will either fill the entire input
capacitance range or reduce sensitivity dramatically. This will also force the designers to
make a trade-off between noise immunity and sensitivity. Just to exemplify, a sensor plate
with 10 mm diameter built on a 1.5 mm FR-4 material with GND on the reverse side will
have a capcitance of approx. 2.2 pF. Reduced dielectric thickness, small GND-separation
and larger sensor plates will easily add up 20 pF which is the maximum value for most of
the touch sensor circuits on the market. The wide range for NXP touch sensors allows
smallest possible GND separation for maximum noise immunity without any
concerns for sensitivity. In addition, a GND layer on the reverse side of the board will
provide high immunity without reducing sensitivity.
As a direct benefit of the wide input capacitance range and auto-calibration feature, a
low-pass filter might be built as illustrated in Figure 1.
AN11157
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 3 February 2012
© NXP B.V. 2012. All rights reserved.
3 of 12


3Pages


AN11157 電子部品, 半導体
NXP Semiconductors
AN11157
Capacitive touch sensing with high EMC performance
GND
Trace
PCB substrate
GND GseNpDaration
separation
GND
013aaa615
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
013aaa616
0.2 0.4 0.6
GND separation [mm]
0.8
1
a. Trace on a PCB
b. Contribution to parasitic capacitance
Fig 3. A traces on a PCB and its contribution to parasitic capacitance
5. IEC testing
Every electronic device is required to comply with certain regulations for appropriate
application areas and level of importance. Thanks to the use of an internal low-frequency
RC oscillator, radiation from the NXP touch sensors is not an issue. However immunity
against conducted and radiated disturbance has to be proven. In order to create the most
immune electronics possible it is also crucial for application board designers to know the
susceptibility of touch sensors for such disturbances. The Ref. 1 “IEC 61000-4-4” and Ref.
2 “IEC 61000-4-6” standards define immunity against bursts of electrical fast transients
(EFT) and injected RF disturbances respectively. In addition to these standards applying
on complete electronic apparatus with power supplies and interface to auxiliary devices,
there are also standards for testing integrated circuits (ICs).
Philips Innovation Services-EMC Center has been chosen for application level testing and
Langer EMV has been chosen for IC level testing.
5.1 ESD testing
ESD testing has been performed according to applicable norms, Human Body Model
(HBM) according to Ref. 6 “JESD22-A114” and is classified for 1.9 kV. Charged device
model (CDM according to JESD22-C101C) level has been tested to be 500 V for all pins
and 750 V for the corner pins.
NXP’s PRTR5V0U8S is an 8 channel (Integrated octal low-capacity ESD protection to IEC
61000-4-2, level 4) ESD protection component from NXP to be recommended for if higher
ESD protection is desired. It provides protection for up to 8 kV discharges from human
bodies.
AN11157
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 3 February 2012
© NXP B.V. 2012. All rights reserved.
6 of 12

6 Page



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