Cypress CapSense CY8C20x36 Guía de usuario descargar pdf (Pagina 51)

Getting Started with CapSense

Document No. 001-64846 Rev. *I 51

3.7 PCB Layout Guidelines

In the typical CapSense application, the capacitive sensors are formed by the traces of a printed circuit board (PCB)

or flex circuit. Following CapSense layout best practices will help your design achieve higher noise immunity, lower

, and higher signal-to-noise ratio (SNR). The CapSense signal drops off at high C

levels due to drive limits of the

internal current sources that are part of the CapSense circuitry. The long time constants associated with high C

are

another reason to avoid high C

3.7.1 Parasitic Capacitance, C

The main components of C

are trace capacitance and sensor capacitance. C

is a nonlinear function of sensor

diameter, trace length, trace width, and the annular gap. There is no simple relation between C

and PCB layout

features, but here are the general trends. An increase in sensor size, an increase in trace length and width, and a

decrease in the annular gap all cause an increase in C

. One way to reduce C

is to increase the clearance between

the sensor and ground. Unfortunately, widening the gap between sensor and ground will decrease noise immunity.

3.7.2 Board Layers

Most applications use a two-layer board with sensor pads and a hatched ground plane on the top side and all other

components on the bottom side. The two-layer stackup is shown in Figure 3-28. In applications where board space is

limited or the CapSense circuit is part of a PCB design containing complex circuitry, four-layer PCBs are used.

Figure 3-28. Two-Layer Stackup for CapSense Boards

3.7.3 Board Thickness

FR4-based PCB designs perform well with board thicknesses ranging from 0.020 inches (0.5 mm) to 0.063 inches

(1.6 mm).

Flex circuits work well with CapSense, and are recommended for curved surfaces. All guidelines presented for PCBs

also apply to flex. Ideally, flex circuits should be no thinner than 0.01 inches (0.25 mm). The high breakdown voltage

of the Kapton

material (290 kV/mm) used for flex circuits provides built in ESD protection for the CapSense sensors.

3.7.4 Button Design

The best shape for buttons is round. Rectangular shapes with rounded corners are also acceptable. Because sharp

points concentrate fields, avoid sharp corners (less than 90º) when designing your sensor pad.

Figure 3-29. Recommended Button Shapes

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Cypress CapSense CY8C20x36 Guía de usuario Pagina 51