Minimize leakage power in embedded SoC designs with Multi-Vt cells

In this Product How-To article, Mark Lee of Cypress uses the company’s CY8C21434 PSoC to explain the methods by which to integrate capacitive sensors into consumer white goods where the ability to operate reliably in wet environments is important.

Capacitive touch sensors are commonly found today in MP3 players and mobile phones. As this sensor technology expands into other product categories, new design challenges are encountered.

With electric ranges, dishwashers and other products in the white goods category, one of these new challenges is operation in a wet environment. This article shows how to design capacitive touch sensors that are water tolerant.

Water tolerant versus waterproof

A waterproof design implies system performance that is totally immune to the effects of water. For a water tolerant design, water levels encountered in normal operation do not interfere with se ***a***nsor operation. Splatters and spills on the touch surface are tolerated, but total immersion is not. Water tolerance is a reasonable and cost effective solution for operation in a wet environment.

In a water tolerant design, only the touch of a finger produces a signal large enough to register as a “touch”. However, if a boiling pot overflows, and the touch surface is submerged in hot liquid, the water tolerant sensor will be challenged to operate normally. Through proper configuration of the sensor array, the submersion can be detected, and the system can be alerted that an abnormal event has occurred.

The safest response to such an event is to turn off the burner until the spill can be cleaned up. In contrast, a waterproof design will continue normal operation after the spill. To turn off the burner, the user of a waterproof system needs to touch the sensor through a coating of hot liquid.

If the liquid is too hot to touch, the burner stays on, and the pot keeps boiling, only making the situation worse. The water tolerant design leads to a system that turns itself off with a major spill. Comparing the two approaches for reacting to a spill of hot liquid, the water tolerant design is the safer and smarter choice.

Classifying the degree of surface wetness

In the following discussion, surface wetness is classified into three categories: Dry, Droplet, and Stream, as shown in Figure 1 below. When liquid is sprayed or splashed onto a dry surface, surface tension causes the liquid to bead up, forming droplets.

Minimize leakage power in embedded SoC designs with Multi-Vt cells

Figure 1. Cross section view of the three categories of surface wetness : a) Dry, b) Droplet, c) Stream.

A water tolerant design needs to operate normally when the surface is covered with droplets. For larger amounts of liquid, the droplets merge together and form a stream if set in motion, or a puddle if ***a***the surface is at a low point.

Special electrodes help in wet environments

Fingers are conductive, so they interact with the electric field that is set up around the touch sensors. Water is conductive, so it interacts with the same electric field when it is lands in the active sensing area.

This can lead to a report of a finger touch when water splashes onto the sensing surface even when no finger is present. Figure 2 below shows an example of drops of water producing the same signal level as a finger for a touch sensor that lacks any features for water tolerance.

Minimize leakage power in embedded SoC designs with Multi-Vt cells

Figure 2. Example of a finger touch and drops of water both producing a signal that crosses the finger detection threshold for a sensor with no water tolerance.

The Raw Count shown in the figure is the unfiltered output from the sensor. The Baseline is a continuously updated estimate of the average Raw Count level when a finger is not present. The Baseline provides a reference point for determining when a finger is present on the sensing surface.

Fingers and water interact in a similar, but not identical, way with electric fields. There is enough difference between the two to make possible techniques for discriminating between a touch and a spill.

On printed circuit boards and flex circuits, a practical level of water tolerance is achieved with the use of a shield electrode and guard sensor. These special electrodes add no material cost to the system since they are incorporated into the same circuit board layout as the touch sensors, as shown in Figure 3 below.

Minimize leakage power in embedded SoC designs with Multi-Vt cells

Figure 3. The shield electrode and guard sensor are added to the printed circuit board layout to add water tolerance to standard touch sensors.

The purpose of the shield electrode is to set up an electric field pattern around the touch sensors that helps attenuate the effects of water. The purpose of the guard sensor is to detect abnormally high liquid levels so the system can react appropriately.