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Holtek's new generation Touch Key Flash series MCUs are fully integrated with touch key functions, communication interfaces and display functions thus reducing greatly the need for external components. With the features of high S/N ratio touch switch control, power fluctuation rejection and excellent noise immunity, the stability of these touch keys has been greatly improved, thus offering solutions over traditional mechanical keys with their inherent weakness of rapid wear and tear and manufacturing complexity. These features combine to ensure effective and rapid product development in areas such as household appliances and other touch key controlled products.

Points to Note Points to Note

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14


Question 1

What are the limitations on the sense PAD shape and size?

Answer

There are no special restrictions on the sense PAD shape. Regarding the PAD size, using a circular share as an example, it is recommended that it has a diameter of 5~10mm, with a minimum size of 4mm. Larger PAD areas will result in higher sensitivities.

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Question 2

Why sometimes is the key reaction speed slow?

Answer

The touch key outer covering/sense distance and the sense PAD size, as well as if there is space between the PAD and the outer covering are all related. For this reason the actual PAD size, and if the actual PAD is firmly bonded to the outer covering and the actual material used for the outer covering must be used for device setup.

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Question 3

Why is a PADCref required? Are their limits on its use?

Answer

A PADCref can be used in place of Cs to reduce the number of external components. When using a PADCref note that it cannot be touched, therefore it must be enough space for it to be concealed.

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Question 4

Why can PADCref not be touched?

Answer

The PADCref is used to change the overall sensitivity of the touch switches. If it is touched or experiences interference, then all the switch sensitivities will increase which could cause erroneous operation.

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Question 5

Can curved touch switch outer coverings be used?

Answer

The sense pad and the external touch key outer covering must be closely bound. If there is space between them, then the touch switch sensitivity will be substantially reduced. For this reason only flat outer coverings should be used. However, if because the product in question requires to use a curved surface design, then the sense pad must still be closely bound to this curved surface.

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Question 6

What can be done if there is no way to fully bond the sense pad the outer covering?

Answer

If due to height restrictions the sense Pad cannot be fully in contact with the outer covering, then a spring (or other conducting material) to implement an extension to the Pad.

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Question 7

Are there any limitations on PAD to PAD spacing ?

Answer

The smallest distance between adjacent pads should be 2mm.

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Question 8

What are the methods of adjusting the touch key sensitivity?

Answer

Factors which affect touch switch sensitivity are the value of Cs, the PAD size, the thickness of the touch key outer covering and the touch key external covering material. The value of Cs should be as close to the capacitance of the sense pad as possible. Larger sense pad areas will result in higher touch key sensitivities, at the same time the Cs value should also be increased, however the related touch switch reaction time will be reduced.

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Question 9

Are there limitations regarding the size of the sense Pad?

Answer

The pad size of course will be limited by the product, however the smallest diameter should not be less than 4mm.

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Question 10

During volume production, how can testing be implemented? How can faults be detected?

Answer

The Holtek touch product production test tools can be consulted for an explanation.

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Question 11

Why sometimes is the key reaction speed slow?

Answer

When the BS80xB enters the low power mode, the first touch reaction time is slower (about 0.4 sec). The time after which the BS80xB will enter the low power mode is shown in the table:

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Question 12

In multi-key applications, is it possible to adjust the sensitivity of individual keys?

Answer

The size of each sense PAD can be changed to adjust its sensitivity?

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Question 13

Are there limits on the material used for the outer covering?

Answer

For materials which are in direct contact with the sense PAD, metallic or carbon containing conducting materials must not be used. The following table (most popular physical dielectric constant table) can be consulted. It is recommended that a material with a dielectric constant, K, of value between 1.5 and 4 is used. Examples would be glass, acrylic, plastic etc.
Dielectric Constant Table:

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Question 14

If the sensitivity is too high, what can be done to reduce the sensitivity?

Answer

a. Reduce the PAD area
b. Reduce the Cs capacitor value
c. Increase the thickness of the external covering
d. Change the covering material
e. An Rs resistor (0Ω~75KΩ) can be placed in series with the line connecting the sense Pad and the BS80xB KIN pin.
f. On the surrounding are of the sense Pad and the connecting layout line a ground net can be added.

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