AN4312 datasheet(18/36 Pages) STMICROELECTRONICS

18 / 36 page

Surface sensor design

AN4312

18/36

DocID024815 Rev 1

Regarding panel thickness, the touchkey must be at least four times as wide as the panel is

thick. For example, a panel which is 1.5 mm thick and has no immediately adjacent ground

layer, must have a touchkey sensor which is at least 6 mm in diameter if the touchkey

sensor is round, or have a 6 mm side if the touchkey sensor is square (see Figure 11).

There are sensitivity issues if dimensions lower than these values are used.

Figure 11. Recommended electrode size

As shown in Equation 4, a capacitor is used to detect the finger touch. The capacitor is

proportional to the size of the electrode. Increasing the electrode area allows the capacitor

to be maximized, but increasing the electrode size above the size of a finger touch only

increases the parasitic capacitance and not the finger touch capacitance, resulting in lower

relative sensitivity. Refer to Section 3.5.4: PCB and layout. There is also a problem of

relative sensitivity: when the electrode size is increased, CT stops increasing while CX

keeps growing. This is because the parasitic capacitance is directly proportional to the

electrode area.

Equation 4

where:

is the touchkey capacitance

is the area with regard to the electrode and the conductive object

is the distance between the electrode and the conductive object (usually the

panel thickness)

εR

is the dielectric relative permittivity

ε0

is the vacuum permittivity

6 mm (min.)

εRε0A

----------------

AN4312 Datasheet(PDF) 18 Page - STMicroelectronics