AK4673 Datasheet, PDF (70/107 Pages) Asahi Kasei Microsystems – Stereo CODEC with MIC/HP-AMP and Touch Screen Controller

[AK4673]
■ The Position Detection of Touch Screen
A position detecting (X, Y position) on the touch panel is selected by the control command via the A2, A1, A0 bits in the
control register. The mode of the position detecting is differential mode, the full scale (ΔVREF) is the differential voltage
between the non-inverting terminal and the inverting terminal of the measured axis (e.g. X-axis measurement: ΔVREF =
V_XP – V_XN). The voltage difference on the A/D converter (ΔAIN) is the voltage between non-inverting terminals of the
non-measured axis and the inverting terminal of the measured axis. (E.g. ΔAIN= (AIN+) - (AIN-) = V_YP-V_XN) The
voltage difference (ΔAIN) is charged to the internal capacitor array during the sampling period. No current flows into the
internal capacitor after the capacitor has been charged completely.
The required settling time to charge the internal capacitor array depends on the source impedance (Rin). If the source
impedance is 600 ohm, the settling time needs at least 2.5μs (1 clock cycle period of SCL 400 KHz)
The position on the touch screen is detected by taking the voltage of one axis when the voltage is supplied between the
two terminals of another axis. At least two A/D conversions are needed to get the two-dimensional (X/Y axis) position.
TSVDD
XP-Driver SW ON
X-Plate
XP
Y-Plate
TSVDD
YP-Driver SW ON
X-Plate
XP
Y-Plate
VREF+
AIN+
ADC
YP
VREF-
AIN-
XN
VREF+
AIN+
ADC
YP
VREF-
AIN-
XN
XN-Driver SW ON
YN
YN Touch Screen
YN-Driver SW ON
a) X-Position Measurement Differential Mode
b) Y-Position Measurement Differential Mode
The X-plate and Y-plate are connected on the dotted line when the panel is touched.
XP
X-Plate (Top side)
XN
YN
Y-Plate (Bottom side)
YP
c) 4-wire Touch Screen Construction
Figure 63 Axis Measurements
The differential mode position detection is typically more accurate than the single-ended mode. As the full scale of
single-ended mode is fixed to the VCC, input voltage may exceed the full-scale reference voltage. This problem does not
occur in differential mode. In addition to this, the differential mode is less influenced by power supply voltage variation
due to the ratio-metric measurement.
MS0670-E-00
- 70 -
2007/09