MAX11800 Datasheet, PDF (20/56 Pages) Maxim Integrated Products – Low-Power, Ultra-Small Resistive Touch-Screen Controllers with I2C/SPI Interface

Low-Power, Ultra-Small Resistive Touch-Screen
Controllers with I²C/SPI Interface
The planar end-to-end resistance included in the cur-
rent path is minimal at this location. Keep the averaging
and panel settling durations to the minimum required
by the application when pressure measurements are
required. Table 3 summarizes the physical panel set-
tings for supported measurement types.
Touch-Detect Modes and Options
Figure 6 shows the internal circuitry in the
MAX11800–MAX11803 used to detect the presence of
a touch on the panel. The selection of the pullup resis-
tance value (R_TD = touch-detect resistance) and the
durations of the rough pullup interval (PUR = low-
impedance pullup) and fine pullup interval (PUF = high-
impedance pullup) are user-defined.
The MAX11800–MAX11803 revert to the low-power
panel setup when placed in touch-detect mode (TDM).
Figure 6 shows the active panel drive switches (YMSW
and XPSW are omitted for simplicity). TSW is a dedicat-
ed pullup switch used in TDM. TSW is also used during
PUF and TDM. XPSW is activated during PUR periods.
TDRSEL allows the selection of an internal pullup resis-
tor value of either 50kΩ or 100kΩ.
The X and Y touch-screen plates create an open circuit
with no current flow in the panel when the panel is not
being touched. In this case, TOUCH (see Figure 6) is
low. When a touch causes contact between the panel X
and Y plates, a current path is created and TOUCH is
pulled high, as long as R_PX + R_PY (the sum of panel
end-to-end resistance) is much lower than R_TD. Typical
open-circuit panel plate resistances range from 200Ω
to 1000Ω.
The MAX11800–MAX11803 enter high-impedance
pullup mode (50kΩ or 100kΩ) when the panel is not
being touched and the device is idle in this mode until
a touch is detected. The YMSW and TSW transistors
are on, and the XPSW and PSW transistors are off. With
no touch present, the Y- input of the TSC is at ground
and the X+ input is at V_DD - V_TN, where V_TN is the
threshold voltage of the TSW nMOS device. This is a
low-power mode in which no current is consumed until
a panel touch occurs. When a touch is present on the
panel, the touch-screen controller (TSC) X+ input is
pulled low by the touch panel plate resistance and the
YMSW transistor. This causes TOUCH to assume a
logic high and the MAX11800–MAX11803 either issue
the TIRQ interrupt for direct conversion modes or begin
self-timed scans for autonomous conversion mode.
The value of the user-defined R_TD depends on the
characteristics of the panel. To ensure reliable detec-
tion values, worst-case panel resistance must be
checked against R_TD. The interaction between R_TD and
the panel (or external noise rejecting) capacitance
determines how quickly the panel can be switched from
measurement modes back to touch monitoring mode
without reporting false touches or erroneous tags due
to panel settling.
Panel touch status is also required to tag data from a
completed scan and measurement operation. Following
each scan operation, the panel must be returned to
TDM to determine if the panel is still being touched and
if the data obtained during the scan operation should
be considered valid. This operation is required since
the panel cannot be monitored for the presence of a
touch during the scan and measurement procedure.
The MAX11800–MAX11803 must return to TDM after
completing a measurement and making a decision on
the touch status of the panel. The measurement proce-
dure is only completed upon resolution of the touch sta-
tus and when data is tagged and available for
Table 3. Summary of Physical Panel Settings for Supported Measurement Types
MODE
X
Y
Z1
Z2
PUR
TDM or PUF
LPM
X+
V_DD
ADC_IN
ADC_IN
U
V_DD (10Ω)
V_DD through
50kΩ or 100kΩ
U
X-
GND
U
GND
GND
U
U
U
Y+
ADC_IN
V_DD
V_DD
V_DD
U
U
U
Y-
U
GND
U
ADC_IN
GND
GND
U
REF+
X+
Y+
Y+
Y+
U
U
U
REF-
X-
Y-
X-
X-
—
—
—
Note: The ADC input is fully differential with the negative input internally connected to GND. The MAX11800–MAX11803 control
access to the PUR, PUF, TDM, and LPM, which do not require setup procedures.
U indicates unconnected node.
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