Differential reference mode always uses the supply voltage,
through the drivers, as the reference voltage for the A/D
converter. V
REF
cannot be used as the reference voltage in
differential mode.
It is possible to use a high-precision reference on V
REF
in
single-ended reference mode for measurements which do
not need to be ratiometric (i.e., battery voltage, temperature
measurement, etc.). In some cases, it could be possible to
power the converter directly from a precision reference. Most
references can provide enough power for the TSC2003, but
they might not be able to supply enough current for the
external load, such as a resistive touch screen.
the Temperature Coefficient (TC) of this voltage is very
consistent at –2.1mV/°C. During the final test of the end
product, the diode voltage would be stored at a known room
temperature, in memory, for calibration purposes by the user.
The result is an equivalent temperature measurement reso-
lution of 0.3°C/LSB.
X+
MUX
A/D
Converter
TOUCH SCREEN SETTLING
In some applications, external capacitors may be required
across the touch screen for filtering noise picked up by the
touch screen (i.e., noise generated by the LCD panel or
backlight circuitry). These capacitors will provide a low-pass
filter to reduce the noise, but they will also cause a settling
time requirement when the panel is touched. The settling
time will typically show up as a gain error. The problem is that
the input and/or reference has not settled to its final steady-
state value prior to the A/D converter sampling the input(s),
and providing the digital output. Additionally, the reference
voltage may still be changing during the measurement cycle.
To resolve these settling time problems, the TSC2003 can be
commanded to turn on the drivers only without performing a
conversion (see Table I). Time can then be allowed before
the command is issued to perform a conversion. Generally,
the time it takes to communicate the conversion command
over the I
2
C bus is adequate for the touch screen to settle.
Temperature Select
TEMP0
TEMP1
FIGURE 6. Functional Block Diagram of Temperature Mea-
surement Mode.
The second mode does not require a test temperature
calibration, but uses a two-measurement method to eliminate
the need for absolute temperature calibration and for achiev-
ing 2°C/LSB accuracy. This mode requires a second conver-
sion with an address of C3 = 0, C2 = 1, C1 = 0, and C0 = 0,
with an 91 times larger current. The voltage difference
between the first and second conversion using 91 times the
bias current will be represented by kT/q • 1n (N), where N is
the current ratio = 91, k = Boltzmann's constant (1.38054 •
10
–23
electrons volts/degrees Kelvin), q = the electron charge
(1.602189 • 10
–19
C), and T = the temperature in degrees
Kelvin. This mode can provide improved absolute tempera-
ture measurement over the first mode, but at the cost of less
resolution (1.6°C/LSB). The equation to solve for
°K
is:
°
K =
where:
∆
V
=
V(I
91
) – V(I
1
) (in mV)
∴
o
K
=
2.573
∆
V
o
K/mV
o
TEMPERATURE MEASUREMENT
In some applications, such as battery recharging, a measure-
ment of ambient temperature is required. The temperature
measurement technique used in the TSC2003 relies on the
characteristics of a semiconductor junction operating at a
fixed current level to provide a measurement of the tempera-
ture of the TSC2003 chip. The forward diode voltage (V
BE
)
has a well-defined characteristic versus temperature. The
temperature can be predicted in applications by knowing the
25°C value of the V
BE
voltage and then monitoring the delta
of that voltage as the temperature changes. The TSC2003
offers two modes of temperature measurement.
The first mode requires calibrations at a known temperature,
but only requires a single reading to predict the ambient
temperature. A diode is used during this measurement cycle.
The voltage across the diode is connected through the MUX
for digitizing the diode forward bias voltage by the A/D
converter with an address of C3 = 0, C2 = 0, C1 = 0, and
C0 = 0 (see Table I and Figure 6 for details). This voltage is
typically 600mV at +25°C, with a 20µA current through it. The
absolute value of this diode voltage can vary a few millivolts;
q
• ∆V
k
•
1n(N)
(1)
C
=
2.573
• ∆
V(mV) – 273
o
K
NOTE: The bias current for each diode temperature mea-
surement is only turned ON during the acquisition mode,
and, therefore, does not add any noticeable increase in
power, especially if the temperature measurement only oc-
curs occasionally.
12
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TSC2003
SBAS162D
TI [ TEXAS INSTRUMENTS ]
