always a percentage of the external resistance, regardless of
how it changes in relation to the on-resistance of the internal
switches. Note that there is an important consideration regarding
power dissipation when using the ratiometric mode of operation
(see the Power Dissipation section for more details).
offers two modes of operation. The first mode requires
calibration at a known temperature, but only requires a single
reading to predict the ambient temperature. A diode is used
(turned on) during this measurement cycle. The voltage
across the diode is connected through the MUX for digitizing
the forward bias voltage by the ADC with an address of
A2 = 0, A1 = 0, and A0 = 0 (see Table I and Figure 6 for
details). This voltage is typically 600mV at +25°C with a 20µA
current through the diode. The absolute value of this diode
voltage can vary a few millivolts. However, the 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 pur-
poses by the user. The result is an equivalent temperature
measurement resolution of 0.3°C/LSB (in 12-bit mode).
As a final note about the differential reference mode, it must
be used with +VCC as the source of the +REF voltage and
cannot be used with VREF. It is possible to use a high-
precision reference on VREF and single-ended reference
mode for measurements which do not need to be ratiometric.
In some cases, it is possible to power the converter directly
from a precision reference. Most references can provide
enough power for the TSC2046, but might not be able to
supply enough current for the external load (such as a
resistive touch screen).
TOUCH SCREEN SETTLING
+VCC
In some applications, external capacitors may be required
across the touch screen for filtering noise picked up by the
touch screen (e.g., noise generated by the LCD panel or
backlight circuitry). These capacitors provide a low-pass filter
to reduce the noise, but cause a settling time requirement
when the panel is touched that typically shows up as a gain
error. There are several methods for minimizing or eliminating
this issue. The problem is the input and/or reference has not
settled to the final steady-state value prior to the ADC sampling
the input(s) and providing the digital output. Additionally, the
reference voltage may still be changing during the measure-
ment cycle. Option 1 is to stop or slow down the TSC2046
DCLK for the required touch screen settling time. This allows
the input and reference to have stable values for the Acquire
period (3 clock cycles of the TSC2046; see Figure 9). This
works for both the single-ended and the differential modes.
Option 2 is to operate the TSC2046 in the differential mode
only for the touch screen measurements and command the
TSC2046 to remain on (touch screen drivers ON) and not go
into power-down (PD0 = 1). Several conversions are made
depending on the settling time required and the TSC2046 data
rate. Once the required number of conversions have been
made, the processor commands the TSC2046 to go into its
power-down state on the last measurement. This process is
required for X-Position, Y-Position, and Z-Position measure-
ments. Option 3 is to operate in the 15 Clock-per-Conversion
mode, which overlaps the analog-to-digital conversions and
maintains the touch screen drivers on until commanded to stop
by the processor (see Figure 13).
TEMP0
TEMP1
MUX
ADC
FIGURE 6. Functional Block Diagram of Temperature Mea-
surement Mode.
The second mode does not require a test temperature calibra-
tion, but uses a two-measurement method to eliminate the
need for absolute temperature calibration and for achieving
2°C accuracy. This mode requires a second conversion with
an address of A2 = 1, A1 = 1, and A0 = 1, with a 91 times larger
current. The voltage difference between the first and second
conversion using 91 times the bias current is represented by
kT/q • ln (N), where N is the current ratio = 91,
k = Boltzmann’s constant (1.38054 • 10–23 electron volts/
degrees Kelvin), q = the electron charge (1.602189 • 10–19 C),
and T = the temperature in degrees Kelvin. This method can
provide improved absolute temperature measurement over
the first mode at the cost of less resolution (1.6°C/LSB). The
equation for solving for °K is:
TEMPERATURE MEASUREMENT
°K = q • ∆V/(k • ln (N))
where, ∆V = V (I91) – V (I1) (in mV)
(1)
In some applications, such as battery recharging, a measure-
ment of ambient temperature is required. The temperature
measurement technique used in the TSC2046 relies on the
characteristics of a semiconductor junction operating at a
fixed current level. The forward diode voltage (VBE) has a
well-defined characteristic versus temperature. The ambient
temperature can be predicted in applications by knowing the
+25°C value of the VBE voltage and then monitoring the delta
of that voltage as the temperature changes. The TSC2046
∴ °K = 2.573 °K/mV • ∆V
°C = 2.573 • ∆V(mV) – 273°K
NOTE: The bias current for each diode temperature mea-
surement is only on for 3 clock cycles (during the acquisition
mode) and, therefore, does not add any noticeable increase
in power, especially if the temperature measurement only
occurs occasionally.
TSC2046
SBAS265C
11
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