TSC2003 datasheet(13/30 Pages) TI2

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TSC2003

SBAS162G

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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.

Differential reference mode always uses the supply voltage,

through the drivers, as the reference voltage for the A/D

converter. VREF cannot be used as the reference voltage in

differential mode.

It is possible to use a high-precision reference on VREF 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.

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 I2C bus is adequate for the touch screen to settle.

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 (VBE)

has a well-defined characteristic versus temperature. The

temperature can be predicted in applications by knowing the

°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;

A/D

Converter

MUX

X+

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 1n(N)

∆V

(1)

where:

∆

V(I ) – V(I ) (in mV)

2.573 V K/mV

2.573

V(mV) – 273 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.

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