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ADS7846N Datasheet(PDF) 11 Page - Texas Instruments

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Part # ADS7846N
Description  TOUCH SCREEN CONTROLLER
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

ADS7846N Datasheet(HTML) 11 Page - Texas Instruments

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ADS7846
11
SBAS125H
www.ti.com
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 12).
TEMPERATURE MEASUREMENT
In some applications, such as battery recharging, a measure-
ment of ambient temperature is required. The temperature
measurement technique used in the ADS7846 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 V
BE voltage and then monitoring the delta
of that voltage as the temperature changes. The ADS7846
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. The PENIRQ
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 20
µA 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).
FIGURE 7. Battery Measurement Functional Block Diagram.
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 mea-
surement over the first mode at the cost of less resolution
(1.6
°C/LSB). The equation for solving for °K is:
°K = q • ∆V/(k • ln (N))
(1)
where,
∆V = V (I
91) – V (I1) (in mV)
∴ °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). Therefore, it does not add any noticeable increase in
power, especially if the temperature measurement only oc-
curs occasionally.
BATTERY MEASUREMENT
An added feature of the ADS7846 is the ability to monitor the
battery voltage on the other side of the voltage regulator (DC/DC
converter), as shown in Figure 7. The battery voltage can vary
from 0.5V to 6V, while maintaining the voltage to the ADS7846
at 2.7V, 3.3V, etc. The input voltage (VBAT) is divided down by
4 so that a 6.0V battery voltage is represented as 1.5V to the
ADC. This simplifies the multiplexer and control logic. In order
to minimize the power consumption, the divider is only on
during the sampling period when A2 = 0, A1 = 1, and A0 = 0
(see Table I for the relationship between the control bits and
configuration of the ADS7846).
+V
CC
V
BAT
7.5k
2.5k
DC/DC
Converter
Battery
0.5V
to
6.0V
0.125V to 1.5V
2.7V
+
FIGURE 6. Functional Block Diagram of Temperature Mea-
surement Mode.
ADC
MUX
PENIRQ
+V
CC
External
Pull-Up
X+
Temperature Select
TEMP0
TEMP1
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 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


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