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Pobierz
Texas Instruments Incorporated
Data Acquisition
Using a touch-screen controller’s
auxiliary inputs
By Wendy Fang,
Precision Analog Applications, High-Performance Analog,
and Tony Chang,
Precision Analog Nyquist, High-Performance Analog
Introduction
Texas Instruments (TI) touch-screen controllers (TSCs),
including the ADS7843/45/46, TSC2046, and TSC2003/4/5/6/7,
have touch-screen input pins and one or more non-touch-
screen or auxiliary analog input pins, such as the battery-
voltage-monitoring pin (V
BAT
) of the TSC2046 or the AUX
pin of the TSC2007. These auxiliary inputs make it possible
to monitor the system’s battery level or other voltage signals
by sharing time with touch-screen inputs or using time
periods when the touch screen is not touched.
The auxiliary analog inputs of different TI TSCs may have
different input ranges and different levels of electrostatic-
discharge (ESD) protection, so certain requirements or
limitations should be considered when they are used. This
article discusses the general and specific features and limita-
tions of the TSC auxiliary (including battery-voltage) inputs.
Auxiliary analog inputs
Table 1 lists auxiliary analog input pins of TI’s current
TSCs. Inside a TSC, a MUX selects and connects one of
the analog inputs to the ADC via commands sent through
the SPI or I
2
C ports. Figure 1 shows an example.
Auxiliary input mode
TI’s TSCs can be operated in either differential or single-
ended (SE) input mode, but auxiliary analog inputs can be
measured and converted only in SE mode (see Figure 2).
Table 1. Auxiliary analog input(s) of TI TSC devices
AUXILIARY ANALOG INPUTS
TSC
V
REF
MAIN FEATURES
1
NUMBER OF INPUTS
PIN NAMES
ADS7843
2
IN3, IN4
External
4-wire, SPI, command-based
ADS7845
1
AUXIN
External
5-wire, SPI, command-based
ADS7846
2
V
BAT
, AUX
Internal
4-wire, SPI, command-based
TSC2046
2
V
BAT
, AUX
Internal
4-wire, SPI, command-based
V
BAT1
, IN1,
TSC2003
4
Internal
4-wire, I
2
C, command-based
V
BAT2
, IN2
4-wire, I
2
C, register-based
TSC2004
1
AUX
External
TSC2005
1
AUX
External
4-wire, SPI, register-based
TSC2006
1
AUX
External
4-wire, SPI, register-based
TSC2007
1
AUX
Shared with V
DD
4-wire, I
2
C, command-based
Figure 2. SE mode of a TSC
Figure 1. Block diagram of TSC’s internal
input circuit
V
REF
TSC
TSC
V
REF
X+
X–
Y+
Y–
Touch-
Screen
Driver’s
Interface
+REF
Analog
Input
+IN
SAR
ADC
SAR ADC
MUX
–
IN
Temp
– REF
AUX
GND
5
Analog Applications Journal
4Q 2007
www.ti.com/aaj
High-Performance Analog Products
Data Acquisition
Texas Instruments Incorporated
A TSC in differential mode does not need any reference,
since the signal’s driver is connected to +REF and –REF
directly. However, the reference voltage, V
REF
, is a must
for SE operation and thus is always needed when an auxil-
iary input, such as the AUX or V
BAT
, is measured.
The reference voltage can be provided to the TSC’s ADC
externally or internally for the ADS7846, TSC2046, and
TSC2003. When an external reference is used, these devices
can be powered up at the V
CC
pin with a supply voltage
between 2.2 and 5.25 V, and an external V
REF
ranging from
1.0 V to V
CC
can be provided. When the TSC’s internal
reference is used, the ADS7846, TSC2046, or TSC2003
should be powered up with a 2.7- to 5.25-V supply to
guarantee that the internal V
REF
will be around 2.5 V
(2.45 V ~ 2.55 V). Other TI TSCs do not have the built-in
voltage reference. In practical applications, one can simply
route the same TSC power supply to the reference.
Auxiliary-input voltage range
The signal range for an auxiliary input should always be
within 0 V to V
REF
. Signals beyond this range will saturate
the ADC and can even increase the temperature, damaging
the input circuitry.
The signal range for a battery-monitor input, say V
BAT
,
can greatly exceed the V
REF
level, since there is a voltage
divider with each V
BAT
pin (see Figure 3). The input
divider circuit limits the range of the signal to the ADC to
within 0 V to V
REF
. Figure 3 shows that the signal to the
ADC is only 25% of the signal at the V
BAT
pin.
As shown in Table 1, the ADS7846, TSC2046, and
TSC2003 have one or two battery-monitor input(s)
designed to monitor/measure signals of up to 6.0 V while
the device is powered with much lower voltage, down to
2.7 VDC.
TSC auxiliary-input features
The main features of TSC auxiliary (including battery)
inputs can be summarized as follows:
Auxiliary input signals can be measured only in SE mode.
A reference voltage, V
REF
, must be provided to the ADC
when an auxiliary input is measured.
The signal range of an auxiliary input must be within 0 V
to V
REF
.
The battery input, V
BAT
, can range from 0 V to as high
as 4
Figure 3. Voltage divider reduces input to ADC
DC/DC
Converter
Battery
+
+V
CC
ADC
7.5 k
V
BAT
2.5 k
Figure 4. Using AUX with external voltage
divider to monitor battery voltage
V
REF
TSC
R1
+
+REF
AUX
Battery
+IN
SAR ADC
R2
–
IN
–
REF
since the battery’s voltage is normally higher than that of
the TSC’s power supply (and thus that of the V
REF
).
A TSC auxiliary pin such as AUX can still be used to
measure/monitor the battery voltage if an external voltage
divider is added between the battery and the AUX, as
shown in Figure 4. R1 and R2 values should be selected
with wide enough margins to ensure that the signal at the
AUX pin is within 0 V to V
REF
. Note that the voltage
divider shown in Figure 4 consumes extra power since a
current of V
CC
/(R1 + R2) continuously drains the battery.
Larger resistance can reduce power consumption.
The V
BAT
input circuit of TI TSCs does not consume
extra power from the battery since there is a switcher
between the divider and the power (ground) (see Figure 3),
and the internal divider is powered up only during the
short period of battery measurement.
V
REF
, up to 6.0 V.
Special applications
The following discussion focuses on the special applica-
tions of the auxiliary analog input pins of TSCs.
Unused input pins
If the TSC’s analog input pins are not used, it is recom-
mended that they be connected directly to an analog
ground.
Using AUX input pin to monitor V
BAT
As Table 1 shows, several TI TSCs are not furnished with a
battery-voltage-monitoring pin (V
BAT
). A regular auxiliary
input cannot be used directly for monitoring the battery,
×
6
Analog Applications Journal
High-Performance Analog Products
www.ti.com/aaj
4Q 2007
Texas Instruments Incorporated
Data Acquisition
Figure 5. Using TSC AUX pin to monitor battery
with controllable switch
Figure 6. External ESD-protection circuit may
supply pseudo power to TSC
V
REF
V
CC
V
REF
TSC
TSC
R1
+
Battery
+REF
AUX
+REF
AUX
+IN
+IN
R2
SAR ADC
SAR ADC
–
IN
–
IN
– REF
–
REF
Analog
Input
Switch
Control
GND
Similarly, while a TSC’s AUX pin is used to monitor
battery voltage, adding a controllable switcher like that
shown in Figure 5 can reduce power consumption and
ensure disconnection of the signal from the AUX pin while
the TSC is powered down.
ESD protection and auxiliary-input requirements
during power down
It is very common in practical applications to power down
a TSC when it is not in use to reduce system power con-
sumption. A question then arises: Can the analog signal
stay connected to the TSC when the TSC’s power source
is removed?
Figure 6 shows one case in which the analog input
should be removed before the TSC is powered down. If
the analog input signal is active (AUX is nonzero) when
the TSC is powered down (V
CC
= 0), the analog signal may
be routed to the V
CC
through the ESD-protection diode
(see the red line in Figure 6). This may partially or fully
power up the TSC, depending upon the amplitude of the
input signal, consuming more power and possibly causing
the TSC to malfunction. Thus the circuit configuration in
Figure 6 should be avoided.
An alternative is to simply use ESD-protection circuitry
as shown in Figure 7. Under extremely large electrostatic
voltages, the circuit in Figure 7a may not be as efficient as
that in Figure 7b due to the limited current flow through
the ESD-protection diodes. On the other hand, the circuit
in Figure 7b is not symmetric and therefore does not pro-
vide equal protection for positive and negative ESD bursts.
Figure 7. ESD-protection circuits for TSC analog input pin
V
REF
V
REF
V
CC
V
CC
TSC
TSC
AUX
+REF
+REF
AUX
+IN
+IN
SAR ADC
SAR ADC
–
IN
–
IN
– REF
Analog
Input
– REF
Analog
Input
GND
GND
(a)
(b)
7
Analog Applications Journal
4Q 2007
www.ti.com/aaj
High-Performance Analog Products
Data Acquisition
Texas Instruments Incorporated
Figure 8. TSC2004/5/6/7 internal ESD protection
for AUX input
Figure 9. Switching off TSC2004/5/6/7 AUX
before TSC is powered down
Off
On
Off
TSC2007
V
CC
TSC
From a GPO at host
V
REF
+REF
On
AUX
+IN
AUX
SAR ADC
Off
–
IN
Analog
Input
– REF
GND
AGND
Some of TI’s new TSCs, including the TSC2004/5/6/7,
were designed with the enhanced, on-chip ESD protection
shown by the simplified circuit in Figure 8. Obviously, for
these devices, the input signal to the TSC’s AUX should be
removed before the TSC’s V
CC
power is shut down. An
example of removing the AUX signal is shown in Figure 9.
Special applications summary
An unused analog input pin should be connected
directly to the analog ground.
Pay attention to the divider’s resistance values and
power consumption when using an AUX to monitor the
battery voltage.
Before removing the TSC’s power, always use software
commands to disable the TSC’s auxiliary functions.
For the ADS7846, TSC2046, and TSC2003, the nonzero
signal to the AUX or V
BAT
pin can stay connected while
the TSC’s power source is removed.
For the TSC2004/5/6/7, the nonzero signal to AUX
should be removed from the analog input pin before the
TSC power is removed.
Reference
For more information related to this article, you can down-
load an Acrobat Reader file at www-s.ti.com/sc/techlit/
litnumber
and replace
“litnumber”
with the
TI Lit. #
for
the materials listed below.
Document Title TI Lit. #
1. Wendy Fang, “Operation Schemes of Touch Screen
Controllers,” Application Report . . . . . . . . . . . .
.slaa359
Related Web sites
dataconverter.ti.com
www.ti.com/sc/device/
partnumber
Replace
partnumber
with
ADS7843,
ADS7845,
ADS7846,
TSC2003,
TSC2004,
TSC2005,
TSC2006,
TSC2007,
or
TSC2046
8
Analog Applications Journal
High-Performance Analog Products
www.ti.com/aaj
4Q 2007
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Internet
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TI Semiconductor KnowledgeBase Home Page
support.ti.com/sc/knowledgebase
Product Information Centers
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