Touch Screen Controller

AD7877


Rev. A
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FEATURES
4-wire touch screen interface
LCD noise reduction feature (STOPACQ pin)
Automatic conversion sequencer and timer
User-programmable conversion parameters
On-chip temperature sensor: −40°C to +85°C
On-chip 2.5 V reference
On-chip 8-bit DAC
3 auxiliary analog inputs
1 dedicated and 3 optional GPIOs
2 direct battery measurement channels (0.5 V to 5 V)
3 interrupt outputs
Touch-pressure measurement
Wake up on touch function
Specified throughput rate of 125 kSPS
Single supply, VCC of 2.7 V to 5.25 V
Separate VDRIVE level for serial interface
Shutdown mode: 1 µA maximum
32-lead LFCSP 5 mm x 5 mm package

APPLICATIONS
Personal digital assistants
Smart hand-held devices
Touch screen monitors
Point-of-sale terminals
Medical devices
Cell phones
Pagers

FUNCTIONAL BLOCK DIAGRAM
19
DIN
26
DCLK
27
DOUT
28
V
DRIVE
18
CS
23
DAV
CONTROL LOGIC AND SERIAL PORT
DAC
REGISTER
CONTROL
REGISTERS
GPIO
REGISTERS
ALERT STATUS/
MASK REGISTER
LIMIT
REGISTERS
LIMIT
COMPARATOR
RESULTS
REGISTERS
SEQUENCER
8-BIT
DAC
29ARNG
30AOUT
4AUX3/GPIO3
5AUX2/GPIO2
6AUX1/GPIO1
31V
REF
PEN INTERRUPT
AND WAKE-UP
ON TOUCH
17 PENIRQ
ALERT
LOGIC
22 ALERT
STOP
ACQ
LOGIC
20 STOPACQ
14 AGND
15 DGND
21 GPIO4
2.5V
REF
BUF
TO
GPIO1-3
ADC DATA
12-BIT SUCCESSIVE
APPROXIMATION ADC
WITH TRACK-AND-HOLD
CLOCK9 TO 1
I/P
MUX
TEMPERATURE
SENSOR
2BAT2
BATTERY
MONITOR
3BAT1
BATTERY
MONITOR
11Y–
13Y+
10X–
12X+
7
V
CC
REF–
IN
REF+
DUAL 3-1
MUX
X– Y– GND X+ Y+ V
REF
AD7877
03796-001
Figure 1.
GENERAL DESCRIPTION
The AD7877 is a 12-bit successive approximation ADC with a
synchronous serial interface and low on resistance switches for
driving touch screens. The AD7877 operates from a single 2.7 V
to 5.25 V power supply (functional operation to 2.2V), and
features throughput rates of 125 kSPS. The AD7877 features
direct battery measurement on two inputs, temperature and
touch-pressure measurement.
The AD7877 also has an on-board reference of 2.5 V. When not
in use, it can be shut down to conserve power. An external
reference can also be applied and can be varied from 1 V to
+VCC, while the analog input range is from 0 V to VREF. The
device includes a shutdown mode, which reduces its current
consumption to less than 1 µA.
To reduce the effects of noise from LCDs, the acquisition phase
of the on-board ADC can be controlled via the STOPACQ pin.
User-programmable conversion controls include variable
acquisition time and first conversion delay. Up to 16 averages
can be taken per conversion. There is also an on-board DAC for
LCD backlight or contrast control. The AD7877 can run in
either slave or master mode, using a conversion sequencer and
timer. It is ideal for battery-powered systems such as personal
digital assistants with resistive touch screens and other portable
equipment.
The part is available in a 32-lead lead frame chip scale package
(LFCSP).