AD7877 Datasheet, PDF (1/44 Pages) Analog Devices – Touch Screen Controller

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
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.
Rev. A
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Touch Screen Controller
AD7877
FUNCTIONAL BLOCK DIAGRAM
V_CC
7
X+ 12
X– 10
Y+ 13
Y– 11
AUX1/GPIO1 6
AUX2/GPIO2 5
AUX3/GPIO3 4
BAT1 3
AD7877
BATTERY
MONITOR
X– Y– GND X+ Y+ V_REF
DUAL 3-1
MUX
REF– REF+
9 TO 1 IN
12-BIT SUCCESSIVE
CLOCK
I/P
APPROXIMATION ADC
MUX
WITH TRACK-AND-HOLD
BAT2 2
BATTERY
MONITOR
SEQUENCER
RESULTS
REGISTERS
STOP
ACQ
LOGIC
LIMIT
COMPARATOR
20 STOPACQ
14 AGND
TEMPERATURE
SENSOR
V_REF 31
2.5V
REF
BUF
LIMIT
REGISTERS
ALERT STATUS/
MASK REGISTER
GPIO
REGISTERS
ALERT
LOGIC
AOUT 30
ARNG 29
8-BIT
DAC
DAC
REGISTER
CONTROL
REGISTERS
TO
GPIO1-3
CONTROL LOGIC AND SERIAL PORT
PEN INTERRUPT
AND WAKE-UP
ON TOUCH
15 DGND
22 ALERT
21 GPIO4
17 PENIRQ
18 19 23
26
27
28
CS DIN DAV DCLK DOUT V_DRIVE
Figure 1.
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).
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]
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