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MXB7846EEE+MAXIMN/a1000avai2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor
MXB7846EEE+T |MXB7846EEETMAXIMN/a1908avai2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor


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MXB7846EEE+-MXB7846EEE+T
2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
Pin Configuration

19-2436; Rev 2; 1/08
General Description

The MXB7846 is an industry-standard 4-wire touch-
screen controller. It contains a 12-bit sampling analog-
to-digital converter (ADC) with a synchronous serial
interface and low on-resistance switches for driving
resistive touch screens. The MXB7846 uses an internal
+2.5V reference or an external reference. The
MXB7846 can make absolute or ratiometric measure-
ments. In addition, this device has an on-chip tempera-
ture sensor, a battery-monitoring channel, and has the
ability to perform touch-pressure measurements without
external components. The MXB7846 has one auxiliary
ADC input. All analog inputs are fully ESD protected,
eliminating the need for external TransZorb™ devices.
The MXB7846 is guaranteed to operate with a supply
voltage down to +2.375V when used with an external
reference or +2.7V with an internal reference. In shut-
down mode, the typical power consumption is reduced
to under 0.5µW, while the typical power consumption at
125ksps throughput and a +2.7V supply is 650µW.
Low-power operation makes the MXB7846 ideal for bat-
tery-operated systems, such as personal digital assis-
tants with resistive touch screens and other portable
equipment. The MXB7846 is available in 16-pin QSOP
and TSSOP packages, and is guaranteed over the
-40°C to +85°C temperature range.
Applications

Personal Digital Assistants
Portable Instruments
Point-of-Sales Terminals
Pagers
Touch-Screen Monitors
Cellular Phones
Features
ESD-Protected ADC Inputs
±15kV IEC 61000-4-2 Air-Gap Discharge
±8kV IEC 61000-4-2 Contact Discharge
Pin Compatible with MXB7843+2.375V to +5.25V Single SupplyInternal +2.5V ReferenceDirect Battery Measurement (0 to 6V)On-Chip Temperature MeasurementTouch-Pressure Measurement 4-Wire Touch-Screen InterfaceRatiometric ConversionSPI™/QSPI™, 3-Wire Serial Interface Programmable 8-/12-Bit ResolutionAuxiliary Analog InputAutomatic Shutdown Between ConversionsLow Power (External Reference)
270µA at 125ksps
115µA at 50ksps
25µA at 10ksps
5µA at 1ksps
2µA Shutdown Current

VDDDCLK
DIN
BUSY
DOUT
PENIRQ
VDD
REF
TOP VIEW
MXB7846
QSOP/TSSOP

GND
BAT
AUX
Ordering Information
PARTTEMP RANGEPIN-
PACKAGE
PKG
CODE

MXB7846EEE-40°C to +85°C16 QSOPE16-6
MXB7846EUE-40°C to +85°C16 TSSOPU16-1
TRANSZORBis a trademark of Vishay Intertechnology, Inc.
SPI/QSPI are trademarks of Motorola, Inc.
Typical Application Circuit appears at end of data sheet.
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
ABSOLUTE MAXIMUM RATINGS

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
VDD, VBAT, DIN, CS, DCLK to GND........................-0.3V to +6V
Digital Outputs to GND...............................-0.3V to (VDD+ 0.3V)
VREF, X+, X-, Y+, Y-, AUX to GND..............-0.3V to (VDD+ 0.3V)
Maximum Current into Any Pin.........................................±50mA
Maximum ESD per IEC-61000-4-2 (per MIL STD-883 HBM)
X+, X-, Y+, Y-, VBAT, AUX......................................15kV (4kV)
All Other Pins..........................................................2kV (500V)
Continuous Power Dissipation (TA= +70°C)
16-Pin QSOP (derate 8.30mW/°C above +70°C).........667mW
16-Pin TSSOP (derate 5.70mW/°C above +70°C).......456mW
Operating Temperature Range...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
ELECTRICAL CHARACTERISTICS

(VDD= 2.7V to 3.6V, VREF= 2.5V, fDCLK= 2MHz (50% duty cycle), fSAMPLE= 125kHz, 12-bit mode, 0.1µF capacitor at REF, TA=
TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETERSYM B O L CONDITIONSMINTYPMAXUNITS
DC ACCURACY (Note 1)

Resolution12Bits
No Missing Codes1112Bits
Relative AccuracyINL(Note 2)±1±2LSB
Differential NonlinearityDNL±1LSB
Offset Error±6LSB
Gain Error(Note 3)±4LSB
NoiseIncluding internal reference70µVRMS
CONVERSION RATE

Conversion TimetCONV12 clock cycles (Note 4)6µs
Track/Hold Acquisition TimetACQ3 clock cycles1.5µs
Throughput RatefSAMPLE16 clock conversion125kHz
Multiplexer Settling Time500ns
Aperture Delay30ns
Aperture Jitter100p s
Channel-to-Channel IsolationVIN = 2.5VP-P at 50kHz100dB
Serial Clock FrequencyfDCLK0.12.0MHz
Duty Cycle4060%
ANALOG INPUT (X+, X-, Y+, Y-, AUX)

Input Voltage Range0VREFV
Input Capacitance25pF
Input Leakage CurrentOn/off leakage, VIN = 0 to VDD±0.1±1µA
SWITCH DRIVERS

Y+, X+7On-Resistance (Note 5)Y-, X-9Ω
INTERNAL REFERENCE

Reference Output VoltageVREFVDD = 2.7V to 5.25V, TA = +25°C2.452.502.55V
REF Output TempcoTCVREF50ppm°/C
REF Short-Circuit Current18mA
REF Output Impedance250Ω
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
ELECTRICAL CHARACTERISTICS (continued)

(VDD= 2.7V to 3.6V, VREF= 2.5V, fDCLK= 2MHz (50% duty cycle), fSAMPLE= 125kHz, 12-bit mode, 0.1µF capacitor at REF, TA=
TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETERSYM B O L CONDITIONSMINTYPMAXUNITS
EXTERNAL REFERENCE (Internal reference disabled, reference applied to REF)

Reference Input Voltage Range(Note 7)1VDDV
Input Resistance1GΩ
fSAMPLE = 125kHz1340
fSAMPLE = 12.5kHz2.5µAInput Current
fDCLK = 0±3
BATTERY MONITOR (BAT)

Input Voltage Range06V
Input ResistanceDuring acquisition10kΩ
VREF = 2.5V±2AccuracyInternal reference±3%
TEMPERATURE MEASUREMENT

Differential method (Note 8)1.6°CResolutionSingle-conversion method0.3°C
Differential method (Note 8)±2°CAccuracySingle-conversion method±3°C
DIGITAL INPUTS (DCLK, CS, DIN)

Input High VoltageVIHVDD ✕ 0.7V
Input Low VoltageVIL0.8V
Input HysteresisVHYST100mV
Input Leakage CurrentIIN±1µA
Input CapacitanceCIN15pF
DIGITAL OUTPUT (DOUT, BUSY)

Output Voltage LowVOLISINK = 250µA0.4V
Output Voltage HighVOHISOURCE = 250µAVDD - 0.5V
PENIRQ Output Low VoltageVOL50kΩ pullup to VDD0.8V
Three-State Leakage CurrentILCS = VDD1±10µA
Three-State Output CapacitanceCOUTCS = VDD15pF
POWER REQUIREMENTS

External reference2.3755.250Supply VoltageVDDInternal reference2.705.25V
fSAMPLE = 125ksps270650
fSAMPLE = 12.5ksps220External
referencefSAMPLE = 0150
fSAMPLE = 125ksps780950
fSAMPLE = 12.5ksps720
Supply CurrentIDD
Internal
referencefSAMPLE = 0650
Shutdown Supply CurrentISHDNDCLK = CS = VDD3µA
Power-Supply Rejection RatioPSRRVDD = 2.7V to 3.6V full scale70dB
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
Note 1:
Tested at VDD= 2.7V.
Note 2:
Relative accuracy is the deviation of the analog value at any code from its theoretical value after the full-scale range has
been calibrated.
Note 3:
Offset nulled.
Note 4:
Conversion time is defined as the number of clock cycles multiplied by the clock period; clock has 50% duty cycle.
Note 5:
Resistance measured from the source to drain of the switch.
Note 6:
External load should not change during conversion for specified accuracy.
Note 7:
ADC performance is limited by the conversion noise floor, typically 300µVP-P. An external reference below 2.5V can com-
promise the ADC performance.
Note 8:
Difference between Temp0 and Temp1. No calibration necessary.
TIMING CHARACTERISTICS (Figure 1)

(VDD= 2.7V to 3.6V, VREF= 2.5V, fDCLK= 2MHz (50% duty cycle), fSAMPLE= 125kHz, 12-bit mode, 0.1µF capacitor at REF, TA=
TMINto TMAX, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETERSYM B O L CONDITIONSMINTYPMAXUNITS
TIMING CHARACTERISTICS (Figure 1)

Acquisition TimetACQ1.5µs
DCLK Clock PeriodtCP500ns
DCLK Pulse Width HightCH200ns
DCLK Pulse Width LowtCL200ns
DIN-to-DCLK Setup TimetDS100ns
DIN-to-DCLK Hold TimetDH0ns
CS Fall-to-DCLK Rise Setup TimetCSS100ns
CS Rise-to-DCLK Rise IgnoretCSH0ns
DCLK Falling-to-DOUT ValidtDOCLOAD = 50pF200ns
CS Rise-to-DOUT DisabletTRCLOAD = 50pF200ns
CS Fall-to-DOUT EnabletDVCLOAD = 50pF200ns
DCLK Falling-to-BUSY RisingtBD200ns
CS Falling-to-BUSY EnabletBDV200ns
CS Rise-to-BUSY DisabletBTR200ns
Typical Operating Characteristics
(VDD= 2.7V, VREF= 2.5VEXTERNAL, fDCLK= 2MHz, fSAMPLE= 125kHz, CLOAD= 50pF, 0.1µF capacitor at REF, TA= +25°C, unless
otherwise noted.)
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
INTEGRAL NONLINEARITY
vs. DIGITAL OUTPUT CODE

MXB7846 toc01
OUTPUT CODE
INL (LSB)
DIFFERENTIAL NONLINEARITY
vs. DIGITAL OUTPUT CODE
MXB7846 toc02
OUTPUT CODE
DNL (LSB)
-1.0
CHANGE IN OFFSET ERROR
vs. SUPPLY VOLTAGE
MXB7846 toc04
SUPPLY VOLTAGE (V)
OFFSET ERROR (LSB)
CHANGE IN OFFSET ERROR
vs. TEMPERATURE
MXB7846 toc05
TEMPERATURE(°C)
OFFSET ERROR FROM +25
C (LSB)5035205-10-25
CHANGE IN GAIN ERROR
vs. SUPPLY VOLTAGE
MXB7846 toc07
SUPPLY VOLTAGE (V)
GAIN ERROR (LSB)
CHANGE IN GAIN ERROR
vs. TEMPERATURE
MXB7846 toc08
TEMPERATURE(°C)
GAIN ERROR FROM +25
C (LSB)5035205-10-25
SWITCH ON-RESISTANCE vs. SUPPLY VOLTAGE
(X+, Y+ : +VDD TO PIN; X-, Y- : TO GND)
MXB7846 toc03
SUPPLY VOLTAGE (V)
2.55.5X+
SWITCH ON-RESISTANCE vs. TEMPERATURE
(X+, Y+ : +VDD TO PIN; X-, Y- : PIN TO GND)

MXB7846 toc06
TEMPERATURE (°C)205-10-25
INTERNAL REFERENCE
vs. SUPPLY VOLTAGE
MXB7846 toc09
SUPPLY VOLTAGE (V)
INTERNAL REFERENCE (V)
CL = 0.1μf
Typical Operating Characteristics (continued)
(VDD= 2.7V, VREF= 2.5VEXTERNAL, fDCLK= 2MHz, fSAMPLE= 125kHz, CLOAD= 50pF, 0.1µF capacitor at REF, TA= +25°C, unless
otherwise noted.)
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
INTERNAL REFERENCE VOLTAGE
vs. TEMPERATURE

MXB7846 toc10
TEMPERATURE (°C)
INTERNAL REFERENCE VOLTAGE (V)655035205-10-25
VDD = 2.7V
CL = 0.1μF
INTERNAL VOLTAGE REFERENCE
vs. TURN-ON TIME

MXB7846 toc11a
TURN-ON TIME (μs)
INTERNAL VOLTAGE REFERENCE (V)
CL = 1μF
(1060μs) 12-BIT SETTLING
INTERNAL VOLTAGE REFERENCE
vs. TURN-ON TIME

MXB7846 toc11b
TURN-ON TIME (μs)
INTERNAL VOLTAGE REFERENCE (V)30252015105
NO CAPACITOR
(30μs) 12-BIT SETTLING
REFERENCE CURRENT
vs. SUPPLY VOLTAGE

MXB7846 toc12
SUPPLY VOLTAGE (V)
REFERENCE CURRENT (
CL = 0.1μF
fSAMPLE = 125kHz
EXTERNAL REFERENCE
REFERENCE CURRENT vs. TEMPERATURE

MXB7846 toc13
TEMPERATURE (°C)
REFERENCE CURRENT (5035205-10-25
VDD = 2.7V
CL = 0.1μF
fSAMPLE = 125kHz
EXTERNAL REFERENCE
REFERENCE CURRENT vs. SAMPLE RATE

MXB7846 toc14
SAMPLE RATE (kHz)
REFERENCE CURRENT (
EXTERNAL REFERENCE
TEMP DIODE VOLTAGE
vs. TEMPERATURE

MXB7846 toc15
TEMPERATURE (°C)
TEMP DIODE VOLTAGE (V)653550-10520-25
TEMP1
TEMP2
TEMP0 DIODE VOLTAGE
vs. SUPPLY VOLTAGE

MXB7846 toc16
SUPPLY VOLTAGE (V)
TEMP0 DIODE VOLTAGE (mV)
TEMP0
TEMP1 DIODE VOLTAGE
vs. SUPPLY VOLTAGE

MXB7846 toc17
SUPPLY VOLTAGE (V)
TEMP1 DIODE VOLTAGE (mV)
TEMP1
Typical Operating Characteristics (continued)
(VDD= 2.7V, VREF= 2.5VEXTERNAL, fDCLK= 2MHz, fSAMPLE= 125kHz, CLOAD= 50pF, 0.1µF capacitor at REF, TA= +25°C, unless
otherwise noted.)
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
SUPPLY CURRENT
vs. SUPPLY VOLTAGE

MXB7846 toc18
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (
fSAMPLE = 12.5kHz
SUPPLY CURRENT vs. TEMPERATURE

MXB7846 toc19
TEMPERATURE (°C)
SUPPLY CURRENT (50-25-1052035
fSAMPLE = 125kHz
VDD = 2.7V
SUPPLY CURRENT vs. SAMPLE RATE

MXB7846 toc20
SAMPLE RATE (kHz)
SUPPLY CURRENT (
VDD = 2.7V
VREF = 2.5V
SHUTDOWN CURRENT
vs. SUPPLY VOLTAGE

MXB7846 toc21
SUPPLY VOLTAGE (V)
SHUTDOWN CURRENT (nA)
DCLK = CS = VDD
SHUTDOWN CURRENT vs. TEMPERATURE

MXB7846 toc22
TEMPERATURE (°C)
SHUTDOWN CURRENT (nA)5035205-10-25
DCLK = CS = VDD = 3V
MAXIMUM SAMPLE RATE
vs. SUPPLY VOLTAGE

MXB7846 toc23
SUPPLY VOLTAGE (V)
SAMPLE RATE (kHz)
2.05.5
Pin Description
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

DCLK
DIN
DOUT
BUSY
tBDV
tDV
tCSStCL
tCH
tDS
tDH
tCP
tDO
tBD
tTR
tBTR
tCSH
PINNAMEFUNCTION

1VDDPositive Supply Voltage. Connect to pin 10.X+X+ Position Input, ADC Input Channel 1Y+Y+ Position Input, ADC Input Channel 2X-X- Position InputY-Y- Position InputGNDGroundBATBattery Monitoring Inputs; ADC Input Channel 3AUXAuxiliary Input to ADC; ADC Input Channel 4REF
Voltage Reference Output/Input. Reference voltage for analog-to-digital conversion. In internal
reference mode, the reference buffer provides a 2.50V nominal output. In external reference mode,
apply a reference voltage between 1V and VDD. Bypass REF to GND with a 0.1µF capacitor.VDDPositive Supply Voltage, +2.375V (2.70V) to +5.25V. External (internal) reference. Bypass with a 1µF
capacitor. Connect to pin 1.PENIRQPen Interrupt Output. Open anode output. 10kΩ to 100kΩ pullup resistor required to VDD.DOUTSerial Data Output. Data changes state on the falling edge of DCLK. High impedance when CS is
HIGH.BUSYBusy Output. BUSY pulses high for one clock period before the MSB decision. High impedance when
CS is HIGH.DINSerial Data Input. Data clocked in on the rising edge of DCLK.CSActive-Low Chip Select. Data is only clocked into DIN when CS is low. When CS is HIGH, DOUT and
BUSY are high impedance.DCLKSerial Clock Input. Clocks data in and out of the serial interface and sets the conversion speed (duty
cycle must be 40% to 60%).
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
Detailed Description

The MXB7846 uses a successive-approximation conver-
sion technique to convert analog signals to a 12-bit digi-
tal output. An SPI/QSPI/MICROWIRE™-compatible serial
interface provides easy communication to a micro-
processor (µP). It features an internal 2.5V reference, an
on-chip temperature sensor, a battery monitor, and a
4-wire touch-screen interface (Functional Diagram).
Analog Inputs

Figure 2 shows a block diagram of the analog input sec-
tion that includes the input multiplexer of the MXB7846,
the differential signal inputs of the ADC, and the differ-
ential reference inputs of the ADC. The input multiplexer
switches between X+, X-, Y+, Y-, AUX, BAT, and the
internal temperature sensor.
In single-ended mode, conversions are performed using
REF as the reference. In differential mode, ratiometric
conversions are performed with REF+ connected to X+ or
Y+, and REF- connected to X- or Y-. Configure the refer-
ence and switching matrix according to Tables 1 and 2.
During the acquisition interval, the selected channel
charges the sampling capacitance. The acquisition
interval starts on the fifth falling clock edge and ends
on the eighth falling clock edge.
The time required for the T/H to acquire an input signal
is a function of how quickly its input capacitance is
charged. If the input signal’s source impedance is high,
the acquisition time lengthens, and more time must be
allowed between conversions. The acquisition time
(tACQ) is the maximum time the device takes to acquire
the input signal to 12-bit accuracy. Calculate tACQwith
the following equation:
where RIN= 2kΩand RSis the source impedance of
the input signal.
Source impedances below 1kΩdo not significantly affect
the ADC’s performance. Accommodate higher source
impedances by either slowing down DCLK or by placing
a 1µF capacitor between the analog input and GND. tRRpFACQSIN . =×+()×8425
+VDDPENIRQ
TEMP1
VREF
A2–A0
(SHOWN 001B)
SER/DFR
(SHOWN HIGH)
GND12-BIT ADC
AUX
VBAT
REF+
REF-
REF ON/OFF
7.5kΩ
2.5kΩ
2.5V
REFERENCE
BATTERY
+IN
-IN
TEMP0
MXB7846
Figure 2. Equivalent Input Circuit
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

6-TO-1
MUX
VDD
REF
12-BIT ADC
PENIRQ
BAT
AUX
SERIAL
DATA
INTERFACE
DIN
DCLK
BUSY
PENIRQ
DOUT
BATTERY
MONITOR
TEMPERATURE
SENSOR
2.5V
REFERENCE
Functional DiagramA1A0MEASUREMENTADC INPUT CONNECTIONDRIVERS ON
00Temp0Temp0—01Y positionX+Y+, Y-10BATBAT—11Z1X+X-, Y+00Z2Y-X-, Y+01X- positionY+X-, X+10AUXAUX—11Temp1Temp1—
Table 1. Input Configuration, Single-Ended Reference Mode (SER/DFRHIGH)A1A0ADC +REF
CONNECTION TO
ADC -REF
CONNECTION TO
ADC INPUT
CONNECTION TO
MEASUREMENT
PERFORMEDDRIVER ON
01Y+Y-X+Y positionY+, Y-11Y+X-X+Z1 positionY+, X-00Y+X-Y-Z2 positionY+, X-01X+X-Y+X positionX+, X-
Table 2. Input Configuration, Differential Reference Mode (SER/DFRLOW)
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
Input Bandwidth and Anti-Aliasing

The ADCs input tracking circuitry has a 25MHz small-
signal bandwidth, so it is possible to digitize high-
speed transient events. To avoid high-frequency sig-
nals being aliased into the frequency band of interest,
anti-alias filtering is recommended.
Analog Input Protection

Internal protection diodes, which clamp the analog input
to VDDand GND, allow the analog input pins to swing
from GND - 0.3V to VDD + 0.3V without damage. Analog
inputs must not exceed VDDby more than 50mV or be
lower than GND by more than 50mV for accurate con-
version. If an off-channel analog input voltage exceeds
the supplies, limit the input current to 50mA. The analog
input pins are ESD protected to ±8kV using the Contact
Discharge method and ±15kV using the Air-Gap
method specified in IEC 61000-4-2.
Touch-Screen Conversion

The MXB7846 provides two conversion methods—differ-
ential and single ended. The SER/DFRbit in the control
word selects either mode. A logic 1 selects a single-
ended conversion, while a logic 0 selects a differential
conversion.
Differential vs. Single Ended

Changes in operating conditions can degrade the accu-
racy and repeatability of touch-screen measurements.
Therefore, the conversion results representing X and Y
coordinates may be incorrect. For example, in single-
ended measurement mode, variation in the touch-screen
driver voltage drops results in incorrect input reading.
Differential mode minimizes these errors.
Single-Ended Mode

Figure 3 shows the switching matrix configuration for
Y-coordinate measurement in single-ended mode. The
MXB7846 measures the position of the pointing device
by connecting X+ to IN+ of the ADC, enabling Y+ and
Y- drivers, and digitizing the voltage on X+. The ADC
performs a conversion with REF+ = REF and REF- =
GND. In single-ended measurement mode, the bias to
the touch screen can be turned off after the acquisition
to save power. The on-resistance of the X and Y drivers
results in a gain error in single-ended measurement
mode. Touch-screen resistance ranges from 200Ωto
900Ω(depending on the manufacturer), whereas the
on-resistance of the X and Y drivers is 8Ω(typ). Limit
the touch-screen current to less than 50mA by using a
touch screen with a resistance higher than 100Ω. The
resistive-divider created by the touch screen and the
on-resistance of the X and Y drivers result in both an
offset and a gain shift. Also, the on-resistance of the X
and Y drivers does not track the resistance of the touch
screen over temperature and supply. This results in fur-
ther measurement errors.
Differential Measurement Mode

Figure 4 shows the switching matrix configuration for
Y-coordinate measurement. The REF+ and REF- inputs
are connected directly to the Y+ and Y- pins, respec-
tively. Differential mode uses the voltage at the Y+ pin
as the REF+ voltage and voltage at the Y- pin as REF-
voltage. This conversion is ratiometric and independent
of the voltage drop across the drivers and variation in
the touch-screen resistance. In differential mode, the
touch screen remains biased during the acquisition and
conversion process. This results in additional supply
current and power dissipation during conversion when
compared to the absolute measurement mode.
PEN Interrupt Request (PENIRQ)

Figure 5 shows the block diagram for the PENIRQfunc-
tion. When used, PENIRQrequires a 10kΩto 100kΩ
pullup to +VDD. If enabled, PENIRQgoes low whenever
the touch screen is touched. The PENIRQoutput can
be used to initiate an interrupt to the microprocessor,
which can write a control word to the MXB7846 to start
a conversion.
Figure 6 shows the timing diagram for the PENIRQpin
function. The diagram shows that once the screen is
touched while CSis high, the PENIRQoutput goes low
after a time period indicated by tTOUCH. The tTOUCH
value changes for different touch-screen parasitic
capacitance and resistance. The microprocessor
receives this interrupt and pulls CSlow to initiate a con-
version. At this instant, the PENIRQpin should be
masked, as transitions can occur due to a selected
input channel or the conversion mode. The PENIRQpin
functionality becomes valid when either the last data bit
is clocked out, or CSis pulled high.
Touch-Pressure Measurement

The MXB7846 provides two methods for measuring the
pressure applied to the touch screen (Figure 7). By
measuring RTOUCH, it is possible to differentiate
between a finger or stylus in contact with the touch
screen. Although 8-bit resolution is typically sufficient,
the following calculations use 12-bit resolution demon-
strating the maximum precision of the MXB7846.
MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor

REF
VDD
GNDREF+
REF-
+IN
-IN
12-BIT ADC
Figure 3. Single-Ended Y-Coordinate Measurement
VDD
GNDREF+
REF-
+IN
-IN
12-BIT ADC
Figure 4. Ratiometric Y-Coordinate Measurement
OPEN CIRCUIT
PENIRQ
ENABLE
PENIRQ
TOUCH SCREEN
+VDD
100kΩ
Figure 5. PENIRQFunctional Block Diagram
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