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MXB7843EEE+-MXB7843EUE+
2.375V to 5.25V, 4-Wire Touch-Screen Controller
General DescriptionThe MXB7843 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 MXB7843 uses an external
reference. The MXB7843 can make absolute or ratio-
metric measurements. The MXB7843 has two auxiliary
ADC inputs. All analog inputs are fully ESD protected,
eliminating the need for external TransZorb™ devices.
The MXB7843 is guaranteed to operate with a single
2.375V to 5.25V supply voltage. In shutdown 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 MXB7843 ideal for bat-
tery-operated systems, such as personal digital assis-
tants with resistive touch screens and other portable
equipment. The MXB7843 is available in 16-pin QSOP
and TSSOP packages, and is guaranteed over the
-40°C to +85°C temperature range.
ApplicationsPersonal Digital Assistants
Portable Instruments
Point-of-Sales Terminals
Pagers
Touch-Screen Monitors
Cellular Phones
FeaturesESD-Protected ADC Inputs
±15kV IEC 61000-4-2 Air-Gap Discharge
±8kV IEC 61000-4-2 Contact DischargePin Compatible with MXB7846+2.375V to +5.25V Single Supply 4-Wire Touch-Screen InterfaceRatiometric ConversionSPI™/QSPI™, 3-Wire Serial Interface Programmable 8-/12-Bit ResolutionTwo Auxiliary Analog InputsAutomatic Shutdown Between ConversionsLow Power
270µA at 125ksps
115µA at 50ksps
25µA at 10ksps
5µA at 1ksps
2µA Shutdown Current
MXB7843
2.375V to 5.25V, 4-Wire Touch-Screen
ControllerVDDDCLK
DIN
BUSY
DOUT
PENIRQ
VDD
REF
TOP VIEW
MXB7843
QSOP/TSSOPGND
IN3
IN4
Pin Configuration
Ordering Information19-2435; Rev 1; 9/05
PARTTEMP RANGEPIN-PACKAGEMXB7843EEE-40°C to +85°C16 QSOP
MXB7843EUE-40°C to +85°C16 TSSOP
TransZorb is a trademark of Vishay Intertechnology, Inc.
SPI/QSPI are trademarks of Motorola, Inc.
Typical Application Circuit appears at end of data sheet.
MXB7843
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
ABSOLUTE MAXIMUM RATINGSStresses 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, DIN, CS, DCLK to GND...................................-0.3V to +6V
Digital Outputs to GND...............................-0.3V to (VDD+ 0.3V)
VREF, X+, X-, Y+, Y-, IN3, IN4 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-, IN3, IN4...........................................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
Noise70µVRMS
CONVERSION RATEConversion 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-, IN3, IN4)Input Voltage Range0VREFV
Input Capacitance25pF
Input Leakage CurrentOn/off-leakage, VIN = 0 to VDD±0.1±1µA
SWITCH DRIVERSY+, X+7On-Resistance (Note 5)Y-, X-9Ω
MXB7843
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
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
REFERENCE (Reference applied to REF)Reference Input Voltage Range(Note 6)1VDDV
Input Resistance5GΩ
fSAMPLE = 125kHz1340
fSAMPLE = 12.5kHz2.5Input Current
fDCLK = 0±3
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 REQUIREMENTSSupply VoltageVDD2.3755.250V
fSAMPLE = 125ksps270650
fSAMPLE = 12.5ksps220Supply CurrentIDD
fSAMPLE = 0150
Shutdown Supply CurrentISHDNDCLK = CS = VDD3µA
Power-Supply Rejection RatioPSRRVDD = 2.7V to 3.6V full scale70dB
MXB7843
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
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:ADC performance is limited by the conversion noise floor, typically 300µVP-P. An external reference below 2.5V can com-
promise the ADC performance.
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
MXB7843
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
INTEGRAL NONLINEARITY
vs. DIGITAL OUTPUT CODEMXB7843 toc01
OUTPUT CODE
INL (LSB)
DIFFERENTIAL NONLINEARITY
vs. DIGITAL OUTPUT CODE
MXB7843 toc02
OUTPUT CODE
DNL (LSB)
-1.0
CHANGE IN OFFSET ERROR
vs. SUPPLY VOLTAGE
MXB7843 toc04
SUPPLY VOLTAGE (V)
OFFSET ERROR (LSB)
CHANGE IN OFFSET ERROR
vs. TEMPERATURE
MXB7843 toc05
TEMPERATURE (°C)
OFFSET ERROR FROM
C (LSB)5035205-10-25
CHANGE IN GAIN ERROR
vs. SUPPLY VOLTAGE
MXB7843 toc07
SUPPLY VOLTAGE (V)
GAIN ERROR (LSB)
CHANGE IN GAIN ERROR
vs. TEMPERATURE
MXB7843 toc08
TEMPERATURE (°C)
GAIN ERROR FROM
°C (LSB)5035205-10-25
SWITCH ON-RESISTANCE vs. SUPPLY VOLTAGE
(X+, Y+ : + VDD TO PIN; X-, Y- : TO GND)
MXB7843 toc03
SUPPLY VOLTAGE (V)
2.55.5X+
SWITCH ON-RESISTANCE vs. TEMPERATURE
(X+, Y+ : + VDD TO PIN; X-, Y- : PIN TO GND)MXB7843 toc06
TEMPERATURE (°C)205-10-25
Typical Operating Characteristics
(VDD= 2.7V, VREF= 2.5V, fDCLK= 2MHz, fSAMPLE= 125kHz, CLOAD= 50pF, 0.1µF capacitor at REF, TA= +25°C, unless otherwise
noted.)
MXB7843
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
Typical Operating Characteristics (continued)(VDD= 2.7V, VREF= 2.5V, fDCLK= 2MHz, fSAMPLE= 125kHz, CLOAD= 50pF, 0.1µF capacitor at REF, TA= +25°C, unless otherwise
noted.)
REFERENCE CURRENT
vs. SUPPLY VOLTAGEMXB7843 toc12
SUPPLY VOLTAGE (V)
REFERENCE CURRENT (
CL = 0.1μF
fSAMPLE = 125kHz
REFERENCE CURRENT vs. TEMPERATUREMXB7843 toc13
TEMPERATURE (°C)
REFERENCE CURRENT (5035205-10-25
VDD = 2.7V
CL = 0.1μF
fSAMPLE = 125kHz
REFERENCE CURRENT vs. SAMPLE RATEMXB7843 toc14
SAMPLE RATE (kHz)
REFERENCE CURRENT (
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MXB7843 toc18
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (
fSAMPLE = 12.5kHz
SUPPLY CURRENT vs. TEMPERATUREMXB7843 toc19
TEMPERATURE (°C)
SUPPLY CURRENT (50-25-1052035
fSAMPLE = 125kHz
VDD = 2.7V
SUPPLY CURRENT vs. SAMPLE RATEMXB7843 toc20
SAMPLE RATE (kHz)
SUPPLY CURRENT (
VDD = 2.7V
VREF = 2.5V
SHUTDOWN CURRENT
vs. SUPPLY VOLTAGEMXB7843 toc21
SUPPLY VOLTAGE (V)
SHUTDOWN CURRENT (nA)
DLCK = CS = VDD
SHUTDOWN CURRENT vs. TEMPERATUREMXB7843 toc22
TEMPERATURE (°C)
SHUTDOWN CURRENT (nA)5035205-10-25
DCLK = CS = VDD
MAXIMUM SAMPLE RATE
vs. SUPPLY VOLTAGEMXB7843 toc23
SUPPLY VOLTAGE (V)
SAMPLE RATE (kHz)
2.05.5
MXB7843
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
Pin Description
PINNAMEFUNCTION1VDDPositive Supply Voltage. Connect to pin 10.
2X+X+ Position Input, ADC Input Channel 1
3Y+Y+ Position Input, ADC Input Channel 2
4X-X- Position Input
5Y-Y- Position InputGNDGroundIN3Auxiliary Input to ADC; ADC Input Channel 3IN4Auxiliary Input to ADC; ADC Input Channel 4REFVoltage Reference Input. Reference voltage for analog-to-digital conversion. Apply a reference
voltage between 1V and VDD. Bypass REF to GND with a 0.1µF capacitor.VDDPositive Supply Voltage, +2.375V to +5.25V. 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%).
MXB7843
Detailed DescriptionThe MXB7843 uses a successive-approximation conver-
sion technique to convert analog signals to a 12-bit digital
output. An SPI/QSPI/MICROWIRE™-compatible serial
interface provides an easy communication to a micro-
processor (µP). It features a 4-wire touch-screen interface
and two auxiliary ADC channels (Functional Diagram).
Analog InputsFigure 2 shows a block diagram of the analog input sec-
tion that includes the input multiplexer of the MXB7843,
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-, IN3, and IN4.
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.
Input Bandwidth and Anti-AliasingThe 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. tRRpFACQSIN . =×+()×8425
2.375V to 5.25V, 4-Wire Touch-Screen
ControllerDCLK
DIN
DOUT
BUSY
tBDV
tDV
tCSS
tCL
tCH
tDStDH
tCP
tDO
tBD
tTR
tBTR
tCSH
Figure 1. Detailed Serial Interface Timing
MICROWIREis a trademark of National Semiconductor Corp.
MXB7843
2.375V to 5.25V, 4-Wire Touch-Screen
Controller6-TO-1
MUX
VDD
REF
12-BIT ADC
IN3
IN4
SERIAL
DATA
INTERFACE
DIN
DCLK
BUSY
PENIRQ
DOUT
Functional DiagramA1A0MEASUREMENTADC INPUT CONNECTIONDRIVERS ON0ReservedReserved—1Y-PositionX+Y+, Y-0IN3IN3—1ReservedReserved—0ReservedReserved—1X-PositionY+X-, X+0IN4IN4—1ReservedReserved—
Table 1. Input Configuration, Single-Ended Reference Mode (SER/DFRHIGH)A1A0ADC +REF
CONNECTION TO
ADC -REF
CONNECTION TO
ADC INPUT
CONNECTION TO
MEASUREMENT
PERFORMEDDRIVER ON1Y+Y-X+Y positionY+, Y-1X+X-Y+X positionX+, X-
Table 2. Input Configuration, Differential Reference Mode (SER/DFRLOW)
MXB7843
Analog Input ProtectionInternal 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 conversions. 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 ConversionThe MXB7843 provides two conversion methods—dif-
ferential and single ended. The SER/DFRbit in the con-
trol word selects either mode. A logic 1 selects a
single-ended conversion, while a logic 0 selects a dif-
ferential 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 ModeFigure 3 shows the switching matrix configuration for
Y-coordinate measurement in single-ended mode. The
MXB7843 measures the position of the pointing device by
connecting X+ to IN+ of the ADC, enabling Y+ and Y- dri-
vers, and digitizing the voltage on X+. The ADC performs
a conversion with REF+ = REF and REF- = GND. In sin-
gle-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-resis-
tance of the X and Y drivers does not track the resistance
of the touch screen over temperature and supply. This
results in further measurement errors.
Differential Measurement ModeFigure 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 MXB7843 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.
External ReferenceDuring conversion, an external reference at REF must
deliver up to 40µA DC load current. If the reference has
a higher output impedance or is noisy, bypass it close
to the REF pin with a 0.1µF and a 4.7µF capacitor.
2.375V to 5.25V, 4-Wire Touch-Screen
Controller
MXB7843
2.375V to 5.25V, 4-Wire Touch-Screen
ControllerVDDPENIRQREF
A2–A0
(SHOWN 001B)
SER/DFR
(SHOWN HIGH)
GNDCONVERTER
IN3
IN4
+REF
-REF
+IN
-IN
Figure 2. Equivalent Input Circuit
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
