MXB7846(2004) 查看數據表（PDF） - Maxim Integrated

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MXB7846
(Rev.:2004)

2.375V to 5.25V, 4-Wire Touch-Screen Controller with Internal Reference and Temperature Sensor

Maxim Integrated 

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 VDD and GND, allow the analog input pins to swing

from GND - 0.3V to VDD + 0.3V without damage. Analog

inputs must not exceed VDD by 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/DFR bit 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 200Ω to

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 PENIRQ func-

tion. When used, PENIRQ requires a 10kΩ to 100kΩ

pullup to +VDD. If enabled, PENIRQ goes low whenever

the touch screen is touched. The PENIRQ output 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 PENIRQ pin

function. The diagram shows that once the screen is

touched while CS is high, the PENIRQ output 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 CS low to initiate a con-

version. At this instant, the PENIRQ pin should be

masked, as transitions can occur due to a selected

input channel or the conversion mode. The PENIRQ pin

functionality becomes valid when either the last data bit

is clocked out, or CS is 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.

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