MXB7846 Ver la hoja de datos (PDF) - Maxim Integrated

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MXB7846

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

the ambient temperature. First, the PENIRQ diode for-

ward bias voltage is measured by the ADC with an

address of A2 = 0, A1 = 0, and A0 = 0 at a known tem-

perature. Subsequent diode measurements provide an

estimate of the ambient temperature through extrapola-

tion. This assumes a temperature coefficient of

-2.1mV/°C. The single conversion method results in a

resolution of 0.3°C/LSB and a typical accuracy of ±3°C.

The differential conversion method uses two measure-

ment points. The first measurement (Temp0) is per-

formed with a fixed bias current into the PENIRQ diode.

The second measurement (Temp1) is performed with a

fixed multiple of the original bias current with an

address of A2 = 1, A1 = 1, and A0 = 1. The voltage dif-

ference between the first and second conversion is

proportional to the absolute temperature and is

expressed by the following formula:

⎡

T(°C) = ⎢2.60

⎣

×

( T1

−

T0)

⎛

⎝⎜

VREF ⎞

4096 ⎠⎟

⎤

× 1000⎥ − 273

⎦

where T0 (Temp0) and T1 (Temp1) are the conversion

results.

This differential conversion method can provide much

improved absolute temperature measurement; however,

the resolution is reduced to 1.6°C/LSB.

BATTERY

0 TO 6.0V

BAT

7.5kΩ

DC/DC

CONVERTER

0 TO 1.5V

+2.375V TO +5.25V

VDD

12-BIT ADC

2.5kΩ

BATTERY

MEASUREMENT ON

Battery Voltage Monitor

A dedicated analog input (BAT) allows the MXB7846 to

monitor the system battery voltage. Figure 8 shows the

battery voltage monitoring circuitry. The MXB7846 mon-

itors battery voltages from 0 to 6V. An internal resistor

network divides down VBAT by 4 so that a 6.0V battery

voltage results in 1.5V at the ADC input. To minimize

power consumption, the divider is only enabled during

the sampling of VBAT.

Internal Reference

Enable the internal 2.5V reference by setting PD1 in the

control byte to a logic 1 (see Tables 3 and 4). The

MXB7846 uses the internal reference for single-ended

measurement mode, battery monitoring, temperature

measurement, and for measurement on the auxiliary

input. To minimize power consumption, disable the inter-

nal reference by setting PD1 to a logic 0 when performing

ratiometric position measurements. The internal 2.5V ref-

erence typically requires 10ms to settle (with no external

load). For optimum performance, connect a 0.1µF capac-

itor from REF to GND. This internal reference can be over-

driven with an external reference. For best performance,

the internal reference should be disabled when the exter-

nal reference is applied. The internal reference of the

MXB7846 must also be disabled to maintain compatibility

with the MXB7843. To disable the internal reference of the

MXB7846 after power-up, a control byte with PD1 = 0 is

required. (See Typical Operating Characteristics for

power-up time of the reference from power down.)

External Reference

Although the internal reference may be overdriven with

an external reference, the internal reference should be

disabled (PD1 = 0) for best performance when using

an external reference. During conversion, an external

reference at REF must deliver up to 40µA DC load cur-

rent. 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. Temperature measurements are

always performed using the internal reference.

Digital Interface

Initialization After Power-Up and Starting a

Conversion

The digital interface consists of three inputs, DIN, DCLK,

CS, and one output, DOUT. A logic-high on CS disables

the MXB7846 digital interface and places DOUT in a

high-impedance state. Pulling CS low enables the

MXB7846 digital interface.

Figure 8. Battery Measurement Functional Block Diagram

14 ______________________________________________________________________________________

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