mxb7846 Maxim Integrated Products, Inc., mxb7846 Datasheet - Page 14

• Manufacturer Part Number: mxb7846
• Description: Mxb7846 2.375v To 5.25v, 4-wire Touch-screen Controller With Internal Reference And Temperature Sensor
• Manufacturer: Maxim Integrated Products, Inc.

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:
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.
Figure 8. Battery Measurement Functional Block Diagram
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
14
______________________________________________________________________________________
T C
BATTERY
0 TO 6.0V
( )
°
=



2 60
.
7.5kΩ
×
2.5kΩ
BAT
(
T
1
−
T
CONVERTER
0
DC/DC
)
0 TO 1.5V



VREF
4096
BATTERY
MEASUREMENT ON

 ×

+2.375V TO +5.25V
1000
12-BIT ADC
V
DD



−
273
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 V
voltage results in 1.5V at the ADC input. To minimize
power consumption, the divider is only enabled during
the sampling of V
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.)
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.
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.
Initialization After Power-Up and Starting a
BAT
.
BAT
Battery Voltage Monitor
by 4 so that a 6.0V battery
External Reference
Internal Reference
Digital Interface
Conversion