MXB7846
2.375V to 5.25V, 4-Wire Touch-Screen Controller
with Internal Reference and Temperature Sensor
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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
(t
ACQ
) is the maximum time the device takes to acquire
the input signal to 12-bit accuracy. Calculate t
ACQ
with
the following equation:
where R
IN
= 2kΩ and R
S
is the source impedance of
the input signal.
Source impedances below 1kΩ do 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.
