MAX1233/MAX1234
±15kV ESD-Protected Touch-Screen
Controllers Include DAC and Keypad Controller
32 ______________________________________________________________________________________
Auxiliary Analog Inputs
Two auxiliary analog inputs (AUX1 and AUX2) allow the
MAX1233/MAX1234 to monitor analog input voltages
from zero to V
REF
. Figure 16 illustrates the process of
auxiliary input reading.
Temperature Measurements
The MAX1233/MAX1234 provide two temperature mea-
surement options: a single-ended conversion method
and a differential conversion method. Both temperature
measurement techniques rely on the semiconductor
junction’s operational characteristics at a fixed current
level. The forward diode voltage (V
BE
) vs. temperature is
a well-defined characteristic. The ambient temperature
can be predicted in applications by knowing the value
of the V
BE
voltage at a fixed temperature and then moni-
toring the delta of that voltage as the temperature
changes. Figure 17 illustrates the functional block of the
internal temperature sensor.
The single conversion method requires calibration at a
known temperature, but only requires a single reading to
predict the ambient temperature. First, the internal diode
forward bias voltage is measured by the ADC at a
known temperature. Subsequent diode measurements
provide an estimate of the ambient temperature through
extrapolation. This assumes a temperature coefficient of
-2.1mV/°C. The single conversion method results in a
resolution of 0.29°C/LSB (2.5V reference) and
0.12°C/LSB (1.0V reference) with a typical accuracy of
±2°C. Figure 18 shows the flowchart for the single tem-
perature measurement.
The differential conversion method uses two measure-
ment points. The first measurement is performed with a
fixed bias current into the internal diode. The second
measurement is performed with a fixed multiple of the
original bias current. The voltage difference between the
first and second conversion is proportional to the
absolute temperature and is expressed by the following
formula:
ΔV
BE
= (kT/q) ✕ ln(N)
where:
ΔV
BE
= difference in diode voltage
N = current ratio of the second measurement to the first
measurement
k = Boltzmann’s constant (1.38 × 10
-23
eV/°Kelvin)
q = electron charge (1.60 × 10
-19
C)
T = temperature in °Kelvin
The resultant equation solving for °K is:
T(°K) = q x ΔV / (k × ln(N))
