Datasheet

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OPERATION - TEMPERATURE MEASUREMENT
X+
MUX
A/D
Converter
Temperature Select
TEMP1 TEMP2
°K 
q V
k n(N)
(7)
V  V

I
82

–V

I
1

(8)
°K 
q V
k n(N)
(9)
TSC2301
SLAS371D – SEPTEMBER 2002 – REVISED AUGUST 2004
In some applications, such as estimating remaining battery life or setting RAM refresh rate, a measurement of
ambient temperature is required. The temperature measurement technique used in the TSC2301 relies on the
characteristics of a semiconductor junction operating at a fixed current level. The forward diode voltage (V
BE
) has
a well-defined characteristic versus temperature. The ambient temperature can be predicted in applications by
knowing the 25 °C value of the V
BE
voltage and then monitoring the delta of that voltage as the temperature
changes.
The TSC2301 offers two modes of temperature measurement. The first mode requires calibration at a known
temperature, but only requires a single reading to predict the ambient temperature. A diode, as shown in
Figure 61 , is used during this measurement cycle. The voltage across this diode is typically 600 mV at 25 °C
while conducting a 20 -µA current. The absolute value of this diode voltage can vary several millivolts, but the
temperature coefficient (TC) of this voltage is very consistent at -2.1 mV/ °C. During the final test of the end
product, the diode voltage would be measured by the TSC2301 ADC at a known room temperature, and the
corresponding digital code stored in system memory, for calibration purposes by the user. The result is an
equivalent temperature measurement resolution of 0.3 °C/LSB. This measurement of what is referred to as
Temperature 1 is illustrated in Figure 62 .
Figure 61. Functional Block Diagram of Temperature Measurement Mode
The second mode does not require a test temperature calibration, but uses a two-measurement (differential)
method to eliminate the need for absolute temperature calibration, and achieves a 2 °C/LSB accuracy. This mode
requires a second conversion with a current 82 times larger than the first 20- µA current. The voltage difference
between the first (TEMP1) and second (Temp2) conversion, using 82 times the bias current, is represented by
kT/q ln (N), where N is the current ratio = 82, k = Boltzmann's constant (1.38054 x 10
-23
electron volts/degree
Kelvin), q = the electron charge (1.602189 x 10
-19
C), and T = the temperature in degrees Kelvin. This method
can provide much improved absolute temperature measurement without calibration, with resolution of 2 °C/LSB.
The resultant equation for solving for °K is:
where:
(in mV)
Temperature 2 measurement is illustrated in Figure 63 .
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