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TSC2200
SBAS191F
FIGURE 18. Functional Block Diagram of Temperature
Measurement Mode.
FIGURE 19. Single Temperature Measurement Mode.
FIGURE 20. Additional Temperature Measurement for Differential
Temperature Reading.
OPERATION
—
TEMPERATURE MEASUREMENT
In some applications, such as battery recharging, a measure-
ment of ambient temperature is required. The temperature
measurement technique used in the TSC2200 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 TSC2200 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 tempera-
ture. A diode, shown in Figure 18, is used during this measure-
ment cycle. This voltage is typically 600mV at +25
°
C with a
20
μ
A current through it. The absolute value of this diode voltage
can vary by a few millivolts; the temperature coefficient (TC) of
this voltage is very consistent at
–
2.1mV/
°
C. During the final test
of the end product, the diode voltage would be stored at a
known room temperature, 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 19.
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 for achieving 2
°
C/LSB accuracy. This mode requires a
second conversion with a 91 times larger current. The
voltage difference between the first (TEMP1) and second
(TEMP2) conversion, using 91 times the bias current, will be
represented by kT/q
ln (N), where N is the current
ratio = 91, k = Boltzmann
’
s constant (1.38054
10
-23
electrons volts/degrees Kelvin), q = the electron charge
(1.602189
10
-19
°
C), and T = the temperature in degrees
Kelvin. This method can provide much improved absolute
temperature measurement, but less resolution of 2
°
C/LSB.
The resultant equation for solving for
°
K is:
°
=
K
q
V
k ln(N)
(6)
where,
=
)
(
)
(
∴°
=
=
2.573
)
°
)
V
V I
K
V I
inmV
2.573 V K/mV
V mV
C
273 K
91
1
Figure 20 shows the Temperature 2 measurement.
A/D
Converter
MUX
X+
Temperature Select
TEMP0
TEMP1
Host Writes
A/D Converter
Control Register
Start Clock
Temperature Input 1
Done
Yes
No
Is Data
Averaging Done
Store Temperature
Input 1 in TEMP1
Register
Power Down
A/D Converter
Power Up
A/D Converter
Power Up Reference
Convert
Temperature Input 1
Issue Data Available
Power Down Reference
Turn Off Clock
Host Writes
A/D Converter
Control Register
Start Clock
Temperature Input 2
Done
Yes
No
Is Data
Averaging Done
Store Temperature
Input 2 in TEMP2
Register
Power Down
A/D Converter
Power Up
A/D Converter
Power Up Reference
Convert
Temperature Input 2
Issue Data Available
Power Down Reference
Turn Off Clock