Datasheet

30
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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
20µA 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
qV
k ln(N)
∆
(6)
where,
∆=
( )
−
( )
( )
∴° = ∆ °
°= •∆
(
)
−°
VVI VI inmV
K 2.573 V K/mV
C 2.573 V mV 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