Datasheet

29
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TSC2000
SBAS257
FIGURE 18. Functional Block Diagram of Temperature Mea-
surement Mode.
FIGURE 19. Single Temperature Measurement Mode.
FIGURE 20. Additional Temperature Measurement for Differ-
ential Temperature Reading.
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
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
A/D
Converter
MUX
X+
Temperature Select
TEMP1 TEMP2
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 TSC2000 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 TSC2000 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, as shown in Figure 18, is used during this
measurement cycle. This voltage is typically 600mV at +25°C
with a 20µA current through it. The absolute value of this diode
voltage can vary 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
elec-
trons 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
See Figure 20 for the Temperature 2 measurement.