TSC2000IPW datasheet(29/34 Pages) TI

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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

Is Data

Averaging Done

Store Temperature

Input 2 in TEMP2

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

Is Data

Averaging Done

Store Temperature

Input 1 in TEMP1

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 (VBE) has a

well-defined characteristic versus temperature. The ambient

temperature can be predicted in applications by knowing the

°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 20

µA 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 ln(N)

∆

(6)

where,

∆=

( ) − ( ) (

)

∴° =

∆ °

°=

• ∆

( ) −°

VV I

V I

in mV

2.573 V K/mV

2.573

V mV

273 K

See Figure 20 for the Temperature 2 measurement.

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