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SS7646

8/21/2004 Rev.2.01

Temperature Measurement

The temperature measurement is based on an on-chip

forward diode measurement. The forward diode

temperature. There are two measurement options for

the

SS7646, a Single Conversion Mode and a

Differential Conversion Mode.

(1) In the Single Conversion Mode, a diode voltage is

first digitized at a fixed calibration temperature (25

during the final test of the end product and this value T1

is stored in the memory, then only a single reading T2 is

required for the users to obtain the am bient

temperature through extrapolation from the calibration

temperature diode result.

This result assumes a diode temperature coefficient

(TC) of approximately –2.1 mV/

°C. This method

provides a resolution of approximately 0.3

°C and a

predicted accuracy of ± 3

°C.

This mode is achieved with an address setting of A2=0,

A1=0, A0=0 (TEMP0). Please refer to Table III and

Figure 8 for details.

(2) The Differential Conversion Mode requires two

points measurement to eliminate the need for absolute

temperature calibration. The first measurement TEMP0

(A2=0, A1=0, A0=0) is performed with a fixed bias

current into a diode and the second measurement

TEMP1 (A2=1, A1=1, A0=1) is performed with N times

(N=120 for SS7646) of the bias current into the

same diode. The voltage difference between first and

s ec ond r eadings is proportional to abs olute

temperature and is given by the following formula:

where V

current multiple, k is Boltzmann’s constant and q is the

electron charge.

Taking Boltzmann’s constant k=1.38054x10

-23

electrons volts/degrees Kelvin, the electron charge

q=1.602189x10

temperature T, in degrees centigrade, would be

calculated as follows:

This method provides a much improved absolute

measurement of temperature of ± 2

°C, however

resolution is reduced to approximately 1.5

°C/LSB.

/PENIRQ

VDD

ADC

MUX

6 Channel

X+

ON

OFF

OFF

IN * I

Figure 8. Block Diagram of Temperature

Measurement Circuit

ADC

DC/DC

Converter

A2A1A0

(Shown 010)

Battery

0V to 6V

ON

ON

7.5K

2.5K

Figure 9. Block Diagram of Battery

Measurement Circuit

ln(N)

x

)

q

kT

(

=

C/LSB

0.3

Resolution

0.61mV

4096

2.5V

LSB

1

°

=

=

=

T1)x0.3

(T2

25

C)

T(

°

−

−

=

°

273

-

(mV)

V

x

2.42

C)

T(

]

ln(120)

x

[k

V

x

(q

K)

T(

BE

BE)

∆

=

°

∆

=

°