and air temperatures. Again, the primary thermal conduction path is through the leads, so the circuit board

surface temperature will contribute to the die temperature much more than the air temperature.

To measure the temperature external to the die use a remote diode. This diode can be located on the die of

the target IC, such as a CPU processor chip, allowing measurement of the IC’s temperature, independent of the

LM64’s temperature. The LM64 has been optimized for use with a MMBT3904 diode-connected transistor.

A discrete diode can also be used to sense the temperature of external objects or ambient air. Remember that a

discrete diode’s temperature will be affected, and often dominated by, the temperature of its leads.

Most silicon diodes do not lend themselves well to this application. It is recommended that a diode-connected

MMBT3904 transistor be used. The base of the transistor is connected to the collector and becomes the anode.

The emitter is the cathode.

8.1.3.1 Diode Non_Ideality

When a transistor is connected to a diode the following relationship holds for Vbe, T, and IF:

S

F

I

I

˜ e

¸¸

¹

·

¨¨

©

§

˜

T

be

V

V

K

1

«

«

¬

ª

»

»

¼

º

(2)

where

V

T = q

kT

(3)

• q = 1.6x10−19 Coulombs (the electron charge)

• T = Absolute Temperature in Kelvin

• k = 1.38x10−23 joules/K (Boltzmann’s constant)

• η is the non-ideality factor of the manufacturing process used to make the thermal diode

• Is = Saturation Current and is process dependent

• If = Forward Current through the base emitter junction

• Vbe = Base Emitter Voltage Drop

In the active region, the −1 term is negligible and may be eliminated, yielding the following equation

S

F

I

I

˜ e

¸¸

¹

·

¨¨

©

§

˜

T

be

V

V

K

«

«

¬

ª

»

»

¼

º

(4)

In the above equation, η and Is are dependent upon the process that was used in the fabrication of the particular

diode. By forcing two currents with a very controlled ratio (N) and measuring the resulting voltage difference, it is

possible to eliminate the Is term. Solving for the forward voltage difference yields the relationship:

N

q

kT

V

be

ln

˜

¸

¨

'

K

¸

¹

·

¨

©

§

(5)

The non-ideality factor, η, is the only other parameter not accounted for and depends on the diode that is used

for measurement. Since ΔVbe is proportional to both η and T, the variations in η cannot be distinguished from

variations in temperature. Since the temperature sensor does not control the non-ideality factor, it will directly

add to the inaccuracy of the sensor.

LM64

SNAS207B – MAY 2004 – REVISED JANUARY 2024

www.ti.com

32

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