Datasheet

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MMBT3904
I
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I
F
LM63
2.2 nF
2
3
D+
D-
LM63
SNAS190E –SEPTEMBER 2002–REVISED MAY 2013 www.ti.com
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
2N3904 transistor be used, as shown in Figure 15. The base of the transistor is connected to the collector and
becomes the anode. The emitter is the cathode.
Figure 15. Processor Connection to LM63
A LM63 with a diode-connected 2N3904 transistor approximates the temperature reading of the LM63 with the
Pentium 4 processor by 1°C.
T
2N3904
= T
PENTIUM 4
− 1°C (4)
DIODE NON-IDEALITY
When a transistor is connected to a diode the following relationship holds for V
be
, T, and I
F
:
where (5)
• 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
• I
s
= Saturation Current and is process dependent
• I
f
= Forward Current through the base emitter junction
• V
be
= Base Emitter Voltage Drop (6)
In the active region, the −1 term is negligible and may be eliminated, yielding the following equation
(7)
In Equation 7, η and I
s
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 I
s
term. Solving for the forward voltage difference yields the relationship:
(8)
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