Datasheet

LTM4628
19
4628fe
For more information www.linear.com/LTM4628
APPLICATIONS INFORMATION
Figure 8. Silicon Diode Voltage V
D
vs Temperature Figure 9. The 1N4148 Diode Voltage V
D
vs Temperature
temperature relationship that makes diodes suitable
temperature sensors. The I
S
term in the equation above
is the extrapolated current through a diode junction when
the diode has zero volts across the terminals. The I
S
term
varies from process to process, varies with temperature,
and by definition must always be less than I
D
. Combining
all of the constants into one term:
K
D
=
η • k
q
where K
D
= 8.62
−5
, and knowing ln(I
D
/I
S
) is always posi-
tive because I
D
is always greater than I
S
, leaves us with
the equation that:
V
D
= T(KELVIN) •K
D
• ln
I
D
I
S
where V
D
appears to increase with temperature. It is com-
mon knowledge
that a silicon diode biased with a current
source has an approximate –2mV/°C temperature rela-
tionship (Figure 8), which is
at odds with the equation. In
fact, the I
S
term increases with temperature, reducing the
ln(I
D
/I
S
) absolute value yielding an approximate –2mV/°C
composite diode voltage slope.
It is important that the bias current source be accurate and
powered from a high impedance source. This is because
the forward voltage drop is also a function of the current
through the diode.
The below equations show that when currents are a de
-
cade apart the V
D
difference is 60mV; therefore the 10µA
current source error will affect the diode forward voltage
at temperature.
kT/q = 26mV
V
D1
– V
D2
= kT/q ln(I
D1
)/(I
D2
)
where V
D1
– V
D2
is the difference in the diode forward
voltage with the I
D1
and I
D2
current difference.
Several 1N4148 diodes were tested with 100µA of current
and Figure 9 shows the results. The 100µA current source
provided the best repeatability for each diode.
The tested diodes are very close to –2.2mV/°C to –2.4
mV/°C
slope
while each are biased with 100µA through a 120k
pull-up resistor to 12V. The Figure 9 graph can be used
to calibrate and measure LTM4628 internal temperature
by measuring the diode V
D
value.
TEMPERATURE (°C)
–273
DIODE VOLTAGE (V
D
)
1.4
1.2
1.0
0.6
0.8
0.2
0.4
0
27 127
4628 F08
227
I
D
= 10µA
–73–173
TEMPERATURE (°C)
–100
1N4148 DIODE VOLTAGE (V
D
)
0.8
0.7
0.6
0.5
0.3
0.1
0.2
0.4
0
50 100 150
4628 F09
2000–50