LM95235EVAL/NOPB National Semiconductor, LM95235EVAL/NOPB Datasheet - Page 22

• Manufacturer Part Number: LM95235EVAL/NOPB
• Description: BOARD EVALUATION LM95235
• Manufacturer: National Semiconductor
• Series: PowerWise®, TruTherm®r

Specifications of LM95235EVAL/NOPB

• Sensor Type: Temperature
• Sensing Range: -40°C ~ 125°C
• Interface: SMBus (2-Wire/I²C)
• Sensitivity: ±1°C
• Voltage - Supply: 3 V ~ 3.6 V
• Embedded: Yes, MCU, 8-Bit
• Utilized Ic / Part: LM95235
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM95235EVAL

www.national.com
3.0 Applications Hints
The LM95235 can be applied easily in the same way as other
integrated-circuit temperature sensors, and its remote diode
sensing capability allows it to be used in new ways as well. It
can be soldered to a printed circuit board, and because the
path of best thermal conductivity is between the die and the
pins, its temperature will effectively be that of the printed cir-
cuit board lands and traces soldered to the LM95235's pins.
This presumes that the ambient air temperature is almost the
same as the surface temperature of the printed circuit board;
if the air temperature is much higher or lower than the surface
temperature, the actual temperature of the LM95235 die will
be at an intermediate temperature between the surface and
air temperatures. Again, the primary thermal conduction path
is through the leads, so the circuit board temperature will con-
tribute to the die temperature much more strongly than will the
air temperature.
To measure temperature external to the LM95235's die, use
a remote diode. This diode can be located on the die of a
target IC, allowing measurement of the IC's temperature, in-
dependent of the LM95235's temperature. 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 an MMBT3904 tran-
sistor base-emitter junction be used with the collector tied to
the base.
The LM95235's TruTherm technology allows accurate sens-
ing of integrated thermal diodes, such as those found on most
processors. With TruTherm technology turned off, the
LM95235 can measure a diode-connected transistor such as
the MMBT3904 or the thermal diode found in an AMD pro-
cessor.
The LM95235 has been optimized to measure the remote
thermal diode integrated in a typical Intel processor on
65 nm or 90 nm process or an MMBT3904 transistor. Using
the Remote Diode Model Select register either pair of remote
inputs can be assigned to be either a typical Intel processor
on 65 nm or 90 nm process or an MMBT3904.
3.1 DIODE NON-IDEALITY
3.1.1 Diode Non-Ideality Factor Effect on Accuracy
When a transistor is connected as a diode, the following re-
lationship holds for variables V
where:
BE
, T and I
F
:
(1)
22
•
•
•
•
•
•
•
In the active region, the -1 term is negligible and may be elim-
inated, yielding the following equation
In Equation 2, η and I
was used in the fabrication of the particular diode. By forcing
two currents with a very controlled ratio(I
ing the resulting voltage difference, it is possible to eliminate
the I
the relationship:
Solving Equation 3 for temperature yields:
Equation 4 holds true when a diode connected transistor such
as the MMBT3904 is used. When this “diode” equation is ap-
plied to an integrated diode such as a processor transistor
with its collector tied to GND as shown in Figure 7 it will yield
a wide non-ideality spread. This wide non-ideality spread is
not due to true process variation but due to the fact that
Equation 4 is an approximation.
TruTherm technology uses the transistor equation, Equation
5, which is a more accurate representation of the topology of
the thermal diode found in an FPGA or processor.
q = 1.6×10
T = Absolute Temperature in Kelvin
k = 1.38×10
η is the non-ideality factor of the process the diode is
manufactured on,
I
I
V
S
f
S
BE
= Forward Current through the base-emitter junction
= Saturation Current and is process dependent,
term. Solving for the forward voltage difference yields
= Base-Emitter Voltage drop
−19
−23
Coulombs (the electron charge),
joules/K (Boltzmann's constant),
S
are dependant upon the process that
F2
/ I
F1
) and measur-
(2)
(3)
(4)
(5)