LM64EVAL National Semiconductor, LM64EVAL Datasheet - Page 26

• Manufacturer Part Number: LM64EVAL
• Description: BOARD EVALUATION LM64
• Manufacturer: National Semiconductor

Specifications of LM64EVAL

• Sensor Type: Temperature, Fan Controller
• Sensing Range: 0°C ~ 85°C
• Interface: SMBus (2-Wire/I²C)
• Sensitivity: ±1°C
• Voltage - Supply: 3 V ~ 3.6 V
• Embedded: Yes, MCU, 8-Bit
• Utilized Ic / Part: LM64
• Lead Free Status / RoHS Status: Not applicable / Not applicable

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3.0 Application Notes
3.2 USE OF THE LOOKUP TABLE FOR NON-LINEAR
PWM VALUES VS TEMPERATURE
The Lookup Table, Registers 50 through 5F, can be used to
create a non-linear PWM vs Temperature curve that could be
used to reduce the acoustic noise from processor fan due to
linear or step transfer functions. An example is given below:
EXAMPLE:
In a particular system it was found that the best acoustic fan
noise performance was found to occur when the PWM vs
Temperature transfer function curve was parabolic in shape.
From 25˚C to 105˚C the fan is to go from 20% to 100%.
Since there are 8 steps to the Lookup Table we will break up
the Temperature range into 8 separate temperatures. For the
80˚C over 8-steps = 10˚C per step. This takes care of the
x-axis.
For the PWM Value, we first select the PWM Frequency. In
this example we will make the PWM Frequency (Register
4C) 20.
For 100% Duty Cycle then, the PWM value is 40. For 20%
the minimum is 40 x (0.2) = 8.
We can then arrange the PWM, Temperature pairs in a
parabolic fashion in the form of y = 0.005 • (x −25)
We can then program the Lookup Table with the temperature
and Closest PWM Values required for the curve required in
our example.
3.3 NON-IDEALITY FACTOR AND TEMPERATURE
ACCURACY
The LM64 can be applied to remote diode sensing in the
same way as other integrated-circuit temperature sensors. 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-
circuit board lands and traces soldered to its pins. This
presumes that the ambient air temperature is nearly 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 LM64 die
will be an intermediate temperature between the surface 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
Temperature
105
25
35
45
55
65
75
85
95
PWM Value
Calculated
10.0
12.5
16.0
20.5
26.0
32.5
40.0
8.0
8.5
(Continued)
Closest PWM
Value
10
13
16
21
26
33
40
8
9
2
+ 8
26
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.
3.3.1 Diode Non_Ideality
When a transistor is connected to a diode the following
relationship holds for V
where
• q = 1.6x10
• T = Absolute Temperature in Kelvin
• k = 1.38x10
• η is the non-ideality factor of the manufacturing process
• I
• I
• V
In the active region, the −1 term is negligible and may be
eliminated, yielding the following equation
In the above equation, η and I
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
difference yields the relationship:
The non-ideality factor, η, is the only other parameter not
accounted for and depends on the diode that is used for
measurement. Since ∆V
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.
used to make the thermal diode
s
f
be
= Forward Current through the base emitter junction
= Saturation Current and is process dependent
= Base Emitter Voltage Drop
−19
−23
Coulombs (the electron charge)
s
joules/K (Boltzmann’s constant)
term. Solving for the forward voltage
be
be
, T, and I
is proportional to both η and T, the
s
F
are dependent upon the
: