MIC502_06 Datasheet, PDF (12/15 Pages) Micrel Semiconductor – Fan Management IC

Micrel, Inc.
MIC502
Design Example
The thermistor-resistor interface network is shown in the
Typical Application drawing. The following example
describes the design process: A thermistor datasheet
specifies a thermistor that is a candidate for this design
as having an R25 resistance of 100kΩ. The datasheet
also supports calculation of resistance at arbitrary tem-
peratures, and it was discovered the candidate
thermistor has a resistance of 13.6k at 70°C (R70).
Accuracy is more important at the higher temperature
end of the operating range (70°C) than the lower end
because we wish the overtemperature fault output
(/OTF) to be reasonably accurate — it may be critical to
operating a power supply crowbar or other shutdown
mechanism, for example. The lower temperature end of
the range is less important because it simply establishes
minimum fan speed, which is when less cooling is
required.
Referring to the “Typical Application,” the following
approach can be used to design the required thermistor
interface network:
let
R1 = ∞
R_T1 = 13.6k
and
(at 70°C)
V_T = 0.7V_DD
since
(70% of VDD)
V_T
=
V_DD × R2
(R_T1 || R1 + R2)
0.7
=
R2
(R_T1 + R2)
0.7R_T1 + 0.7R2 = R2
0.7R_T1 = 0.3R2
and
R2 = 2.33R_T1 = 2.33 × 13.6k = 31.7k ≈ 33k
Let’s continue by determining what the temperature-
proportional voltage is at 25°C.
let
R1 = ∞
and
R_T1 = 100k
from
(at 25°C).
V_T
=
V_DD × R2
(R_T1 + R2)
V_T
=
V_DD × 33k
(100k + 33k)
V_T = 0.248V_DD
Recalling from above discussion that the desired V_T for
25°C should be about 40% of V_DD, the above value of
24.8% is far too low. This would produce a voltage that
would stop the fan (recall from the above that this occurs
when V_T is about 30% of V_DD. To choose an appropriate
value for R1 we need to learn what the parallel
combination of R_T1 and R1 should beat 25°C:
again
V_T
=
V_DD × R2
(R_T1 || R1 + R2)
0.4
=
(R _T1
R2
|| R1 + R2)
0.4(R_T1 || R1) + 0.4R2 = R2
0.4(R_T1 || R1) = 0.6R2
and
R_T1 || R1 = 1.5R2 = 1.5 × 33k = 49.5k
since
R_T1 = 100k
and
R_T1 || R1 = 49.5k ≈ 50k
let
R1 = 100k
While that solves the low temperature end of the range,
there is a small effect on the other end of the scale. The
new value of V_T for 70°C is 0.734, or about 73% of V_DD.
This represents only a 3% shift from the design goal of
70% of V_DD. In summary, R1 = 100k, and R2 = 33k. The
candidate thermistor used in this design example is the
RL2010-54.1K-138-D1, manufactured by Keystone
Thermometrics.
The R25 resistance (100kΩ) of the chosen thermistor is
probably on the high side of the range of potential
thermistor resistances. The result is a moderately high-
impedance network for connecting to the V_T1 and/or V_T2
input(s). Because these inputs can have up to 1µA of
leakage current, care must be taken if the input network
impedance becomes higher than the example. Leakage
current and resistor accuracy could require consideration
in such designs. Note that the V_SLP input has this same
leakage current specification.
Secondary Fan-Control Input
The above discussions also apply to the secondary fan-
control input, V_T2, pin 5. It is possible that a second
thermistor, mounted at another temperature-critical
location outside the power supply, may be appropriate.
There is also the possibility of accommodating the NLX
“FanC” signal via this input. If a second thermistor is the
desired solution, the V_T2 input may be treated exactly
like the V_T1 input. The above discussions then apply
directly. If, however, the NLX FanC signal is to be
November 2006
12
M9999-112206