MIC502 Datasheet, PDF (10/11 Pages) Micrel Semiconductor – Fan Management IC Advance Information

MIC502
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 V_DD)
( ) ^{VT =}
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-propor-
tional voltage is at 25°C.
Let
R1 = ∞
and
R_T1 = 100k
From
(at 25°C).
( ) ^{VT =}
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 be
at 25°C:
Again
( ) ^{VT =}
V_DD × R2
R_T1 || R1+ R2
( ) ^{0.4 =}
R2
R_T1 || 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
Micrel
R_T1 = 100k
and
RT1 || 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). Be-
cause 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 accu-
racy 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, VT2, pin 5. It is possible that a second ther-
mistor, mounted at another temperature-critical location out-
side 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 VT2
input may be treated exactly like the VT1 input. The above
discussions then apply directly. If, however, the NLX FanC
signal is to be incorporated into the design then the operating
voltage (V_DD = 5V vs. V_DD = 12V) becomes a concern. The
FanC signal is derived from a 12V supply and is specified to
swing at least to 10.5V. A minimum implementation of the
FanC signal would provide the capability of asserting full-
speed operation of the fan; this is the case when 10.5V ≤
FanC ≤ 12V. This FanC signal can be applied directly to the
VT2 input of the MIC502, but only when its V_DD is 12V. If this
signal is required when the MIC502 V_DD = 5V a resistor
divider is necessary to reduce this input voltage so it does not
exceed the MIC502 V_DD voltage. A good number is 4V
(80%V_DD).
Because of input leakage considerations, the impedance of
the resistive divider should be kept at ≤ 100kΩ. A series
resistor of 120kΩ driven by the Fan C signal and a 100kΩ
shunt resistor to ground make a good divider for driving the
V_T2 input.
Transistor and Base-Drive Resistor Selection
The OUT motor-drive output, pin 7, is intended for driving a
medium-power device, such as an NPN transistor. A rather
ubiquitous transistor, the 2N2222A, is capable of switching
up to about 400mA. It is also available as the PN2222A in a
plastic TO-92 package. Since 400mA is about the maximum
current for most popular computer power supply fans (with
many drawing substantially less current) and since the MIC502
provides a minimum of 10mA output current, the PN2222A,
with its minimum β of 40, is the chosen motor-drive transistor.
MIC502
172
May 1999