MIC502 Micrel Semiconductor, MIC502 Datasheet - Page 10

• Manufacturer Part Number: MIC502
• Description: Fan Management IC Advance Information
• Manufacturer: Micrel Semiconductor

MIC502
Referring to the “Typical Application,” the following approach
can be used to design the required thermistor interface
network:
Let
and
Since
and
Let’s continue by determining what the temperature-propor-
tional voltage is at 25 C.
Let
and
From
Recalling from above discussion that the desired V
should be about 40% of V
too low. This would produce a voltage that would stop the fan
(recall from the above that this occurs when V
of V
learn what the parallel combination of R
at 25 C:
Again
and
Since
MIC502
DD
R1 =
R
V
0.7R
0.7R
R2 = 2.33R
R1 =
R
V
0.4(R
0.4(R
R
V =
0.7 =
V =
V =
V =
0.4 =
. To choose an appropriate value for R1 we need to
T
T
T1
T1
T1
T
T
T
T
= 0.7V
= 0.248V
= 13.6k
= 100k
|| R1 = 1.5R2 = 1.5
T1
T1
T1
T1
V
R
R
R
100k + 33k
R
R
V
DD
+ 0.7R2 = R2
= 0.3R2
T1
T1
T1
V
DD
V
|| R1) + 0.4R2 = R2
|| R1) = 0.6R2
T1
T1
DD
R2
DD
DD
|| R1+ R2
+ R2
|| R1+ R2
+ R2
|| R1+ R2
T1
R2
R2
DD
33k
= 2.33
R2
R2
(at 70 C)
(70% of V
(at 25 C).
DD
, the above value of 24.8% is far
13.6k = 31.7k
33k = 49.5k
DD
)
T1
and R1 should be
T
33k
is about 30%
T
for 25 C
172
and
let
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
only a 3% shift from the design goal of 70% of V
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 (100k ) 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
cause these inputs can have up to 1 A 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
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
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
signal is required when the MIC502 V
divider is necessary to reduce this input voltage so it does not
exceed the MIC502 V
(80%V
Because of input leakage considerations, the impedance of
the resistive divider should be kept at
resistor of 120k
shunt resistor to ground make a good divider for driving the
V
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 of 40, is the chosen motor-drive transistor.
T2
T
input.
SLP
for 70 C is 0.734, or about 73% of V
R
RT1 || R1 = 49.5k
R1 = 100k
DD
T1
input has this same leakage current specification.
).
= 100k
DD
= 5V vs. V
driven by the Fan C signal and a 100k
DD
DD
50k
= 12V) becomes a concern. The
voltage. A good number is 4V
T1
and/or V
DD
DD
100k . A series
. This represents
= 5V a resistor
DD
T2
input(s). Be-
is 12V. If this
May 1999
DD
Micrel
. In