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MIC502_11 Datasheet, PDF (11/15 Pages) Micrel Semiconductor – Fan Management IC
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Micrel, Inc.
Application Information
The Typical Application drawing on page 1 illustrates a
typical application circuit for the MIC502. Interfacing the
MIC502 with a system consists of the following steps:
1. Selecting a temperature sensor
2. Interfacing the temperature sensor to the VT1 input
3. Selecting a fan-drive transistor, and base-drive
current limit resistor
4. Deciding what to do with the Secondary Fan-
Control Input
5. Making use of the Overtemperature Fault Output
Temperature Sensor Selection
Temperature sensor T1 is a negative temperature
coefficient (NTC) thermistor. The MIC502 can be
interfaced with either a negative or positive tempco
thermistor; however, a negative temperature coefficient
thermistor typically costs less than its equivalent positive
tempco counterpart. While a variety of thermistors can
be used in this application, the following paragraphs
reveal that those with an R25 rating (resistance at 25°C)
of from about 50kΩ to 100kΩ lend themselves nicely to
an interface network that requires only a modest current
drain. Keeping the thermistor bias current low not only
indicates prudent design; it also prevents self-heating of
the sensor from becoming an additional design
consideration. It is assumed that the thermistor will be
located within the system power supply, which most
likely also houses the speed-controlled fan.
Temperature Sensor Interface
As shown by the Electrical Characteristics table, the
working voltage for input VT1 is specified as a
percentage of VDD. This conveniently frees the designer
from having to be concerned with interactions resulting
from variations in the supply voltage. By design, the
operating range of VT1 is from about 30%of VDD to about
70% of VDD.
VPWM(min) = V PWM(min) – VPWM(span)
When VT1 = VPWM(max) ≈ 0.7VDD, a 100% duty-cycle
motor-drive signal is generated. Conversely, when VT1 =
VPWM(min) ≈ 0.3VDD, the motor-drive signal has a 0% duty
cycle. Resistor voltage divider R1 || T1, R2 in the Typical
Application diagram is designed to preset VT1 to a value
of VPWM that corresponds to the slowest desired fan
speed when the resistance of thermistor T1 is at its
highest (cold) value. As temperature rises the resistance
of T1 decreases and VT1 increases because of the
parallel connection of R1 and T1.
Since VT1 = VPWM(min) represents a stopped fan (0% duty-
cycle drive), and since it is foreseen that at least some
cooling will almost always be required, the lowest
voltage applied to the VT1 input will normally be
MIC502
somewhat higher than 0.3VDD (or >VPWM(min)). It is
assumed that the system will be in sleep mode rather
than operate the fan at a very low duty cycle (<25%).
Operation at very low duty cycle results in relatively little
airflow. Sleep mode should be used to reduce acoustic
noise when the system is cool. For a given minimum
desired fan speed, a corresponding VT1(min) can be
determined via the following observation:
since
VPWM(max) = 70% of VDD ∝ 100% RPM
and
VPWM(min) = 30% of VDD ∝ 0% RPM
then
VPWM(span) = 40% of VDD ∝ 100% RPM range.
Figure 6 shows the following linear relationship between
the voltage applied to the VT1 input, motor drive duty
cycle, and approximate motor speed.
since
VT1 = 0.7VDD ∝ 100% PWM
then
VT1 = 0.6VDD ∝ 75% PWM
and
VT1 = 0.5VDD ∝ 50% PWM
and
VT1 = 0.4VDD ∝ 25% PWM
In addition to the R25 thermistor rating, sometimes a
datasheet will provide the ratio of R25/R50 (resistance at
25°C divided by resistance at 50°C) is given. Sometimes
this is given as an R0/R50 ratio. Other datasheet
contents either specify or help the user determine device
resistance at arbitrary temperatures. The thermistor
interface to the MIC502 usually consists of the thermistor
and two resistors.
100
80
60
40
20
0
0 2 0 40 60 80 100
VT1/SUPPLY VOLTAGE (%)
Figure 6. Control Voltage vs. Fan Speed
November 2006
11
M9999-112206
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