MIC502 datasheet(11/15 Pages) MICREL

11 / 15 page

Micrel, Inc.

MIC502

November 2006

M9999-112206

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

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

2 040

6080

100

T1/SUPPLY VOLTAGE (%)

Figure 6. Control Voltage vs. Fan Speed

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