TC646 datasheet(16/28 Pages) MICROCHIP

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TC646

DS21446C-page 16

 2002 Microchip Technology Inc.

Design Example

Step 1. Calculate R1 and R2 based on using an NTC

having a resistance of 10 k

Ω at TMIN (25°C)

and 4.65 k

Ω at TMAX (45°C) (See Figure 5-9).

R1 = 20.5 kΩ

R2 = 3.83 kΩ

Step 2. Set auto-shutdown level VAS = 1.8V.

Limit the divider current to 100 µA from which

R5 = 33 kΩ

R6 = 18 kΩ

Step 3. Design the output circuit.

Maximum fan motor current = 250 mA. Q1

beta is chosen at 50 from which R7 = 800 Ω.

FIGURE 5-9:

Design Example.

5.8

TC646 as a Microcontroller

Peripheral

In a system containing a microcontroller or other host

intelligence, the TC646 can be effectively managed as

a CPU peripheral. Routine fan control functions can be

performed by the TC646 without controller intervention.

The microcontroller receives temperature data from

one or more points throughout the system. It calculates

a fan operating speed based on an algorithm specifi-

cally designed for the application at hand. The proces-

sor controls fan speed using complementary port bits

I/O1 through I/O3. Resistors R1 through R6 (5% toler-

ance) form a crude 3-bit DAC that translates the 3-bit

code from the processor's outputs into a 1.6V DC con-

trol signal. A monolithic DAC or digital pot may be used

instead of the circuit shown in Figure 5-10.

With VAS set at 1.8V, the TC646 enters auto-shutdown

when the controller's output code is 000[B]. Output

codes 001[B] to 111[B] operate the fan from roughly

40% to 100% of full speed. An open-drain output from

the processor (I/O0) can be used to reset the TC646

following detection of a fault condition. The FAULT out-

put can be connected to the controller's interrupt input,

or to another I/O pin, for polled operation.

FAULT

SENSE

NTC

10 k

Ω

@ 25˚C

20.5 k

Ω

3.83 k

Ω

33 k

Ω

800

Ω

GND

Reset

Shutdown

(Optional)

+12V

+5V

VDD

VIN

VAS

VOUT

RSENSE

2.2

Ω

CSENSE

0.1

µF

TC646

Fan

Fan/Thermal

Fault

0.01

µF

0.01

µF

18 k

Ω

Open-Drain

Device

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