TC642EPA datasheet(15/28 Pages) MICROCHIP

15 / 28 page

 2002 Microchip Technology Inc.

DS21444C-page 15

TC642

5.7

Power Supply Routing and

Bypassing

Noise present on the VIN and VMIN inputs may cause

erroneous operation of the FAULT output. As a result,

these inputs should be bypassed with a 0.01

µF

capacitor mounted as close to the package as is possi-

ble. This is particularly true of VIN, which is usually

driven from a high impedance source (such as a ther-

mistor). In addition, the VDD input should be bypassed

with a 1 µF capacitor. Grounds should be kept as short

as possible. To keep fan noise off the TC642 ground

pin, individual ground returns for the TC642 and the low

side of the fan current sense resistor should be used.

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 minimum fan speed VMIN = 1.8V.

Limit the divider current to 100 µA from which

R5 = 33 kΩ and 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.

FAULT

SENSE

NTC

10 k

Ω

@ 25˚C

20.5 k

Ω

3.83 k

Ω

33 k

Ω

800

Ω

18 k

Ω

10 k

Ω

GND

Fan

Shutdown

(Optional)

+12V

+5V

VDD

VIN

VMIN

VOUT

RSENSE

2.2

Ω

CSENSE

0.1

µF

TC642

Fan

0.01 µF

0.01

µF

Fan/

Thermal Fault

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