TC649EPA datasheet(15/28 Pages) MICROCHIP

15 / 28 page

 2002 Microchip Technology Inc.

DS21449C-page 15

TC649

5.6

Latch-up Considerations

As with any CMOS IC, the potential exists for latch-up

if signals are applied to the device which are outside

the power supply range. This is of particular concern

during power-up if the external circuitry (such as the

sensor network, VAS divider or shutdown circuit) are

Care should be taken to ensure that the TC649’s VDD

supply powers up first. If possible, the networks

attached to VIN and VAS should connect to the VDD sup-

ply at the same physical location as the IC itself. Even

if the IC and any external networks are powered by the

same supply, physical separation of the connecting

points can result in enough parasitic capacitance and/

or inductance in the power supply connections to delay

one power supply “routing” versus another.

5.7

Power Supply Routing and

Bypassing

Noise present on the VIN and VAS 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 possible.

This is especially true of VIN, which is usually drive from

a high impedance source (such as a thermistor). Addi-

tionally, the VDD input should be bypassed with a 1 µF

capacitor with grounds being kept as short as possible.

To keep fan noise off the TC649 ground pin, individual

ground returns for the TC649 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 auto-shutdown Level.

VAS = 1.8V.

Limit the divider current to 100 µA

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.

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

1 µF

TC649

Fan

Fan Fault

0.01 µF

1 µF

18 k

Ω

Open-

Drain

Device

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