TC648

DS21448C-page 14

 2002 Microchip Technology Inc.

FIGURE 5-7:

Powering the Fan from a -12V Supply.

5.5

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

powered by a supply different from that of the TC648.

supply powers up first. If possible, the networks

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.

5.6

Power Supply Routing and

Bypassing

erroneous operation of the OTF output. As a result,

these inputs should be bypassed with a 0.01 µF capac-

itor mounted as close to the package as possible. This

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

capacitor and grounds should be kept as short as pos-

sible. To keep fan noise off the TC648 ground pin, indi-

vidual ground returns for the TC648 and the low side of

the fan drive device should be used.

Auto-Shutdown Mode Design Example

having a resistance of 10 k

and 4.65 k

Step 2. Set auto-shutdown level.

Limit the divider current to 100 µA

Step 3. Design the output circuit

Maximum fan motor current = 250 mA.

5.7

Minimum Speed Mode Design

Example

Given:

Minimum speed = 40%(1.8V)

Thermistor = 100 k

Ω at 25°C

7.87 k

Ω)

Calculate R2:

4.02 k

Ω)

GND

+5V

-12V

TC648

Fan

2.2 k

Ω

NOTE: *Value depends on the specific application and is shown for example only.

12.0V

Zener

10 k

Ω