TC649B datasheet(13/36 Pages) MICROCHIP

13 / 36 page

 2003 Microchip Technology Inc.

DS21755B-page 13

TC646B/TC648B/TC649B

For the TC646B, TC648B and TC649B devices, the VIN

pin is also used as the shutdown pin. The VSHDN and

VREL threshold voltages are characterized in the “Elec-

trical Characteristics Table” of Section 1.0. If the VIN pin

voltage is pulled below the VSHDN threshold, the device

will shut down (VOUT output goes to a low state, the

FAULT/OTF pin is inactive). If the voltage on the VIN pin

then rises above the release threshold (VREL), the

device will go through a power-up sequence (assuming

that the VIN voltage is also higher than the voltage at

the VAS pin). The power-up sequence is shown later in

the “Behavioral Algorithm Flowcharts” of Section 4.9.

FIGURE 4-4:

VIN Voltage vs. PWM Duty

Cycle (Typical).

4.5

Auto-Shutdown Mode (VAS)

For the TC646B, TC648B and TC649B devices, pin 3

is the VAS pin and is used for setting the auto-shutdown

threshold voltage.

The auto-shutdown function provides a way to set a

threshold voltage (temperature) at which the fan will be

shut off. This way, if the temperature in the system

reaches a threshold at which the fan(s) no longer needs

to operate, the fan can be shutdown automatically.

The voltage range for the VAS pin is the same as the

voltage range for the VIN pin (1.20V to 2.60V). The volt-

age at the VAS pin is set in this range so that when the

voltage at the VIN pin decreases below the voltage at

the VAS pin (signifying that the threshold temperature

has been reached), the VOUT output is shut off (goes to

a low state). In auto-shutdown, the FAULT/OTF output

is inactive (high-impedance). Auto-shutdown mode is

exited when the VIN voltage exceeds the VAS voltage

by the auto-shutdown hysteresis voltage (VHAS). Upon

exiting auto-shutdown mode, the start-up timer is

triggered and the device returns to normal operation.

4.6

VOUT Output (PWM Output)

The VOUT output is a digital output designed for driving

the base of a transistor or the gate of a MOSFET. The

VOUT output is designed to be able to quickly raise the

base current or the gate voltage of the external drive

device to its final value.

When the device is in shutdown/auto-shutdown mode,

the VOUT output is actively held low. The output can be

varied from 0% (full off) to 100% duty cycle (full on). As

previously discussed, the duty cycle of the VOUT output

is controlled via the VIN input voltage and can be termi-

nated based on the VAS voltage.

A base current-limiting resistor is required when using

a transistor as the external drive device in order to limit

the amount of drive current that is drawn from the VOUT

output.

The VOUT output can be directly connected to the gate

of an external MOSFET. One concern when doing this,

though, is that the fast turn-off time of the fan drive

MOSFET can cause a problem because the fan motor

looks like an inductor. When the MOSFET is turned off

quickly, the current in the fan wants to continue to flow

in the same direction. This causes the voltage at the

drain of the MOSFET to rise. If there aren’t any clamp

diodes internal to the fan, this voltage can rise above

the drain-to-source voltage rating of the MOSFET. For

this reason, an external clamp diode is suggested. This

is shown in Figure 4-5.

FIGURE 4-5:

Clamp Diode for Fan.

4.7

FAULT/OTF Output

The FAULT/OTF output is an open-drain, active-low

output. For the TC646B and TC649B devices, pin 6 is

labeled as the FAULT output and indicates when a fan

fault condition has occurred. For the TC646B device,

the FAULT output also indicates when an over-temper-

ature (OTF) condition has occurred. For the TC648B

device, pin 6 is the OTF output that indicates an over-

temperature (OTF) condition has occurred.

100

1.2

1.4

1.6

1.8

2.2

2.4

2.6

2.8

VIN (V)

GND

RSENSE

VOUT

Q1: N-Channel MOSFET

FAN

Clamp Diode

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