TC642B datasheet(13/34 Pages) MICROCHIP

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 2003 Microchip Technology Inc.

DS21756B-page 13

TC642B/TC647B

FIGURE 4-4:

VIN voltage vs. PWM duty

cycle(Typical).

The PWM duty cycle is also controlled by the VMIN pin

See Section 4.5, “Minimum Speed (VMIN Pin)”, for

more details on this function.

4.5

Minimum Speed (VMIN Pin)

For the TC642B and TC647B devices, pin 3 is the VMIN

pin. This pin is used for setting the minimum fan speed

threshold.

The minimum fan speed function provides a way to set

a threshold for a minimum duty cycle on the VOUT out-

put. This in turn produces a minimum fan speed for the

user. The voltage range for the VMIN pin is the same as

that for the VIN pin (1.20V to 2.60V). The voltage at the

VMIN pin is set in this range so that as the voltage at the

VIN pin decreases below the VMIN voltage, the output

duty cycle will be controlled by the VMIN voltage. The

following equation can be used to determine the neces-

sary voltage at VMIN for a desired minimum duty cycle

on VOUT.

EQUATION

VMIN VOLTAGE

Example: If a minimum duty cycle of 40% is desired,

the VMIN voltage should be set to:

EXAMPLE 4-1:

If the voltage at the VIN pin falls below 1.76V, the duty

cycle of the VOUT output will not decrease below the

40% value that is now set by the voltage at the VMIN

pin. In this manner, the fan will continue to operate at

40% speed even when the temperature (voltage at VIN)

continues to decrease.

For the TC642B and TC647B devices, the VMIN pin is

also used as the shutdown pin. The VSHDN and VREL

threshold voltages are characterized in the “Electrical

Characteristics” table of Section 1.0. If the VMIN pin

voltage is pulled below the VSHDN threshold, the device

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

FAULT pin is inactive). If the voltage on the VMIN pin

then rises above the release threshold (VREL), the

device will go through a Power-Up sequence. The

Power-Up sequence is shown later in Figure 4-9.

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 mode, the VOUT output

is actively held low. The output can be varied from 0%

duty cycle (full off) to 100% duty cycle (full on). As pre-

viously discussed, the duty cycle of the VOUT output is

controlled via the VIN input voltage along with the VMIN

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. 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.

100

1.2

1.4

1.6

1.8

2.2

2.4

2.6

2.8

VIN (V)

VMIN (V) = (DC * 1.4) + 1.20

DC = Desired Duty Cycle

100

VMIN (V) = (40 * 1.4) + 1.20

100

VMIN = 1.76V

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