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TC646B

PWM Fan Speed Controllers With Auto-Shutdown, Fan Restart and FanSense™ Technology for Fault Detection

Microchip Technology 

TC646B/TC648B/TC649B

12V

VDD

FAN

TC646B VOUT

TC648B

TC649B

D

G

QDRIVE

S

GND

FIGURE 4-2:

PWM Fan Drive.

By modulating the voltage applied to the gate of the

MOSFET (QDRIVE), the voltage that is applied to the

fan is also modulated. When the VOUT pulse is high, the

gate of the MOSFET is turned on, pulling the voltage at

the drain of QDRIVE to zero volts. This places the full

12V across the fan for the ton period of the pulse. When

the duty cycle of the drive pulse is 100% (full on,

ton = t), the fan will run at full speed. As the duty cycle

is decreased (pulse on time “ton” is lowered), the fan

will slow down proportionally. With the TC646B,

TC648B and TC649B devices, the duty cycle is con-

trolled by the VIN input and can also be terminated by

the VAS input (auto-shutdown). This is described in

more detail in Section 5.5, “Output Drive Device

Selection”.

4.3 Fan Start-up

Often overlooked in fan speed control is the actual

start-up control period. When starting a fan from a non-

operating condition (fan speed is zero revolutions per

minute (RPM)), the desired PWM duty cycle or average

fan voltage cannot be applied immediately. Since the

fan is at a rest position, the fan’s inertia must be over-

come to get it started. The best way to accomplish this

is to apply the full rated voltage to the fan for a minimum

of one second. This will ensure that in all operating

environments, the fan will start and operate properly.

An example of the start-up timing is shown in

Figure 1-1.

A key feature of the TC646B/TC648B/TC649B devices

is the start-up timer. When power is first applied to the

device, or when the device is brought out of the shut-

down/auto-shutdown modes of operation, the VOUT

output will go to a high state for 32 PWM cycles (one

second for CF = 1 µF). This will drive the fan to full

speed for this time frame.

During the start-up period for the TC646B and TC649B

devices, the SENSE pin is being monitored for fan

pulses. If pulses are detected during this period, the fan

speed controller will then move to PWM operation. If

pulses are not detected during the start-up period, the

DS21755B-page 12

start-up timer is activated again. If pulses are not

detected at the SENSE pin during this additional

period, the FAULT output will go low to indicate that a

fan fault condition has occurred. See Section 4.7,

“FAULT/OTF Output”, for more details.

4.4 PWM Frequency & Duty Cycle

Control (CF & VIN Pins)

The frequency of the PWM pulse train is controlled by

the CF pin. By attaching a capacitor to the CF pin, the

frequency of the PWM pulse train can be set to the

desired value. The typical PWM frequency for a 1.0 µF

capacitor is 30 Hz. The frequency can be adjusted by

raising or lowering the value of the capacitor. The CF

pin functions as a ramp generator. The voltage at this

pin will ramp from 1.20V to 2.60V (typically) as a saw-

tooth waveform. An example of this is shown in

Figure 4-3.

2.8

CF = 1 µF

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0

20

VCMAX

VCMIN

40

60

80

100

Time (msec)

FIGURE 4-3:

CF Pin Voltage.

The duty cycle of the PWM output is controlled by the

voltage at the VIN input pin. The duty cycle of the PWM

output is produced by comparing the voltage at the VIN

pin to the voltage ramp at the CF pin. When the voltage

at the VIN pin is 1.20V, the duty cycle will be 0%. When

the voltage at the VIN pin is 2.60V, the PWM duty cycle

will be 100% (these are both typical values). The

VIN-to-PWM duty cycle relationship is shown in

Figure 4-4.

The lower value of 1.20V is referred to as “VCMIN” and

the 2.60V threshold is referred to as “VCMAX”. A calcu-

lation for duty cycle is shown in the equation below. The

voltage range between VCMIN and VCMAX is character-

ized as “VCSPAN“ and has a typical value of 1.4V, with

minimum and maximum values of 1.3V and 1.5V,

respectively.

EQUATION

PWM DUTY CYCLE

Duty Cycle (%) = (VIN - VCMIN) * 100

VCMAX - VCMIN

 2003 Microchip Technology Inc.

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