TC647BEOATR datasheet(12/34 Pages) MICROCHIP

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TC642B/TC647B

DS21756B-page 12

 2003 Microchip Technology Inc.

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 TC642B and

TC647B devices, the duty cycle is controlled by either

the VIN or VMIN input, with the higher voltage setting the

duty cycle. 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-oper-

ating condition (fan speed is zero revolutions per minute

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

voltage can not be applied immediately. Since the fan is

at a rest position, the fan’s inertia must be overcome 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 environ-

ments, the fan will start and operate properly. An exam-

ple of the start-up timing is shown in Figure 1-1.

A key feature of the TC642B/TC647B device is the

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

(when the device is brought out of the shutdown mode

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, 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 (see Section 4.5, “Minimum Fan

Speed”, for more details on operation when coming out

of start-up). If pulses are not detected during the start-

up period, the start-up timer is activated again. If pulses

are not detected at the SENSE pin during this addi-

tional start-up period, the FAULT output will go low to

indicate that a fan fault condition has occurred. See

Section 4.7, “FAULT 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.

FIGURE 4-3:

CF Pin Voltage.

The duty cycle of the PWM output is controlled by the

voltage at the VIN input pin (or the VMIN voltage, which-

ever is greater). 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 volt-

age 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 calcula-

tion 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

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

406080

100

Time (msec)

CF = 1 µF

VCMAX

VCMIN

Duty Cycle (%) =

VCMAX - VCMIN

(VIN - VCMIN) * 100

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