TC646VOA datasheet(8/28 Pages) MICROCHIP

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TC646

DS21446C-page 8

 2002 Microchip Technology Inc.

4.0

SYSTEM BEHAVIOR

The flowcharts describing the TC646’s behavioral

algorithm are shown in Figure 4-1. They can be

summarized as follows:

4.1

Power-Up

(1) Assuming the device is not being held in auto-shut-

down mode (VIN > VAS)..........

(2) Turn VOUT output on for 32 cycles of the PWM

clock. This ensures that the fan will start from a

dead stop.

(3) During this Start-up Timer, if a fan pulse is

detected, branch to Normal Operation; if none are

received…

(4) Activate the 32-cycle Start-up Timer one more time

and look for fan pulse; if a fan pulse is detected,

proceed to Normal Operation; if none are

received…

(5) Proceed to Fan Fault.

(6) End.

4.2

Normal Operation

“Normal Operation” is an endless loop which may only

be exited by entering shutdown mode, auto-shutdown

mode or Fan Fault. The loop can be thought of as

executing at the frequency of the oscillator and PWM.

(1) Reset the missing pulse detector.

(2) Is the TC646 in shutdown or auto-shutdown

mode?

If so...

a. VOUT duty cycle goes to zero.

b. FAULT is disabled.

c. Exit the loop and wait for VIN > (VAS + VHAS) to

resume operation.

(3) If an over-temperature fault occurs (VIN > VOTF),

activate FAULT; release FAULT when

VIN< VOTF.

(4) Drive VOUT to a duty cycle proportional to VIN on

a cycle by cycle basis.

(5) If a fan pulse is detected, branch back to the

start of the loop (1).

(6) If the missing pulse detector times out …

(7) Activate the 3-cycle Diagnostic Timer and look

for pulses; if a fan pulse is detected, branch

back to the start of the loop (1); if none are

received…

(8) Activate the 32-cycle Start-up Timer and look for

pulses; if a fan pulse is detected, branch back to

the start of the loop (1); if none are received…

(9) Quit Normal Operation and go to Fan Fault.

(10) End.

4.3

Fan Fault

Fan fault is an infinite loop wherein the TC646 is

latched in shutdown mode. This mode can only be

released by a reset (i.e., VIN being brought below

VSHDN, then above (VAS + VHAS), or by power-cycling).

(1) While in this state, FAULT is latched on (low) and

the VOUT output is disabled.

(2) A reset sequence applied to the VIN pin will exit the

loop to Power-up.

(3) End.

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