TC642 datasheet(5/28 Pages) MICROCHIP

5 / 28 page

 2002 Microchip Technology Inc.

DS21444C-page 5

TC642

3.0

DETAILED DESCRIPTION

3.1

PWM

The PWM circuit consists of a ramp generator and

threshold detector. The frequency of the PWM is deter-

mined by the value of the capacitor connected to the CF

input. A frequency of 30 Hz is recommended

(CF = 1 µF). The PWM is also the time base for the

Start-up Timer (see Section 3.4, “Start-Up Timer”). The

PWM voltage control range is 1.25V to 2.65V (typical)

for 0% to 100% output duty cycle.

3.2

FAULT Output

The TC642 detects faults in two ways.

First, pulses appearing at SENSE due to the PWM

turning on are blanked, with the remaining pulses

filtered by a missing pulse detector. If consecutive

pulses are not detected for 32 PWM cycles (

≅1 Sec if

CF = 1 µF), the Diagnostic Timer is activated, and VOUT

is driven high continuously for three PWM cycles

(

≅100 msec if CF = 1 µF). If a pulse is not detected

within this window, the Start-up Timer is triggered (see

Section 3.4). This should clear a transient fault condi-

tion. If the missing pulse detector times out again, the

PWM is stopped and FAULT goes low. When FAULT is

activated due to this condition, the device is latched in

shutdown mode and will remain off indefinitely.

The TC642 may be configured to continuously attempt

fan restarts, if so desired.

Continuous restart mode is enabled by connecting the

FAULT output to VMIN through a 0.01 µF capacitor, as

shown in Figure 3-1. When connected in this manner,

the TC642 automatically attempts to restart the fan

every time a fault condition occurs. When the FAULT

output is driven low, the VMIN input is momentarily

pulled below VSHDN, initiating a reset and clearing the

fault condition. Normal fan start-up is then attempted as

previously described. The FAULT output may be

connected to external logic (or the interrupt input of a

microcontroller) to shut the TC642 down if multiple fault

pulses are detected at approximately one second

intervals. Diode D1, capacitor C1 and resistors R5 and

R6 are provided to ensure fan restarts are the result of

a fan fault and not an over-temperature fault. A CMOS

logic OR gate may be substituted for these

components, if available.

FIGURE 3-1:

Fan Fault Output Circuit.

Note:

At this point, action must be taken to restart

the fan by momentarily pulling VMIN below

VSHDN, or cycling system power. In either

case, the fan cannot remain disabled due

to a fault condition, as severe system dam-

age could result. If the fan cannot be

restarted, the system should be shut down.

FAULT

SENSE

10k

Ω

GND

From

System

Shutdown

Controller

(Optional)

*The parallel combination of R3 and R4 must be >10 kΩ.

+12V

+5V

VDD

VIN

VMIN

VOUT

RBASE

Ω

VDD

RSENSE

CSENSE

µF

TC642

Fan

0.01

µF

From

Temp

Sensor

+5V

0.01

µF

TC642

RESET

Fault

Detected

TC642 Datasheet(PDF) 5 Page - Microchip Technology

TC642 Datasheet(HTML) 5 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL