TC652 datasheet(6/14 Pages) MICROCHIP

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TC652/TC653

DS21453B-page 6

3.2

Start-Up Timer

To ensure reliable fan start-up, the Start-up Timer turns

PWM high for about 2 seconds whenever the fan is

started from the off state. This occurs at power-up and

when coming out of Shutdown mode.

3.3

SENSE Input

(FanSense™ Technology)

The SENSE input, Pin 5, is connected to a low value

current sensing resistor in the ground return leg of the

fan circuit through the capacitor. During normal fan

operation, commutation occurs as each pole of the fan

is energized. This causes brief interruptions in the fan

current, seen as pulses across the sense resistor. If the

device is not in Shutdown mode, and pulses are not

appearing at the SENSE input, a FAULT exists. The

short, rapid change in fan current (high di/dt) causes a

corresponding dv/dt across the sense resistor, RSENSE.

The waveform on RSENSE is differentiated and con-

verted to a logic-level pulse-train by CSENSE and the

internal signal processing circuitry. The presence and

frequency of this pulse-train is a direct indication of fan

operation.

3.4

FAULT

This pin goes low to indicate a fan FAULT condition.

Pulses appearing at SENSE pin due to the PWM turn-

ing on are blanked and the remaining pulses are

filtered by a Missing Pulse Detector. If consecutive

pulses are not detected for 32 PWM cycles (about 2

sec), the PWM is Low and FAULT goes low. FAULT can

be disabled by momentarily toggling SHDN or VDD pin,

or cycling system power. FAULT remains high during

Shutdown mode.

3.5

Over-Temperature Alert (TOVER)

This pin goes low when the TH set point is exceeded

by 10°C (typical). This indicates that the fan is at

maximum drive, and the potential exists for system

overheating: either heat dissipation in the system has

gone beyond the cooling system's design limits, or

some FAULT exists such as fan bearing failure or an

airflow obstruction. This output may be treated as a

“System Overheat” warning and used to trigger system

shutdown, or bring other fans to full speed in the

system. The fan will continue to run at 100% speed

while TOVER is asserted. Built-in hysteresis prevents

TOVER from “chattering” when measured temperature

is at or near the TH + 10°C trip point. As temperature

falls through the TH + 10°C trip point, hysteresis

maintains the TOVER output low until measured

temperature is 5°C above the trip point setting.

3.6

Shutdown (SHDN)

The fan can be unconditionally shutdown by pulling low

the SHDN pin. During shutdown, FAULT output is high

and PWM output is low. This is ideal for notebook

computers and other portable applications when you

need to change batteries and must not have the fan

running at that time. Thermal monitoring and TOVER are

still in operation during shutdown. IDD shutdown current

is around 50

µA.

3.7

Auto-Shutdown Mode

The TC653 has auto-shutdown. If the temperature is

below the factory set point at minimum speed (TL),

PWM is low and the fan is automatically shut off (Auto-

shutdown mode). This feature is ideal for notebook

computers and other portable applications that need to

conserve as much battery power as possible and thus

run a fan when it is only absolutely needed. The TC653

will continue to be active so as to monitor temperature

for TOVER. The TC653 exits Auto-shutdown mode

when the temperature rises above the factory set point

(T1).

3.8

Temperature Selection Guide

(Minimum Fan Speed/Full Speed)

The five temperature regions defined by the six

thresholds are defined in the TC652/TC653 by means

of factory trimming. Once a TL and TH are set, the T1 –

T4 thresholds are automatically equally spaced

between TL and TH.

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