TC664 datasheet(13/36 Pages) MICROCHIP

13 / 36 page

 2002 Microchip Technology Inc.

DS21737A-page 13

TC664/TC665

See Section 7.4 for more details on selecting the

appropriate current sense resistor and coupling

capacitor values.

FIGURE 4-7:

Fan Current Sensing.

By selecting FPPR (bits 2-1<01>) in the Configuration

grammed to calculate RPM data for fans with 1, 2, 4 or

8 current pulses per rotation. The default state

assumes a fan with 2 pulses per rotation.

The measured RPM data is then stored in the RPM-

OUTPUT (RPM) Register. This register is a 9-Bit read

only register which stores RPM data with 25 RPM res-

olution. By setting RES (bit 6<0>) of the Configuration

25 RPM resolution. If this bit is left in the default state

of '0', the RPM data will only be readable with resolu-

tion of 50 RPMs, which represents 8-bit data.

The maximum fan RPM reading is 12775 RPM. If this

value is exceeded, a counter overflow bit in the Status

represents the RPM counter overflow bit for the RPM-

OUTPUT Register. This bit will automatically be reset

to zero if the fan RPM reading has been below the max-

imum value of 12775 RPM for 2.4 seconds.

See Table 6-1 for RPM and Status Register command

byte assignments.

4.8

Fan Fault Threshold and

Indication (FAULT)

For the TC664/TC665 devices, a fault condition exists

whenever a fan’s sensed RPM level falls below the

user programmable threshold. The RPM threshold

value for fan fault detection is set in the FAN_FAULT

The RPM threshold represents the fan speed at which

the TC664/TC665 devices will indicate a fan fault. This

threshold can be set at lower levels to indicate fan

locked rotor conditions, or set to higher levels to give

indications for predictive fan failure. It is recommended

that the RPM threshold be at least 10% lower than the

minimum fan speed which occurs at the lowest duty

cycle set point. The default value for the fan RPM

threshold is 500 RPM. If the fan's sensed RPM is less

than the fan fault threshold for 2.4 seconds (typical), a

fan fault condition is indicated.

When a fault condition, due to low fan RPM, occurs, a

logic low is asserted at the FAULT output and the FF

(bit 0<0>) in the Status Register is set to '1'. The FAULT

output and the fault bit in the Status Register can be

reset by setting FFCLR (bit 7<0>) in the Configuration

For the TC665 device, a fault condition is also indicated

when an Over Temperature Fault condition occurs.

This condition occurs when the VOUT duty cycle

exceeds the 100% value, indicating that no additional

cooling capability is available. For this condition, a logic

low is asserted at the FAULT output and OTF (bit 5<X>)

of the Status Register, the over temperature fault indi-

cator, is set to a ‘1’ (The TC664 also indicates an over

temperature condition via the OTF bit in the status reg-

ister). If the duty cycle then decreases below 100%, the

FAULT output will be released and OTF (bit 5<X>) of

the Status Register will be reset to ‘0’.

4.9

Low Power Shutdown Mode

Some applications may have operating conditions

where fan cooling is not required as a result of low

ambient temperature or light system load. During these

times it may be desirable to shut the fans down to save

power and reduce system noise.

The TC664/TC665 devices can be put into a low power

shutdown mode by setting SDM (bit 0<0>) in the Con-

figuration Register to a ‘1’ (this bit is the shutdown bit).

When the TC664/TC665 devices are in shutdown

mode, all functions except for the SMBus interface are

suspended. During this mode of operation, the TC664

and TC665 devices will draw a typical supply current of

only 5 µA. Normal operation will resume as soon as Bit

0 in the Configuration Register is reset to ‘0’.

When the TC664/TC665 devices are brought out of a

shutdown mode by resetting SDM (bit 0<0>) in the

Configuration Register, all of the registers, except for

the Configuration and FAN_FAULT Registers assume

SENSE

VOUT

FAN

RISO

RSENSE

CSENSE

TC664

TC665

GND

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