TC654 datasheet(13/38 Pages) MICROCHIP

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DS20001734C-page 13

TC654/TC655

peak of the voltage pulse at the SENSE pins must

exceed the 100 mV (typical) threshold in order for the

pulse to be counted in the fan RPM measurement.

See Section 7.4 “FanSense Network (RSENSE &

CSENSE)” for more details on selecting the appropriate

current sense resistor and coupling capacitor values.

FIGURE 4-7:

Fan Current Sensing.

By selecting F1PPR (bits 2-1<01>) and F2PPR (bits 4-

3<01>) in the Configuration Register, the TC654 and

TC655 can be programmed 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-

OUTPUT1 (RPM1, for SENSE1 input) and RPM-OUT-

PUT2 (RPM2, for SENSE2 input) Registers. These

registers are 9-Bit Read-Only registers which store

RPM data with 25 RPM resolution. By setting RES (bit

6<0>) of the Configuration Register to a ‘1’, the RPM

data can be read with 25 RPM resolution. If this Bit is

left in the default state of '0', the RPM data will only be

readable with resolution of 50 RPMs, which represents

8-Bit data.

The maximum fan RPM reading is 12775 RPM. If this

value is exceeded, counter overflow bits in the Status

in the Status Register represent the RPM1 and RPM2

counter overflow bits for the RPM1 and RPM2 regis-

ters, respectively. These bits 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 RPM1, RPM2 and Status Register

command byte assignments.

4.8

Fan Fault Threshold and

Indication (FAULT)

For the TC654 and TC655 devices, a fault condition

exists whenever a fan’s sensed RPM level falls below

the user programmable threshold. The RPM threshold

values for fan fault detection are set in the FAN_-

FAULT1 and FAN_FAULT2 Registers (8-bit, read/

write).

The RPM threshold represents the fan speed at which

the TC654/TC655 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

thresholds 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. F1F (bit

0<0>) and F2F (bit 1<0>) in the Status Register are set

to ‘1’ for respective low RPM levels on the SENSE1

and SENSE2 inputs. The FAULT output and the fault

bits in the Status Register can be reset by setting

FFCLR (bit 7<0>) in the Configuration Register to a ‘1’.

For the TC655 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 TC654 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 TC654/TC655 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 TC654/TC655 devices are in Shutdown

mode, all functions except for the SMBus interface are

suspended. During this mode of operation, the TC654

and TC655 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’.

SENSE2

SENSE1

VOUT

FAN

RISO1

RISO2

RSENSE2

RSENSE1

CSENSE2

CSENSE1

TC654/

TC655

GND

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