EMC2101-R-ACZL-TR datasheet(27/57 Pages) SMSC

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SMBus Fan Control with 1°C Accurate Temperature Monitoring

Datasheet

SMSC EMC2101

Revision 2.54 (06-16-09)

DATASHEET

5.6

Fan Control

The EMC2101 includes either a PWM or a linear DAC based fan driver on the shared FAN pin. Both

PWM and DAC use the Fan Control Look-Up Table and/or Fan Setting Register interchangeably as

well as the Spin-Up Routine.

In addition, the EMC2101 can monitor the fan speed using the ALERT / TACH pin.

5.6.1

DAC Driver

The Linear DAC driver included in the EMC2101 has 6-bits of resolution based on the supply voltage

and is used for linear drive fan circuits. Its advantages over PWM drive circuits include reduced circuit

complexity at the expense of reduced effective signal range.

APPLICATION NOTE: When using the DAC Driver, the pull-up resistor on the FAN pin should be removed.

APPLICATION NOTE: The DAC driver output voltage is controlled by either the Fan Setting Register (see

Section 6.18) or the Fan Control Look-Up Table Registers (see Section 6.22). It is also

controlled by the POLARITY bit (see Section 6.16). The PWM Frequency Register (see

Section 6.19) and PWM Divider Register (see Section 6.20) have no effect on the DAC’s

output voltage range, resolution, or response.

5.6.2

PWM Driver

The PWM driver included in the EMC2101 has, at most, 64 steps equalling 1.5% resolution. The

effective resolution, duty cycle, and frequency are all adjustable based on programmed values. It’s

advantages over linear drive circuits include a large signal range (0% to 100% duty cycle) at the

expense of added complexity on the drive circuit.

The PWM output is open drain and requires a pull-up resistor to VDD.

5.6.3

TACH Monitor

The TACH monitor counts the number of clock pulses that occur between five edges of the TACH

signal. The monitor assumes that the tachometer signal is always valid (such as generated from a 4-

wire fan or a direct drive fan) and that the tachometer signal generates 2 TACH pulses per fan

revolution.

5.6.4

Fan Control Look-Up Table

The EMC2101 uses an 8 entry look-up table to apply a user-programmable fan control profile based

on measured temperature. The user programs the Fan Control Look-Up Table using incrementally

higher temperatures and the desired fan output that should be set when that temperature is reached.

If the measured temperature on the External Diode channel exceeds any of these temperature

thresholds, the fan output will be automatically programmed to the desired setting corresponding to the

exceeded temperature. When the measured temperature drops to a point below any lower threshold

minus the hysteresis value, the fan output will be set to the corresponding lower set point.

Figure 5.7 shows an example of this operation.

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