SMBus Fan Control with 1°C Accurate Temperature Monitoring
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
5.6.4
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.
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.
Revision 2.53 (03-13-07)
SMSC EMC2101
DATA2S6HEET
SMSC [ SMSC CORPORATION ]
