MAX31785ETL+ Maxim Integrated Products, MAX31785ETL+ Datasheet - Page 22

• Manufacturer Part Number: MAX31785ETL+
• Description: Board Mount Temperature Sensors 6Ch Intelligent Fan Controller
• Manufacturer: Maxim Integrated Products

Specifications of MAX31785ETL+

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Table 7. Fan Control Operation Modes
Note: The RPM modes should only be used with fans that provide a tachometer output.
6-Channel Intelligent Fan Controller
Fan control has four operational modes. The mode is
determined by the combination of FAN_COMMAND_1
and bit 6 of FAN_CONFIG_1_2 (see Table 7). Fan control
can be disabled by setting bit 7 in FAN_CONFIG_1_2 to
zero.
In dual fan applications operating in RPM mode, the
tachometer selected when TACHSEL = 0 is close-
loop-controlled to the target RPM. Once PWM ramp-
ing is complete, TACHSEL toggles between the two
tachometers every 500ms for monitoring purposes. The
slower of the two tachometer signals is reported by
READ_FAN_SPEED_1 and is used as a comparison for
fan faults and warning. If one of the two tachometer sig-
nals operate at a slower speed, it is recommended that
the slower tachometer be presented to the TACH input
when TACHSEL = 0. If more than one fan channel is
configured for dual fan operation, these fans must com-
plete ramping to the target fan speed before the device
begins to toggle the TACHSEL output and monitor both
tachometers.
In the automatic mode, the fan is controlled in a closed
loop based on the controlling temperature (the highest
postnormalized temperature reading) and the associ-
ated fan control PWM duty cycle (in %) or fan speed
(in RPM). These parameters are assigned in the fan
lookup table (LUT). See the MFR_FAN_LUT description
for configuration details. When a controlling temperature
22
FAN CONTROL
Automatic PWM
Automatic RPM
Manual PWM
Manual RPM
MODE
Automatic Fan Control Operation
External host controls the fan speed by directly setting the fan
PWM duty-cycle values.
External host controls the fan speed by setting target fan speed
values. The device reads the actual fan speed and close loop
adjusts the output fan PWM to match the target fan speed.
The device sets the output PWM based on the fan LUT that
maps the temperature sensor readings to the required fan PWM
duty-cycle values.
The device reads the actual fan speed and close loop adjusts
the output fan PWM to match the target fan speed based on
the fan LUT that maps the temperature sensor readings to the
required fan speed.
Fan Control Operation
Dual Fan Applications
FAN OPERATIONAL DETAILS
exceeds the temperature level programmed in the LUT,
the device outputs a PWM duty cycle or adjusts the fan
speed associated with that temperature. See Figure 1 for
an example.
One or all of the 11 available temperature sensors can
be used to control the fan speed. Each temperature
sensor has an offset adjustment that allows monitoring
specific temperature zones with different thermal char-
acteristics. In Figure 1, remote diode temperature sensor
0 is monitoring a zone that is 15NC more sensitive than
the zone that the I
To keep the audible noise and fan power consumption
as low as possible, the device allows each temperature
sensor to have a temperature offset added. This allows
temperature zones with different thermal profiles to con-
trol the fan at the lowest possible speed to maintain the
required temperature.
If no temperature sensors are assigned to control the fan,
the output fan PWM signal is ramped to 100% duty cycle.
In Figure 2, at temperature sample 1, the required fan
speed is at the level associated with temperature level
2 and since temperature sample 1 is above temperature
level 3, the fan PWM duty cycle needs to be increased
to increase the fan speed. The device increases the fan
PWM duty cycle at a rate controlled by the RAMP bits in
the MFR_FAN_CONFIG command code. If the PWM duty
cycle has not reached the target value before the tem-
perature sample detects that a new PWM target value is
needed, the device stops moving toward the old target
and starts moving to the new target according to the
programmed ramp rate.
FAN_CONFIG_1_2
2
C temperature sensor 3 is measuring.
BIT 6 OF
0
1
0
1
FAN_COMMAND_1
0000h to 7FFFh
0000h to 7FFFh
8000h to FFFFh
8000h to FFFFh
VALUE IN