MAX31785 MAXIM [Maxim Integrated Products], MAX31785 Datasheet - Page 42

• Manufacturer Part Number: MAX31785
• Description: 6-Channel Intelligent Fan Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

6-Channel Intelligent Fan Controller
Table 26. MFR_FAN_CONFIG (continued)
Note: It is recommended that the fan be disabled prior to changing MFR_FAN_CONFIG.
42
BIT
7:5
1:0
4
3
2
ROTOR_HI_LO
BIT NAME
RAMP[2:0]
SPIN[1:0]
HEALTH
ROTOR
The RAMP bits select how fast the device ramps the PWM from one duty cycle to another (either up
or down). In PWM mode, the following table always applies. In RPM mode, the fan speed is read
either every 200ms or 1000ms and when the reported fan speed is within 20% of the target speed,
the maximum allowed PWM duty cycle change is set to 1%. In RPM mode, the PWM duty cycle is
not changed as long as the fan is within Q5% of the target speed.
Determines if a locked rotor indication is active low or active high. This bit is ignored if
ROTOR = 0.
0 = ROTOR is active low (TACH input is low if the rotor stops).
1 = ROTOR is active high (TACH input is high if the rotor stops).
The ROTOR bit selects if the fan does not have a tachometer but rather a stalled (or locked)
rotor output.
The SPIN bits determine how the device spins up (or starts) the fan from a dead stop. To over-
come the initial mechanical fan inertia, the device can be programmed to drive the fan at 100%
duty cycle until a programmable number of fan revolutions (cumulative count) is detected or a
locked rotor signal is negated. The device allows a 2s startup period during which the fan speed
monitors are disabled. If after 2s the fan does not respond, the PWM output remains at 100%
duty cycle (if TACHO = 0) or goes to 0% duty cycle (if TACHO = 1).
The HEALTH bit controls the automatic checking of the fan health. The fan-health diagnostic can
be enabled by setting this bit to 1.
0 = Health meter function is disabled.
1 = Health meter function is enabled.
RAMP2
ROTOR
SPIN1
0
0
0
0
1
1
1
1
0
1
0
0
1
1
RAMP1
Stalled/locked rotor
0
0
1
1
0
0
1
1
FAN OUTPUT
Tachometer
detect
SPIN0
RAMP0
0
1
0
1
0
1
0
1
0
1
0
1
CYCLE UPDATE
TACH input expects fan RPM
TACH input expects locked rotor signal. The polarity is
selected with the ROTOR_HI_LO bit (also set
MFR_FAN_FAULT_LIMIT = 0001h).
Automatic spin-up disabled
Two revolutions or locked rotor negated
Four revolutions or locked rotor negated
Eight revolutions or locked rotor negated
PWM DUTY
RATE (ms)
MEANING
1000
1000
1000
200
200
200
200
200
SPIN-UP RELAXATION CRITERIA
MAX31785 CONFIGURATION
MAX PWM DUTY
CYCLE CHANGE
ALLOWED (%)
1
2
3
1
2
3
4
5
100% PWM DUTY
CYCLE (seconds)
TIME TO RAMP
FROM 40% TO
2.4
60
30
20
12
6
4
3