Datasheet

6-Channel Intelligent Fan Controller

MAX31785

Table 30. MFR_FAN_PWM2RPM

MFR_FAN_RUN_TIME (F7h)

The MFR_FAN_RUN_TIME command returns the number of hours of fan lifetime operation. Once every hour, the device

automatically stores this counter in flash memory. The counter is a 16-bit value that rolls over. The latest stored value

is automatically recalled from flash memory upon device power cycle or RST action or a soft reset. This count can be

reset to zero by writing a sequence of all zeros (0000h), followed by all ones (FFFFh), followed by all zeros (0000h)

within 8ms. This value is not updated when the fan is turned off (0% PWM duty cycle).

MFR_FAN_PWM_AVG (F8h)

The MFR_FAN_PWM_AVG command returns the lifetime average of the fan PWM duty cycle in % duty cycle. This aver-

age is not updated when the fan is turned off (0% PWM duty cycle). When combined with the MFR_FAN_RUN_TIME

information, this duty cycle helps predict the remaining lifetime of the fan. Once every hour, the device automatically

stores this value in flash memory. The 2 data bytes are in DIRECT format. The latest stored value is automatically

recalled from flash memory upon device power cycle or RST action or a soft reset. This count can be reset to zero by

writing a sequence of all zeros (0000h), followed by all ones (FFFFh), followed by all zeros (0000h) within 8ms.

MFR_FAN_PWM2RPM (F9h)

The MFR_FAN_PWM2RPM command is used to configure the table the device uses to drive the fan-health status bits in

the lower nibble of STATUS_FAN_1_2. The table maps four preassigned PWM duty cycles to the expected fan speed

in RPM. Table 30 describes the MFR_FAN_PWM2RPM command.

RPM@PWM: Expected Fan Speed at a Certain PWM Duty Cycle: The expected fan speed at four PWM duty cycles

is used to determine the fan health. The fan speed is set in RPM at each PWM duty-cycle step breakpoint. The valid

fan-speed range is 0 to 32,767 (inclusive). The 2 data bytes are in DIRECT format.

Applications Information

Power-Supply Decoupling

To achieve the best results when using the device,

decouple the V

power supply with a 0.1FF capacitor.

Use a high-quality, ceramic, surface-mount capacitor

if possible. Surface-mount components minimize lead

inductance, which improves performance, and ceram-

ic capacitors tend to have adequate high-frequency

response for decoupling applications.

Decouple the REG25 and REG18 regulator outputs using

1FF and 10nF capacitors (one each per output).

Open-Drain Pins

MSDA, MSCL, SCL, SDA, FAULT, and ALERT are open-

drain pins and require external pullup resistors con-

nected to V

to realize high logic levels.

BYTE NUMBER WORD NAME MEANING

6-7 RPM@100% PWM Expected fan speed for a PWM duty cycle of 100%.

4-5 RPM@80% PWM Expected fan speed for a PWM duty cycle of 80%.

2-3 RPM@60% PWM Expected fan speed for a PWM duty cycle of 60%.

0-1 RPM@40% PWM Expected fan speed for a PWM duty cycle of 40%.