ADM1027 datasheet(35/56 Pages) AD

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REV. A

ADM1027

–35–

Step 7

Determine THYST for Each Temperature Channel

THYST is the amount of extra cooling a fan provides after the

measured temperature has dropped back below TMIN before the

fan turns off. The premise for temperature hysteresis (THYST)

is that without it, the fan would merely chatter, or cycle on and

off regularly, whenever the temperature is hovering about the

TMIN setting.

The THYST value chosen will determine the amount of time

needed for the system to cool down or heat up, as the fan is

turning on and off. Values of hysteresis are programmable in

the range 1

∞C to 15∞C. Larger values of THYST prevent the fans

chattering on and off as previously described. The THYST default

value is 4

∞C.

Hysteresis Registers

Reg. 0x6D Remote 1, Local Hysteresis Register

<7:4> = Remote 1 temperature hysteresis (default = 4

∞C)

<3:0> = Local temperature hysteresis (default = 4

∞C)

Reg. 0x6E Remote 2 Temperature Hysteresis Register

<7:4> = Remote 2 temperature hysteresis (default = 4

∞C)

Note that in some applications it is required that the fans not

turn off below TMIN, but remain running at PWMMIN. Bits <7:5>

of Enhance Acoustics Register 1 (Reg. 0x62) allow the fans to

be turned off or kept spinning below TMIN. If the fans are always

on, the THYST value has no effect on the fan when the tempera-

ture drops below TMIN.

Enhance Acoustics Register 1 (Reg. 0x62)

Bit 7 (MIN3) = 0, PWM3 is OFF (0% PWM duty cycle) when

temperature is below TMIN – THYST.

Bit 7 (MIN3) = 1, PWM3 runs at PWM3 minimum duty cycle

below TMIN – THYST.

Bit 6 (MIN2) = 0, PWM2 is OFF (0% PWM duty cycle) when

temperature is below TMIN – THYST.

Bit 6 (MIN2) = 1, PWM2 runs at PWM2 minimum duty cycle

below TMIN – THYST.

Bit 5 (MIN1) = 0, PWM1 is OFF (0% PWM duty cycle) when

temperature is below TMIN – THYST.

Bit 5 (MIN1) = 1, PWM1 runs at PWM1 minimum duty cycle

below TMIN – THYST.

100%

RANGE

HYST

MIN

TTHERM

TACHOMETER 1

MEASUREMENT

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

TACHOMETER 2

MEASUREMENT

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

TACHOMETERS 3

AND 4

MEASUREMENT

PWM

GENERATOR

PWM

GENERATOR

RAMP

CONTROL

(ACOUSTIC

ENHANCEMENT)

PWM

GENERATOR

PWM

CONFIG

PWM

MIN

MUX

THERMAL CALIBRATION

100%

TMIN

TRANGE

THERMAL CALIBRATION

100%

TMIN

TRANGE

THERMAL CALIBRATION

100%

TMIN

TRANGE

PWM

MIN

PWM

MIN

PWM

CONFIG

PWM

CONFIG

REMOTE 1 =

AMBIENT TEMP

LOCAL =

VRM TEMP

REMOTE 2 =

CPU TEMP

PWM1

TACH1

CPU

FANSINK

FRONT

CHASSIS

REAR

CHASSIS

PWM3

TACH3

PWM2

TACH2

Figure 36. The THYST Value Applies to Fan On/Off Hysteresis and THERM Hysteresis

ADM1027 Datasheet(PDF) 35 Page - Analog Devices

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