EVAL-ADM1026EB datasheet(24/56 Pages) AD

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ADM1026

Rev. A | Page 24 of 56

Automatic Fan Speed Control

The ADM1026 offers a simple method of controlling fan speed

according to temperature without intervention from the host

processor. Monitoring must be enabled by setting Bit 0 of

Configuration Register 1 (Address 00h), to enable automatic fan

speed control. Automatic fan speed control can be applied to the

DAC output, the PWM output, or both, by setting Bit 5 and/or

Bit 6 of Configuration Register 1.

The TMIN registers (Addresses 10h to 12h) contain minimum

temperature values for the three temperature channels (on-chip

sensor and two remote diodes). This is the temperature at which

a fan starts to operate when the temperature sensed by the

controlling sensor exceeds TMIN. TMIN can be the same or

different for all three channels. TMIN is set by writing a twos

complement temperature value to the TMIN registers. If any

sensor channel is not required for automatic fan speed control,

TMIN for that channel should be set to 127°C (01111111).

In automatic fan speed control mode, (as shown Figure 42 and

Figure 43) the four MSBs of the DAC control register (Address

04h) and PWM control register (Address 05h) set the minimum

values for the DAC and PWM outputs. Note that, if both DAC

control and PWM control are enabled (Bits 5 and 6 of

Configuration Register 1 = 1), the four MSBs of the DAC

control register (Address 04h) define the minimum fan speed

values for both the DAC and PWM outputs. The value in the

PWM control register (Address 05h) has no effect.

Minimum DAC Code DACMIN = 16 × D

256

Code

Voltage

Output

DAC

Minimum PWM Duty Cycle PWMMIN = 6.67 × D

where D is the decimal equivalent of Bits 7 to 4 of the register.

When the temperature measured by any of the sensors exceeds

the corresponding TMIN, the fan is spun up for 2 seconds with

the fan drive set to maximum (full scale from the DAC or 100%

PWM duty cycle). The fan speed is then set to the minimum as

previously defined. As the temperature increases, the fan drive

increases until the temperature reaches TMIN + 20°C.

The fan drive at any temperature up to 20°C above TMIN is

given by

()

100

MIN

ACTUAL

MIN

PWM

−

()

240

MIN

ACTUAL

MIN

DAC

−

For simplicity of the automatic fan speed algorithm, the DAC

code increases linearly up to 240, not its full scale of 255.

However, when the temperature exceeds TMIN +20°C, the DAC

output jumps to full scale. To ensure that the maximum cooling

capacity is always available, the fan drive is always set by the

sensor channel demanding the highest fan speed.

If the temperature falls, the fan does not turn off until the

temperature measured by all three temperature sensors has

fallen to their corresponding TMIN − 4°C. This prevents the fan

from cycling on and off continuously when the temperature is

close to TMIN.

Whenever a fan starts or stops during automatic fan speed

control, a one-off interrupt is generated at the INT output. This

is described in more detail in the section on the ADM1026

Interrupt Structure.

PWM

OUTPUT

MIN

TEMPERATURE

TMIN

SPIN UP FOR 2 SECONDS

100%

TMIN – 4°C

TMIN + 20°C

Figure 42. Automatic PWM Fan Control Transfer Function

DAC

OUTPUT

MIN

TEMPERATURE

TMIN

255

TMIN – 4°C

TMIN + 20°C

SPIN UP FOR 2 SECONDS

240

Figure 43. Automatic DAC Fan Control Transfer Function

Fan Inputs

Pins 3 to 6 and 9 to 12 may be configured as fan speed

measuring inputs by clearing the corresponding bit(s) of

Configuration Register 2 (Address 01h), or as general-purpose

logic inputs/outputs by setting bits in this register. The power-

on default value for this register is 00h, which means all the

inputs are set for fan speed measurement.

EVAL-ADM1026EB Datasheet(PDF) 24 Page - Analog Devices

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