ADM1029
http://onsemi.com
20
Speed Control Mode by writing to the three Temperature
Cooling Action Registers (Registers 0x48, 0x49, 0x4A).
The device powers up in Automatic Fan Speed Control
Mode by default, as long as the T
MIN
/Install pin (Pin 18)
does not have the disable option selected (T
MIN
/Install pin
tied low or high). The default setting is that both fans will run
at the fastest speed calculated by all three temperature
channels. The control mode offers flexibility in that the user
can decide which temperature channel/channels control
each fan (five options).
Table 11. AUTOMATIC MODE FAN BEHAVIOR
Option Temperature Cooling Action
1 Bit 0 Register 0x49 and/or Bit 1 Reg 0x4A =
Remote Temp 1 Controls Fan 1, Remote Temp 2
Controls Fan 2
2 Bit 0 Register 0x48 and Bit 1 Register 0x48 = 1
Local Temp Controls Fan 1 and/or Fan 2
3 Bit 0 Register 0x49 and Bit 1 Register 0x49 =
Remote Temp 1 Controls Fan 1 and/or Fan 2
4 Bit 0 Register 0x4A and Bit 1 Register 0x4A =
Remote Temp 2 Controls Fan 1 and/or Fan 2
5 Bits 0, 1 Reg 0x48, 0x49, 0x4A = 1 Max Speed
Calculated by Local and Remote Temperature
Channels Controls Fans 1 and/or 2
When Option 5 is chosen, this offers increased flexibility.
The Local and Remote temperature channels can have
independently programmed control loops with different
control parameters. Whichever control loop calculates the
fastest fan speed based on the temperature being measured,
drives both fans.
Figures 32 and 33 show how the fan’s PWM duty cycle is
determined by two independent control loops. This is the
type of Automode Fan Behavior seen when Bits 0 and 1 of
all three Temperature Cooling Action Registers = 11.
Figure 32 shows the control loop for the Local Temperature
channel. Its T
MIN
value has been programmed to 20C, and
its T
RANGE
value is 40C.
33
40
47
53
60
66
73
80
87
93
100
Figure 32. Max Speed Calculated by Local
Temperature Control Loop Drive Fan
LOCAL TEMPERATURE (C)
PWM DUTY CYCLE (%)
T
MAX
= T
MIN
+ T
RANGE
T
MIN
02040 60
T
RANGE
= 40C
Figure 33. Max Speed Calculated by Remote
Temperature Control Loop Drive Fan
33
40
47
53
60
66
73
80
87
93
100
PWM DUTY CYCLE (%)
REMOTE TEMPERATURE (C)
T
MAX
= T
MIN
+ T
RANGE
T
MIN
02040 7080
The local temperature’s T
MAX
will thus be 60C.
Figure 33 shows the control loop for the Remote 1
Temperature channel. Its T
MIN
value has been set to 0C,
while its T
RANGE
= 80C. Therefore, the Remote 1
Temperature’s T
MAX
value will be 80C.
If both temperature channels measure 40C, both control
loops will calculate a PWM duty cycle of 66%. Therefore,
the fans will be driven at 66% duty cycle.
If both temperature channels measure 20C, the local
channel will calculate 33% PWM duty cycle, while the
Remote 1 channel will calculate 50% PWM duty cycle.
Thus, the fans will be driven at 50% PWM duty cycle.
Consider the local temperature measuring 60C, while the
Remote 1 temperature is measuring 70C. The PWM duty
cycle calculated by the local temperature control loop will
be 100% (since the temperature = T
MAX
). The PWM duty
cycle calculated by the Remote 1 temperature control loop
at 70C will be approximately 90%. So the fans will run full
speed (100% duty cycle). Remember that the fan speed will
be based on the fastest speed calculated, and is not
necessarily based on the highest temperature measured.
Depending on the control loop parameters programmed, a
lower temperature on one channel may actually calculate a
faster speed than a higher temperature on another channel.
Programming the Automatic Fan Speed Control Loop
1. Program a value for T
MIN
2. Program a value for the slope T
RANGE
3. T
MAX
= T
MIN
+ T
RANGE
4. Program a value for Fan Spin-up Time
5. Program the desired Automatic Fan Speed Control
Mode Behavior, i.e., which temperature channel
controls each fan
Other Control Loop Parameters?
Having programmed all the above loop parameters, are
there any other parameters to worry about?
