ADM1031
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14
Fan Spin-Up
As mentioned in the How Does the Control Loop Work?
section, once the temperature being measured exceeds the
T
MIN
value programmed, the fan turns on at minimum speed
(default = 33% duty cycle). However, the problem with fans
being driven by PWM is that 33% duty cycle is not enough
to reliably start the fan spinning. The solution is to spin the
fan up for a predetermined time, and once the fan has spun
up, its running speed can be reduced in line with the
temperature being measured.
The ADM1031 allows fan spin-up times between 200 ms
and 8 seconds. Bits <2:0> of Fan Characteristics Register 1
(Register 020) and Fan Characteristic Register 2 (Register
021) program the fan spin-up times.
Table 9. FAN SPIN-UP TIMES
Bits 2:0
Spin-Up Time
(Fan Characteristics Registers 1, 2)
000 200 ms
001 400 ms
010 600 ms
011 800 ms
100 1 sec
101 2 sec (Default)
110 4 sec
111 8 sec
Once the automatic fan speed control loop parameters
have been chosen, the ADM1031 device can be
programmed. The ADM1031 is placed into automatic fan
speed control mode by setting Bit 7 of Configuration
Register 1 (Register 000). The device powers up in
automatic fan speed control mode by default. The control
mode offers further flexibility in that the user can decide
which temperature channel/channels control each fan.
Table 10. AUTO MODE FAN BEHAVIOR
Bits 6, 5 Control Operation (Configuration Register 1)
00 Remote Temperature 1 Controls Fan 1
Remote Temperature 2 Controls Fan 2
01 Remote Temperature 1 Controls Fan 1 and 2
10 Remote Temperature 2 Controls Fan 1 and 2
11 Maximum Speed Calculated by Local and Remote
Temperature Channels Controls Fans 1 and 2
When Bit 5 and Bit 6 of Configuration Register 1 are both
set to 1, increased flexibility is offered. 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 the fans.
Figures 24 and 25 show how the fan’s PWM duty cycle is
determined by two independent control loops. This is the
type of auto mode fan behavior seen when Bit 5 and Bit 6 of
Configuration Register 1 are set to 11. Figure 24 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. The local temperature’s T
MAX
is thus 60C.
Figure 25 shows the control loop for the remote temperature
channel. Its T
MIN
value has been set to 0C, while its
T
RANGE
=80C. Therefore, the remote temperature’s
T
MAX
value is 80C.
Consider if both temperature channels measure 40C.
Both control loops calculate a PWM duty cycle of 66%.
Therefore, the fan is driven at 66% duty cycle. If both
temperature channels measure 20C, the local channel
calculates 33% PWM duty cycle, while the Remote 1
channel calculates 50% PWM duty cycle. Thus, the fans are
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 is 100%
(because the temperature = T
MAX
). The PWM duty cycle
calculated by the Remote 1 temperature control loop at 70C
is approximately 90%. Therefore, the fan runs full-speed
(100% duty cycle). Remember, that the fan speed is 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, can actually calculate a faster speed than a higher
temperature on the other channel.
33
40
47
53
60
66
73
80
87
93
100
Figure 24. Max Speed Calculated by Local
Temperature Control Loop Drives Fan
LOCAL TEMPERATURE (C)
PWM DUTY CYCLE (%)
T
MAX
= T
MIN
+ T
RANGE
T
MIN
02040 60
T
RANGE
= 40C
