ADT7476A
www.onsemi.com
43
Selecting T
RANGE
The T
RANGE
value can be selected for each temperature
channel: Remote 1, Local, and Remote 2 temperature.
Bits [7:4] (T
RANGE
) of Register 0x5F to Register 0x61
define the T
RANGE
value for each temperature channel.
Table 47. SELECTING A T
RANGE
VALUE
Bits [7:4] (Note 1)
T
RANGE
(
5
C)
0000
2
0001
2.5
0010
3.33
0011
4
0100
5
0101
6.67
0110
8
0111
10
1000
13.33
1001
16
1010
20
1011
26.67
1100
32 (Default)
1101
40
1110
53.33
1111
80
1. Register 0x5F configures Remote 1 T
RANGE
; Register 0x60
configures Local T
RANGE
; Register 0x61 configures Remote 2
T
RANGE
.
Actual Changes in PWM Output
(Advanced Acoustics Settings)
While the automatic fan control algorithm describes the
general response of the PWM output, it is also necessary to
note that the enhance acoustics registers (0x62 and 0x63)
can be used to set/clamp the maximum rate of change of
PWM output for a given temperature zone. This means that
if T
RANGE
is programmed with an AFC slope that is quite
steep, a relatively small change in temperature could cause
a large change in PWM output and possibly an audible
change in fan speed, which can be noticeable/ bothersome
to end users.
Decreasing the speed the PWM output changes by
programming the smoothing on the appropriate temperature
channels (Register 0x62 and Register 0x63) changes how
fast the fan speed increases/decreases in the event of a
temperature spike. The PWM duty cycle increases slowly
until the PWM duty cycle reaches the appropriate duty cycle
as defined by the AFC curve.
Figure 61 shows PWM duty cycle vs. temperature for
each T
RANGE
setting. The lower graph shows how each
T
RANGE
setting affects fan speed vs. temperature. As can be
seen from the graph, the effect on fan speed is nonlinear.
Figure 61. T
RANGE
vs. Actual Fan Speed
(Not PWM Drive) Profile
2
5
C
80
5
C
53.3
5
C
40
5
C
32
5
C
26.6
5
C
20
5
C
16
5
C
13.3
5
C
10
5
C
8
5
C
6.67
5
C
5
5
C
4
5
C
3.33
5
C
2.5
5
C
2
5
C
80
5
C
53.3
5
C
40
5
C
32
5
C
26.6
5
C
20
5
C
16
5
C
13.3
5
C
10
5
C
8
5
C
6.67
5
C
5
5
C
4
5
C
3.33
5
C
2.5
5
C
TEMPERATURE ABOVE T
MIN
0
PWM DUTY CYCLE (%)
0
20
40
60
80
100
120
10
20
30
40
50
60
70
80
90
100
20
30
40
50
60
70
80
TEMPERATURE ABOVE T
MIN
F
AN SPEED (% OF MAX)
0
20
40
60
80
100
120
10
90
100
0
The graphs in Figure 61 assume that the fan starts from
0% PWM duty cycle. Clearly, the minimum PWM duty
cycle, PWM
MIN
, needs to be factored in to see how the loop
actually performs in the system. Figure 62 shows how
T
RANGE
is affected when the PWM
MIN
value is set to 20%.
It can be seen that the fan actually runs at about 45% fan
speed when the temperature exceeds T
MIN
.
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