Analog Devices ADT7473 Manual page 40

ADT7473
T is implemented as a slope, which means that as PWM
RANGE
is changed, T changes, but the actual slope remains the
RANGE
same. The higher the PWM
MIN
T , that is, the fan reaches full speed (100%) at a lower
RANGE
temperature. Figure 55 shows how increasing PWM
the effective T .
RANGE
100%
50%
33%
25%
10%
0%
T
MIN
Figure 55. Increasing PWM
For a given T value, the temperature at which the fan runs
RANGE
at full speed for different PWM
calculated as follows:
T = T + (Max DC − Min DC) × T
MAX MIN
where:
T is the temperature at which the fan runs full speed.
MAX
T is the temperature at which the fan turns on.
MIN
Max DC is the maximum duty cycle (100%) = 255 decimal.
Min DC is equal to PWM
MIN
T is the duty PWM duty cycle vs. temperature slope.
RANGE
Example 1: Calculate T, given that T
and PWM = 10% duty cycle = 26 (decimal).
MIN
T = T + (Max DC − Min DC) × T
MAX MIN
T = 30°C + (100% − 10%) × 40°C/170
MAX
T = 30°C + (255 − 26) × 40°C/170
MAX
T = 84°C (effective T
MAX
Example 2: Calculate T , given that T
MAX
40°C, and PWM = 25% duty cycle = 64 (decimal).
MIN
T = T + (Max DC − Min DC) × T
MAX MIN
T = 30°C + (100% − 25%) × 40°C/170
MAX
T = 30°C + (255 − 64) × 40°C/170
MAX
T = 75°C (effective T
MAX
value, the smaller the effective
changes
MIN
30°C
40°C
45°C
54°C
Changes Effective T
MIN RANGE
values can be easily
MIN
/170
RANGE
.
= 30°C, T = 40°C,
MIN RANGE
/170
RANGE
= 54°C)
RANGE
= 30°C, T
MIN RANGE
/170
RANGE
= 45°C)
RANGE
Example 3: Calculate T
MIN
40°C, and PWM
T = T
MAX
T = 30°C + (100% − 33%) × 40°C/170
MAX
T = 30°C + (255 − 85) × 40°C/170
MAX
T = 70°C (effective T
MAX
Example 4: Calculate T
40°C, and PWM
T = T
MAX
T = 30°C + (100% − 50%) × 40°C/170
MAX
T = 30°C + (255 − 128) × 40°C/170
MAX
T = 60°C (effective T
MAX
Selecting a T
The T value can be selected for each temperature channel:
RANGE
Remote 1, local, and Remote 2 temperature. Bits <7:4> (T
of Registers 0x5F to 0x61 define the T
temperature channel.
Table 12. Selecting a T
1
Bits <7:4>
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
1
Register 0x5F configures Remote 1 T
T ; Register 0x61 configures Remote 2 T
RANGE
Summary of T
When using the automatic fan control function, the
temperature at which the fan reaches full speed can be
=
calculated by
T = T
MAX
Equation 1 holds true only when PWM
PWM duty cycle.
Rev. 0 | Page 40 of 76
, given that T
MAX MIN
= 33% duty cycle = 85 (decimal).
MIN
+ (Max DC − Min DC) × T
MIN
= 40°C)
RANGE
, given that T
MAX MIN
= 50% duty cycle = 128 (decimal).
MIN
+ (Max DC − Min DC) × T
MIN
= 30°C)
RANGE
Slope
RANGE
RANGE
Value
RANGE
T (°C)
RANGE
2
2.5
3.33
4
5
6.67
8
10
13.33
16
20
26.67
32 (default)
40
53.33
80
; Register 0x60 configures local
RANGE
.
RANGE
Function
RANGE
+ T
MIN RANGE
MIN
= 30°C, T =
RANGE
/170
RANGE
= 30°C, T =
RANGE
/170
RANGE
)
RANGE
value for each
(1)
is equal to 33%