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ADT7473
STEP 10: HIGH AND LOW LIMITS FOR
TEMPERATURE CHANNELS
The low limit defines the temperature at which the T
starts to be increased, if temperature falls below this value. This
has the net effect of reducing the fan speed, allowing the system
to get hotter. An interrupt can be generated when the tempera-
ture drops below the low limit.
The high limit defines the temperature at which the T
starts to be reduced, if temperature increases above this value.
This has the net effect of increasing fan speed to cool down the
system. An interrupt can be generated when the temperature
rises above the high limit.
Programming High and Low Limits
There are six limit registers; a high limit and low limit are
associated with each temperature channel. These 8-bit registers
allow the high and low limit temperatures to be programmed
with 1°C resolution.
Temperature Limit Registers
Reg. 0x4E, Remote 1 Temperature Low Limit = 0x01
Reg. 0x4F, Remote 1 Temperature High Limit = 0x7F
Reg. 0x50, Local Temperature Low Limit = 0x01
Reg. 0x51, Local Temperature High Limit = 0x7F
Reg. 0x52, Remote 2 Temperature Low Limit = 0x01
Reg. 0x53, Remote 2 Temperature High Limit = 0x7F
How Dynamic T
Control Works
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The basic premise is as follows:
1.
Set the target temperature for the temperature zone, which
could be, for example, the Remote 1 thermal diode. This
value is programmed to the Remote 1 operating
temperature register.
2.
As the temperature in that zone (Remote 1 temperature)
rises toward and exceeds the operating point temperature,
T
is reduced, and the fan speed increases.
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3.
As the temperature drops below the operating point
temperature, T
is increased, and the fan speed is
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reduced.
However, the loop operation is not as simple as described in
these steps. A number of conditions govern the situations in
which T
can increase or decrease.
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Short Cycle and Long Cycle
The ADT7473 implements two loops: a short cycle and a long
cycle. The short cycle takes place every n monitoring cycles.
value
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The long cycle takes place every 2n monitoring cycles. The
value of n is programmable for each temperature channel. The
bits are located at the following register locations:
Remote 1 = CYR1 = Bits <2:0> of Calibration Control
Register 2 (Address = 0x37).
value
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Local = CYL = Bits <5:3> of Calibration Control Register 2
(Address = 0x37).
Remote 2 = CYR2 = Bits <7:6> of Calibration Control Register
2 and Bit 0 of Calibration Control Register 1 (Address = 0x36).
Table 14. Cycle Bit Assignments
Code
000
001
010
011
100
101
110
111
Care should be taken when choosing the cycle time. A long
cycle time means that T
has very fast temperature transients, the dynamic T
loop will always be lagging. If a cycle time is chosen that is too
fast, the full benefit of changing T
needs to change again on the next cycle; in effect, it is over-
shooting. It is necessary to carry out some calibration to
identify the most suitable response time.
Figure 65 shows the steps taken during the short cycle.
CURRENT
TEMPERATURE
MEASUREMENT
T1(n)
OPERATING
POINT
TEMPERATURE
OP1
PREVIOUS
TEMPERATURE
MEASUREMENT
T1 (n – 1)
Rev. 0 | Page 48 of 76
Short Cycle
Long Cycle
8 cycles
(1 sec)
16 cycles
16 cycles
(2 sec)
32 cycles
32 cycles
(4 sec)
64 cycles
64 cycles
(8 sec)
128 cycles
128 cycles
(16 sec)
256 cycles
256 cycles
(32 sec)
512 cycles
512 cycles
(64 sec)
1024 cycles
1024 cycles
(128 sec)
2048 cycles
is updated less often. If your system
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might not be realized and
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WAIT n
MONITORING
CYCLES
IS T1(n) >
NO
(OP1 – HYS)
YES
YES
IS T1(n) – T1(n – 1)
≤ 0.25°C
NO
IS T1(n) – T1(n – 1) = 0.5 – 0.75°C
IS T1(n) – T1(n – 1) = 1.0 – 1.75°C
IS T1(n) – T1(n – 1) > 2.0°C
Figure 65. Short Cycle Steps
(2 sec)
(4 sec)
(8 sec)
(16 sec)
(32 sec)
(64 sec)
(128 sec)
(256 sec)
control
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DO NOTHING
DO NOTHING
(SYSTEM IS
COOLING OF
FOR CONSTANT)
DECREASE T
BY 1°C
MIN
DECREASE T
BY 2°C
MIN
DECREASE T
BY 4°C
MIN
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