Step 10: High And Low Limits For Temperature Channels - Analog Devices dBCool ADT7473 Manual

ADT7473
STEP 10: HIGH AND LOW LIMITS FOR
TEMPERATURE CHANNELS
T he low limit defines the temperature at which the T
s tarts 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 temper
ture drops below the low limit.
The high limit defines the temperature a
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
Register 0x4E, Remote 1 Temperature Low Limit = 0x01
Register 0x4F, Remote 1 Temperature High Limit = 0x7F
Register 0x50, Local Temperature Low Limit = 0x01
Register 0x51, L ocal Temperature High Limit = 0x7F
Reg ster 0x52, Remote 2 Temperature Low Limit = 0x01 i
Re gist er 0x53, Remote 2 Temperature High Limit = 0x7F
How Dynamic T Control Works
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Th basic premise is as follows: e
1. Set the tar get 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 temperatur
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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value
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a-
t which the T value
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e,
Rev. A | Page 50 of 76
Short Cycle and Long Cycle
The ADT7473 implements t
cycle. The short cycle takes place every n monitoring cycles.
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 Dynamic T
Register 2 (0x37).
Local = CYL = Bits [5:3] f Dynamic T o
(0x37).
Remote 2 = CYR2 = Bits [7:6] of Dynamic T
Register 2 (0x37) and Bit 0 of Dynamic T
(0x36).
Table 16. Cycle Bit Assignments
Code Short Cycle
000 8 cycles 1 sec
001 16 cycles 2 sec
010 32 cycles 4 sec
011 64 cycles 8 sec
100 128 cycles 16 sec
101 256 cycles 32 sec
110 512 cycles 64 sec
111 1024 cycles 128 sec
Care should be taken when choosing the cycle time. A long
cycle time means that T is updated less often. If your system
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has very fast temperature transients, the dynamic T
loop is always lagging. If a cycle time is chosen that is t
the full benefit of changing T
needs to change again on the next cycle; in effect, it is over-
shootin g. 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.
WAIT n
MONITORING
CYCLES
CURRENT
TEMPERATURE
MEASUREMENT
T1(n)
IS T1(n) >
(OP1 – HYS)
OPERATING
POINT
TEMPERATURE
YES
OP1
PREVIOUS
TEMPERATURE
IS T1(n) – T1(n – 1)
MEASUREMENT
≤ 0.25°C
T1 (n – 1)
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
wo loops: a short cycle and a long
Control
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Control Re gister 2
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Control
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Control Register 1
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Long Cycle
16 cycles 2 sec
32 cycles 4 sec
64 cycles 8 sec
128 cycles 16 sec
256 cycles 32 sec
512 cycles 64 sec
1024 cycles 128 sec
2048 cycles 256 sec
control
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oo fast,
might not be realized and
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NO
DO NOTHING
DO NOTHING
YES (SYSTEM IS
COOLING OFF
FOR CONSTANT)
DECREASE T BY 1°C
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DECREASE T BY 2°C
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DECREASE T BY 4°C
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