ADM1024(2000) データシートの表示（PDF） - Analog Devices
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ADM1024 Datasheet PDF : 28 Pages
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ADM1024
6. In all these circuits, the output transistor must have an ICMAX
greater than the maximum fan current, and be capable of
dissipating power due to the voltage dropped across it when
the fan is not operating at full speed.
7. If the fan motor produces a large back e.m.f when switched
off, it may be necessary to add clamp diodes to protect the
output transistors in the event that the output goes very
quickly from full scale to zero.
FAULT-TOLERANT FAN CONTROL
The ADM1024 incorporates a fault-tolerant fan control capability
that can override the setting of the analog output and force it to
maximum to give full fan speed in the event of a critical over-
temperature problem even if, for some reason, this has not been
handled by the system software.
There are four temperature set points that will force the analog
output to FFh if any one of them is exceeded for three or more
consecutive measurements. Two of these limits are programmable
by the user and two are hardware limits intended as must not exceed
limits that cannot be changed.
The analog output will be forced to FFh if:
The temperature measured by the on-chip sensor exceeds the limit
programmed into register address 13h.
or
The temperature measured by either of the remote sensors exceeds
the limit programmed into address 14h.
or
The temperature measured by the on-chip sensor exceeds 70°C,
which is hardware programmed into a read-only register at
address 17h.
or
The temperature measured by either of the remote sensors exceeds
85°C, which is hardware programmed into a read-only register
at address 18h.
Once the hardware override of the analog output is triggered,
it will only return to normal operation after three consecutive
measurements that are 5 degrees lower than each of the above
limits.
The analog output can also be forced to FFh by pulling the
THERM pin (Pin 2) low.
The limits in registers 13h and 14h can be programmed by the
user. Obviously these limits should not exceed the hardware values
in registers 17h and 18h, as they would have no effect. The power-
on default values of these registers are the same as the two
hardware registers, 70°C and 85°C respectively, so there is no
need to program them if these limits are acceptable.
Once these registers have been programmed, or if the defaults
are acceptable, the values in these registers can be locked by
writing a 1 to Bits 1 and 2 of Configuration Register 2 (address
4Ah). This prevents any unauthorized tampering with the limits.
These lock bits can only be written to 1 and can only be cleared
by power-on reset or by taking the RESET pin low, so registers
13h and 14h cannot be written to again unless the device is
powered off, then on.
LAYOUT AND GROUNDING
Analog inputs will provide best accuracy when referred to a clean
ground. A separate, low-impedance ground plane for analog
ground, which provides a ground point for the voltage dividers
and analog components, will provide best performance but is
not mandatory.
The power supply bypass, the parallel combination of 10 µF
(electrolytic or tantalum) and 0.1 µF (ceramic) bypass capacitors
connected between Pin 9 and ground, should also be located as
close as possible to the ADM1024.
FAN INPUTS
Pins 5 and 6 may be configured as analog inputs or fan speed
inputs by programming Bits 0 and 1 of the Channel Mode
Register. The power-on default for these bits is all zeroes, which
makes Pins 5 and 6 fan inputs.
Signal conditioning in the ADM1024 accommodates the slow
rise and fall times typical of fan tachometer outputs. The
maximum input signal range is 0 to VCC. In the event that these
inputs are supplied from fan outputs that exceed 0 V to 6.5 V,
either resistive attenuation of the fan signal or diode clamping
must be included to keep inputs within an acceptable range.
Figures 17a to 17d show circuits for most common fan tach
outputs.
If the fan tach output has a resistive pull-up to VCC it can be directly
connected to the fan input, as shown in Figure 17a.
12V
VCC
PULL-UP
4.7k⍀
TYP
FAN1 OR
FAN2
TACHO
OUTPUT
160k⍀
FAN SPEED
COUNTER
Figure 17a. Fan with Tach. Pull-Up to +VCC
If the fan output has a resistive pull-up to 12 V (or other voltage
greater than 6.5 V), the fan output can be clamped with a Zener
diode, as shown in Figure 17b. The Zener voltage should be
chosen so it is greater than VIH but less than 6.5 V, allowing
for the voltage tolerance of the Zener. A value of between 3 V
and 5 V is suitable.
12V
VCC
PULL-UP
4.7k⍀
TYP
FAN1 OR
FAN2
TACHO
OUTPUT
ZD1*
ZENER
160k⍀
FAN SPEED
COUNTER
*CHOOSE ZD1 VOLTAGE APPROX. 0.8 ؋ VCC
Figure 17b. Fan with Tach. Pull-Up to Voltage >6.5 V
(e.g., 12 V ) Clamped with Zener Diode
REV. 0
–15–
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